Warehousing, Handling, and Picking
Systems 6
Learning Objectives
Storage and warehousing are central logistics tasks. They require logistics
systems which are to a great extent technology-driven and which any logistics
expert needs to work with in everyday practice. This chapter aims to familiarize
the reader with the basics of warehousing, handling and sorting systems. In so
doing, it will address the basic methods to organize storage areas and storage
processes. Furthermore, tried and tested solutions for warehouse equipment will
be explained. The main focus will be to elucidate the business-economical
contexts and effects associated with different solutions in order to point out
the diverse options to the readers and to assist them in the assessment thereof.
Keywords
¢ Functions of warehousing
¢ Storage systems and conveyors
¢ Technological and organizational warehousing designs
¢ Task areas within a warehouse
¢ Transport and handling of goods
¢ Integrating a warehouse into transport chains
H. Gleissner and J.C. Femerling, Logistics, Springer Texts in Business and Economics,
DOI 10.1007/978-3-319-01769-3_6, # Springer International Publishing Switzerland 2013
95
6.1 Warehousing Basics
Storage is a key element of the logistical system of services and occurs in all stages
of a supply chain and on all levels of the value-adding process and production
process. Storage is of particular importance for distribution logistics in the context
of end-customer delivery of goods. Apart from the actual storing of goods, additional functions, such as picking, packaging, or return processing need to be
fulfilled in this area.
There is a direct link between warehousing and inventory management (see
Chap. 7). At a warehouse, goods are prepared or transshipped for on-carriage to the
ultimate consumer and may undergo quality-checks and maintenance in cases of
long storage periods.
Warehouses also fulfill allocative functions within the distribution process to
direct shipments to their destinations in the right composition. Accordingly,
warehouses serve to bring together flows of goods from different sources and
places, e. g. production sites or suppliers. In one warehouse, an entire range of
products may thus be available, which can be transported to the respective distribution area or from which actual customer orders can be compiled. The on-carriage
may be organized in a way which utilizes the capacities of the outgoing loading or
transport unit most efficiently (see Chap. 5).
The Deutsches Institut fu¨r Normung, DIN e. V. (German Institute for
Standardization) and the Verein Deutscher Ingenieure (Association of German
Engineers) have developed several definitions with regard to the most important
warehousing terminology. Storage is any planned waiting time for goods or work
objects in the flow of materials and goods.1 Throughout the literature, the term flow
of materials is more commonly used for production processes in the industry sector.
In the distribution and trading sector, however, the term flow of goods is more
common.
A warehouse is a room or an area in which goods are stored in solid (general
cargo or bulk cargo), liquid, or gaseous state and where their quantity and value is
usually recorded. In this context, the term general cargo characterizes goods which
can be handled or carried by conventional conveyors either directly or in their
respective packaging. Bulk cargo consists of very small, loose, and pourable
conveying goods. Storage, in the economic sense, is a deliberate process. However,
only if the goods stored are documented the term storage actually applies.
Conveyance occurs if goods are moved within a storage area or plant, e.g. from
or towards the productions site. Thus, conveyance mainly refers to in-plant flows of
materials and goods. Within the confines of such a system, conveyance may also
comprise the movement of people, besides the movement of goods. The conveyance of goods from one system to another (warehouses, production sites etc.) over
long distances is referred to as transport.
1 Cf. VDI code No. 2411.
96 6 Warehousing, Handling, and Picking Systems
A package is the most frequently occurring element in both warehousing and
transport. A package consists of the packaged good (the commodity) and its
packaging, which should also be suitable for conveyance and transport.2 Several
packages or goods with the same destination may be compiled to loading units
for the purpose of efficient conveyance, transshipment, and transport. Loading
aids are usually used to form loading units. Pallets are the most commonly used
loading aid.3
6.2 Storage Facilities
Apart from the basic storage function that warehouses assume, they also fall into
different operational categories for storage, buffering, and distribution. Depending
on warehouse type, they primarily serve to bridge the time or to change the
composition of a package, loading unit, or consignment between receipt and
dispatch at the warehouse. If there is a focus on offsetting fluctuations in demand
over more or less long periods of time, the warehouse is referred to as a storage
warehouse (procurement logistics). Innovative logistics solutions aim to minimize
the inventory in such storage warehouses (see Chap. 7). A buffer warehouse serves
to offset imbalances between receipt and dispatch for relatively short periods of
time. This type of warehouse is especially important in production logistics, where
the output and input for individual, consecutive production steps need to be
balanced. It is the task of production planning to minimize these buffer stocks in
order to reduce the amount of fixed capital in the production process. Distribution
warehouses (distribution logistics) serve to direct the flow of goods to the end
customer, to make goods quickly available in the respective sales regions, and to
compose a consignment according to the customer’s specifications.
Different storage systems are available to render the storing process efficient.
Which kind of storage system is utilized is dependent on the goods and on the
market’s requirement for availability (service level). Furthermore, costs are an
important factor when choosing a storage system. Maximum utilization of storage
area or space volume, small investments for warehouse equipment, as well as
simple and cost-efficient processes for storage, dispatch, and picking with high
output are the main factors to consider. Thus, storage systems fall into three
categories. In a static storage system the packages or loading units remain in one
place without being moved between admission to the warehouse and taking out of
the warehouse. This does not include movements as a result of a temporary
withdrawal to remove individual parts of a loading unit or to re-stock the loading
unit in another place. In dynamic storage systems the goods are moved within the
storage area during their storage time. These movements are triggered by specific
storage techniques, which, for example, enable goods to move forward as soon as
2 Cf. DIN code no. 55405.
3 Cf. DIN code no. 30781.
6.2 Storage Facilities 97
one unit is removed at the front. Another distinction can be made according to the
accessibility of storage goods. In cases where line storage is employed, all loading
units can be accessed at any time, which is not possible if the loading units are
stacked in compact block storage. Finally, we distinguish between storage systems
with storage racks and without storage racks.
6.2.1 Static Storage Systems
Block storage is the simplest form of storage. The packages or loading units are in
this case stored in large blocks on the floor. If they are stable enough they may also
be stacked on top of one another. Only those loading units which are on top along
the aisles are directly accessible, which corresponds to the system of stacking. Most
block storage systems work this way and thus achieve high storage space utilization. The greatest disadvantage here is the limited accessibility of all storage goods,
which precludes the use of storage strategies such as First In-First Out (FiFo) (see
Sect. 6.6). Block storage may also be designed as a line storage system, to provide
better accessibility. In this way, however, much storage space is taken up by the
aisles between the lines. Block storage is mainly used for storing large-volume,
heavy and sturdy goods and is often found in outdoor storage areas (see Fig. 6.1).
Since block storage does not necessitate any rack and shelving systems, no
further investments need to be made and the storage space can be used flexibly
for various storage goods and transshipment or retrieval options. The possible
stacking heights vary according to the goods being stored. In most cases, however,
they do not reach the same level of space utilization as rack and shelving systems
do. The level of space utilization results from the share of the storage volume
(including operating aisles between racks) in the overall warehouse volume.
Pallet racks are the most commonly used storage system. Using the loading aid
of the pallet, goods and loading units are stored on especially designated shelves.
We distinguish between one-unit and multi-unit space systems. Multi-unit systems
Fig. 6.1 Block storage figure
98 6 Warehousing, Handling, and Picking Systems
are designed to store more than one pallet next to each other on the shelf. Pallets can
have varying measurements, but in most cases they are rectangular. Therefore, they
can either be stored deep (mostly three next to each other), which results in greater
capacity utilization, or they may be stored horizontally if goods are picked directly
from the pallet. Pallets can also be stored one behind the other. If a pallet rack
exceeds a height of approximately 15 m, it is called high rack.4 Depending on type
of goods and picking frequency the pallet racks may be separated by broad operating
aisles or they can be designed as a narrow-aisle construction (see Fig. 6.2).
In contrast to block storage systems, pallet racks provide direct accessibility to
each loading unit, as long as they are stored next to each other on the shelf. If
standard pallet types are used, pallet racks also guarantee relatively high flexibility
and transshipment performance with regard to goods types and picking
requirements. The high storage volume utilization which can be achieved using
this type of storage system is especially advantageous in metropolitan areas where
storage space is costly. The ground on which a pallet rack is set up needs to meet
certain constructional requirements irrespective of whether the construction is
located outdoors or in a warehouse. In comparison to a block storage system,
additional costs may arise for investments in shelving technologies and possibly
also for technologically more advanced conveyor techniques used in the storage
and retrieval processes.
Instead of using pallets, rack shelves may also serve to store small parts in
containers of different sizes. To this end, the space between the shelving levels can
be reduced. This storage type is termed container shelving. The storage of goods on
flat, trough-like plates (trays) is referred to as tray storage.
Drive-in racks or drive-in shelves combine pallet shelving and block storage and
are used for storing palletized goods. In principle, they are similar to one-unit pallet
shelves. With this storage system, however, several pallets can be stored deep one
Fig. 6.2 Pallet rack
4 Cf. Schulte (2009), p. 234.
6.2 Storage Facilities 99
behind the other. This type of shelf can only be operated from one front side.
Storage and retrieval is carried out by forklifts, which drive between the rack
supports.
The storage principle of drive-in racks greatly limits the accessibility of the
pallets (see Fig. 6.3). Beginning at the back, the loading units need to be stored from
the top to the bottom, while they have to be retrieved the other way round. This
means that merely the LiFo (Last in-First out) principle can be implemented. At
first glance, the storage space utilization seems to be higher in comparison to pallet
rack systems. In practice, though, this can only be achieved with a high number of
articles coupled with a small range of articles.5 Accordingly, drive-in racks are used
for articles which are heavy or pressure-sensitive and therefore cannot be stored in a
block storage system.
If the articles can be taken out on the other side, the drive-in rack is referred to as
a drive-through rack. These racks allow for the realization of the FiFo principle,
which is in many cases more cost-effective.
Bay shelves are made up of vertical posts and shelves which can be mounted or
screwed into the construction. They are primarily operated manually without the
use of ground conveyors (see Fig. 6.4).
Since the shelves can be easily mounted on the posts by means of hole pins, this
storage system offers great flexibility. Additionally, it provides high storage space
utilization as the stored articles determine which kind of shelf will be used. This
type of shelf is mainly used to store loose small parts or small parts in boxes or outer
packing. Outer packaging is an additional piece of packaging – such as plastics or
straps –, which serves to keep the goods together in their original wrapping. Since
each of the shelves is accessible, a broad range of articles can be stored. Picking
position and distance are decisive for quick accessibility in the picking process.
Fig. 6.3 Drive-in rack and drive-through rack (Cf. Ju¨nemann and Schmidt (2000), p. 52.)
5 Cf. Ju¨nemann and Schmidt (2000), p. 51.
100 6 Warehousing, Handling, and Picking Systems
Articles that are stored on top or at the bottom are more difficult to pick than those
stored at breast height.
Cantilever shelves consist of vertical posts on arms jutting out laterally. Long
loads, such as tubes, profiles, bars, beams, and boards are stored on these arms. The
number of arms on top of each other and the arm length are determined by the
measurements and weight of the storage goods (see Fig. 6.5).
If the articles are supposed to be individually accessible, the number of articles
per cantilever arm is limited.
6.2.2 Dynamic Storage Systems
Live storage shelving is the most commonly used dynamic storage system. The
loading units are stored on slightly inclined roller lanes and are fed in at the higher
Fig. 6.5 Cantilever shelf
Fig. 6.4 Bay shelf
6.2 Storage Facilities 101
end of the lane and taken out at the lower end. As soon as one loading unit is
removed, the remaining units move forward automatically. Live storage shelves
require the articles to be stored on suitable load carriers or to be packed in stable
cardboard boxes (see Fig. 6.6).
The moving load carriers enable the implementation of a FiFo principle. High
picking and withdrawal performance can be achieved if each shelf stores one
article. The main advantages of this storage system are the separation between
the feeding and exit aisles as well as its high storage space utilization if the lanes are
full.
Push-back shelving uses the same construction as live storage shelving. The
articles, however, are both fed in and taken out at the lower end of the lane. This
kind of shelf is only used for palletized articles. The first pallet is put on the roller
lane and pushed back as the second pallet is fed in (see Fig. 6.7).
In line with the LiFo principle, the goods are fed in and taken out at the same
end. The pallets are supposed to push back those pallets which have already been
Fig. 6.6 Live storage shelf
Fig. 6.7 Push-back shelf
102 6 Warehousing, Handling, and Picking Systems
stored on the shelf. This needs to be taken into account when opting for a shelving
system. Furthermore, the pallets are prone to canting.
Satellite storage systems are a variation to push-back shelving. With this storage
system, the pallets are put on self-propelled carts which independently drive into
the lanes. Subsequently, the carts drive back to the point of loading. This system’s
advantage over push-back systems is the fact that forklifts or shelving operating
devices can proceed with other tasks while the pallet is still being stored.
A vertical carousel (paternoster) is a type of shelving storage system in which
the individual shelves are attached to two vertically circulating chains. Using a
control panel, the required shelf is moved to the discharge opening (see Fig. 6.8).
In order to avoid full circulations when retrieving goods, the same articles should
ideally be grouped together and stored on different levels. To this end, the shelves
are partitioned and allow for separation according to article type. The handling
performance is dependent on the height of the carousel and on the range of articles.
Paternosters offer a higher percentage of storage space utilization than bay shelving
systems while providing the same accessibility. The system is lockable and is well
suited for the storage of small to medium-sized goods and high-value goods.
Manual operation, e.g. in the case of a system failure, is not possible.
A horizontal carousel is another type of shelving storage system. A horizontal
overhead conveyor moves the individual shelves, which are attached to the conveyor. Using a control panel, the requested shelves are moved to the side of
discharge (see Fig. 6.9). Thus, the goods-to-man principle (see Sect. 6.5) applies
both to vertical and horizontal carousels.
This shelving type is primarily used in picking warehouses. Employment of this
solution begs the question whether the picking person would have access to the
needed article more quickly on foot or using this type of system. Ideally, several
carousels should be inter-connected in one picking zone so that the requested article
can be retrieved during the picking person’s transit time. Transit time describes the
time that is needed for a person to move from one picking point to another.
Fig. 6.8 Vertical carousel
6.2 Storage Facilities 103
With slide shelving, the pallet shelves or bay shelves are mounted on slide racks,
which are operated either manually or electrically. Only one or two aisles are open
along the entire shelving block. The remaining shelves are densely put together. If
goods are needed from one specific aisle, the shelves are moved sideward to open
up a new aisle (see Fig. 6.10).
Highly cost-effective storage and warehouse space utilization is contrasted by
limited simultaneous accessibility to goods stored in different aisles. For this
reason, this type of storage system is predominantly used for archiving or for
storing spare parts, e.g. in workshops. Among the system’s advantages are optional
lockability and accessibility to individual goods.
Case Study 6.1: Alternative Storage Systems
A mail-order company is planning to set up an additional auxiliary warehouse
for cardboard boxes palletized on euro pallets. This is to be implemented on the
existing company grounds, which cannot be expanded any further. The company
offers a large range of articles and each article must be accessible at all times.
Furthermore, it is imperative that opened pallets can be re-stored in their open
Fig. 6.10 Slide shelves
Fig. 6.9 Horizontal carousel
104 6 Warehousing, Handling, and Picking Systems
state. The average height of one pallet is 1 m and the pallets are not stackable. To
cover the long-term storage requirements, the new warehouse is supposed to
have more than 6,000 pallet spaces available. Constructional restrictions by the
responsible building authority allow an indoor clearance of 13 m.
The planning department is discussing three alternative storage systems:
(a) Pallet shelving (storage space utilization 45 %),
(b) Live storage shelving (storage space utilization 65 %),
(c) Block storage (storage space utilization 80 %).
The level of storage space utilization is the share of the storage area (including operational aisles between shelves but excluding main traffic areas and zones
for receiving, issuing, and provisioning goods) in the total area of the warehouse.
For safety purposes and to ensure sufficient free scope for storing and retrieving,
a clearance height of 0.5 m per pallet needs to be factored in for pallet shelving.
Other areas, such as goods receiving, goods issuing, and main traffic areas may
be disregarded in the calculation.
Calculate the space requirements for each of the three alternatives.
Considering the circumstances, which storage system would you
recommend?
6.3 Conveyors
To enable conveyance, i.e. the movement of goods within systems over short
distances, technological aids – so-called conveyors or conveyor systems – need to
be employed.6 Conveyors transport goods within locally determined operational
units, e.g. within production sites, warehouses, transshipment terminals or, airports.
Besides their main task of conveying, conveyor technologies serve additional
purposes such as distributing or sorting (sorting out of shipments), collecting
(merging of individually incoming packages), buffering (transit time from
dispatching location to destination or accumulation zones at the end of conveyor),
or picking (sorting of goods for outgoing shipments or customer shipments). If
some of the conveying goods can be discharged at certain points along the conveyor, we also talk about sorters or sorting technology.
Conveyor systems can be distinguished according to the following criteria:
¢ Discontinuous conveyors produce an intermittent stream of conveying goods
and operate in individual working cycles (so-called intermittent conveyors).
¢ Continuous conveyors convey bulk or general cargo continuously.
¢ Floor-bound conveyors drive on the floor or on installations that are flushmounted in the floor.
¢ Supported conveyors move above the floor on supports or on supported rails.
¢ Suspended conveyors move along rails on the ceiling.
6 Cf. DIN 30781.
6.3 Conveyors 105
6.3.1 Discontinuous Conveyors
Manual lift trucks are manually-operated conveyors for the movement of single
pallets on short distances. They are used for loading and un-loading vehicles (trucks
or trains wagons).
Manual lift trucks can only lift a pallet to a height which enables them to drive
carrying the pallet. For this reason they cannot be used to stack goods. Due to their
technical simplicity, sturdiness, and good maneouvrability, manual lift trucks are
suited to cover short distances in cramped spaces (see Fig. 6.11).
Lift pallet trucks, which belong to the category of stackers, are a variation of the
manual lift truck. The base lift functionality as a so called low lift pallet truck
enables them to stack pallets at a low height, around 1 m, which is why they are
frequently used in the goods receiving and goods outgoing departments where goods
are stacked up. Lift pallets trucks may also operate in storage areas. There are also
high lift pallet trucks available to reach stacking highs up to 5 m (see Fig. 6.12).
Both manual lift trucks and lift pallet trucks can be fitted with an engine. This
may serve to support staff members in pulling or pushing the truck, or the truck may
be designed as a driver-seated lift truck. Both trucks are mainly used indoors since
they cannot be driven on uneven surfaces due to their small wheels.
Fig. 6.11 Manual lift truck
Fig. 6.12 Lift pallet truck
106 6 Warehousing, Handling, and Picking Systems
There are different types of stackers to be utilized according to goods
requirements and varying weights, heights, or ground conditions. Their main
differentiators are their engine type (diesel, LPG, or electric), add-on devices
(e.g. fork, clamp, mandrel, gripper, and lifting platforms), steering (four-wheel
drive and three-wheel drive) and tires (large and small pneumatic tires or solid
rubber tires).7 Examples of the most commonly used types of stackers are explained
below.
A counterbalance forklift or frontload truck carries the load unsupportedly in
front of the front wheels. These kinds of stackers therefore require a certain overall
length to counterbalance the weight. The reachable lifting height depends on the
construction of the stacker but is in most cases higher than with a lift pallet truck
(see Fig. 6.13).
Due to their robust construction and relatively large pneumatic-tired wheels,
counterbalance forklifts can be operated outdoors to transport medium-sized loads
at high speeds. Since these characteristics render it a truck for various purposes, its
lifting height and maneuverability are limited, which makes it unsuitable for narrow
operating aisles. The most common types are the usually smaller and more versatile
three-wheel stacker as well as the four-wheel stacker, which is more robust and
hence more suited to transport heavier loads.
Stackers with outriggers counterbalance the transported load by means of
outriggers. Among these, the most important type is the moving-mast reach truck.
It is the most frequently used stacker for in-house storage of pallets since the
required aisle width is smaller than it is for counterbalance forklifts. This type of
stacker is a mixture of a counterbalance forklift and a forklift with outriggers. For
loading and unloading, the mast moves forward to the front wheels. During
transport the mast is retracted which reduces the length of the stacker. Movingmast reach trucks are used for medium-sized loads. Their load-carrying capacity
decreases with increasing lifting height (see Fig. 6.14).
Fig. 6.13 Counterbalance
forklift or frontload truck
7 Cf. Schulte (2009), p. 161.
6.3 Conveyors 107
Their protruding outriggers render these trucks unsuitable for lifting up pallets
from the floor since the pallets must be hoisted over the outriggers and the mast
needs to be retracted towards the driver’s cabin before driving. Neither are movingmast reach trucks very well suited for outdoor conditions due to their small wheels.
They are mainly employed for stacking rather than for horizontal load handling.
Other types of stackers with outriggers include the pantograph reach truck.
Unlike the moving-mast reach truck, on which the mast can be moved, this type of
stacker moves its fork horizontally. If the outriggers are able to go underneath the
rack, pantograph reach trucks, as opposed to moving-mast reach trucks, may
perform storage tasks several loading units deep into the rack without the need
for an extended fork.
The type of stacker that requires the narrowest aisle width is the 4-way reach
truck. It features the construction of a moving-mast reach truck but is additionally
fitted with all-wheel steering. Since it is thus possible to drive sideways, this type of
stacker is especially suitable for storing long loads.
Narrow aisle trucks are exclusively used for shelving operations. An aisle width
of a mere 1.5 m or more is sufficient for them to be employed. Within the aisle,
narrow aisle trucks run on rails alongside the shelves. Outside the aisle they change
into unguided drive.
Owing to their lateral operability, narrow aisle trucks can move pallets both
horizontally and vertically (diagonally), which leads to quicker cycle times. The
cycle time is the time required for a single cycle or a double cycle. A single cycle
comprises the pick-up and the storage of goods as well as the empty return run of
the conveyor. Double-cycle runs also include the pick-up of another good from the
same aisle and its transport to the point of departure. There should be a preference
for double-cycle runs to ensure better time efficiency.
Narrow aisle trucks are capable of positioning heavy loads precisely at great
heights. This necessitates solid rubber tires and thus a floor of good quality (see
Fig. 6.15).
Narrow aisle trucks come in different forms. On a very basic level we can
distinguish between man up trucks and man down trucks. With the man up
Fig. 6.14 Reach truck
108 6 Warehousing, Handling, and Picking Systems
principle, the operator elevates with the load for increased fine-tuning of the fork
and direct accessibility to the articles on the shelf. Man-down trucks are employed
only if entire pallets are being handled. In this case the driver needs a positioning
aid for precise pick-up of the pallet.
Stacker cranes have guide rails on the floor and for precise and safe at the top of
the mast lateral operability. These conveyors serve to operate the bays of a storage
rack either manually or automatically. They are predominantly floor-travelling and
guided by rails. Only in exceptional cases, mostly with outdated installations, they
Fig. 6.16 Stacker crane
Fig. 6.15 Narrow aisle truck
Fig. 6.17 Automated
Guided Vehicle (AGV)
6.3 Conveyors 109
Fig. 6.18 Comparison of common conveyors
110 6 Warehousing, Handling, and Picking Systems
are suspended or attached to the racks.8 In any case, however, they need to be
guided from the ceiling or the upper edge of the rack. Stacker cranes cannot be
driven outside the storage zone. The changing of the aisles is implemented by
means of curves and switches or using a separate transfer carrier which picks up the
entire unit, drives out of the alley, and changes aisles (see Fig. 6.16).
The additional guiding rail makes quick diagonal steering in high warehouses
possible. Storage heights of 40 m can thus be reached without any problems. The
stacker crane shown above features a telescopic fork to be used for pickup. Stacker
cranes are often automated but also all types of conveyors covered so far can be
operated automatically.
Automated Guided Vehicle systems (AGV) convey goods automatically throughout the entire interior area of the warehouse. Automated conveyors navigate
through the warehouse autonomously using radio or infrared signals once they
have received control data. To this end, orientation points are mostly used along
which the AGV determines its position (see Fig. 6.17).
Depending on the controlling system, it may be difficult to change the routes
(steered by induction loop), which renders the system rather inflexible. Compared
to manually operated conveyors an AGV’s conveying capacity is low. This is
because its speed is limited to about 1 m/s so that in an emergency case it can
come to a standstill as quickly as possible. AGVs used in closed-off indoor or
outdoor areas reach much higher speeds. The fact that their operation is unmanned
is of great advantage in cases where these conveyors are used for hazardous or highprecision tasks. Standardized load carriers and measurements for operating aisles
are prerequisite for the use of Automated Guided Vehicles.
Figure 6.18 summarizes the most important differentiators of aisle width and
stacking height.
Case Study 6.2: Conveying Capacity
Wilhelm Karmann GmbH – an automobile supplier from Osnabru¨ck –
specializes in the development and production of roofing systems. Additionally,
the company is responsible for the production and assembly of complete series
of models from various OEMs (Original Equipment Manufacturer). OEMs
manufacture ready-for-sale products (automobiles in this case). A new goods
receiving warehouse is being planned for the production of a new series of
models. The supplied components (interior components) are unloaded from the
truck using forklift trucks. How many forklift trucks are required if the following
task is to be completed?
¢ Conveyance of 5.190 pallets per week over a distance of 311 m on average
¢ Conveying speed: 2 m/s
¢ Conveying capacity utilization: 40 %
¢ Working hours: 15 h/day; 5 day/week
8 Cf. Ju¨nemann and Schmidt (2000), p. 119.
6.3 Conveyors 111
6.3.2 Continuous Conveyors
Continuous conveyors can be classified as non-floor-bound, floor-mounted, and
floor-bound devices. In most cases they are mechanized or automated, which allows
them to be integrated in different conveyor systems. Continuous conveyors are
mostly stationary, which means their layout is not very flexible, and they may pose
an obstruction to other conveyors or work equipment.9
Roller conveyors are the type of continuous conveyor most commonly used in
warehouses. They consist of many successively arranged, freely rotating rolls
which are fitted between two steel profiles. Roller conveyors are available with or
without an engine. Those equipped with an engine are moved by chains if heavy
loads are conveyed, while belts are sufficient for the conveyance of smaller loads.
Most roller conveyors are floor-mounted although they may also be non-floorbound as a suspending conveyor from the warehouse ceiling (see Fig. 6.19).
Roller conveyors have proven to be a reliable, low-maintenance type of conveyor for indoor and outdoor use. They are, however, only suitable for packaged
goods with at least one even surface. Changes in direction may be realized by curve
structures or turntables. Using special roller track components the conveying goods
can be accumulated to alter the distance between the goods or to singularize
individual goods.
Chain conveyors are a variation of the roller conveyor. They convey the goods
on engine-driven chains located on the side. Only goods with standardized
measurements, such as pallets, can be moved on these kinds of conveyors.
Since box sizes can vary greatly, chain conveyors are more suited to transport
pallets while roller conveyors lend themselves to the conveyance of boxes (see
Fig. 6.20).
Belt conveyors are used to convey goods whose nature does not allow transport
on rolls (e.g. bulk goods). Belt conveyors are comprised of a circulating conveyor
belt made of rubber, synthetic, or India rubber. The belt runs on rolls or glides along
a surface. At least one powered roll moves the conveyor, while the number of
Fig. 6.19 Roller conveyor
9 Cf. DIN 15201.
112 6 Warehousing, Handling, and Picking Systems
powered rolls is dependent on the length and weight of the conveying good (see
Fig. 6.21).
Belt conveyors generally reach higher speeds than roller conveyors. What is
more, the dampening effect of the conveyor belt as well as the missing relative
motion of the conveying good to the conveyor belt result in less noise being
generated. Curves can be formed on the belt conveyor by using tapered rolls.
Conveying routes can be generated by joining several separately powered belt
conveyors together, which makes it possible to accumulate and sort out goods.
The advantages of roller and belt conveyors can be brought together in a
combination of both these conveyor systems. Thus, the more inexpensive roller
conveyors may be used for straight sections while belt conveyors are employed for
curves and slopes since they provide better adhesion for the goods conveyed.
Circular conveyors transport general cargo by means of a suspension gear. The
suspension devices are connected to a permanently running chain. Circular
conveyors may run horizontally, vertically, or ascendingly. Consequently, this
allows for an almost unrestricted layout in path configuration. The suspension
gears can take different forms and may be designed for the particular load to be
handled. Thus, they are available as platforms, trays, boxes, forks, or hooks, to
name but a few.10 At the points of pickup and deposit, the conveyor is pulled
downwards, while for the rest of the conveying path it runs above the work area.
Fig. 6.21 Belt conveyor
Fig. 6.20 Chain conveyor
10 Cf. Bode and Preuß (2004), p. 227.
6.3 Conveyors 113
This way, the conveyor does not take up any floor space and obstructions can be
avoided (see Fig. 6.22).
Circular conveyors run continuously and they need to form a closed loop.
Power and Free conveyors are a variation of the circular conveyor. They feature
separate tracks for drivers and suspension gears. If needed, the suspension gears are
hooked into the drivers, which pull them along. This mechanism enables discontinuous conveyance and allows for switches to be integrated in the conveyor’s path.
Electric telpher lines also belong to the category of circular conveyors. This
conveyor need not run in a closed loop since each suspending device is selfpropelled. On the basis of the same throughput, an electric telpher line is more
lucrative over longer distances than a power and free conveyor. The latter is more
suited for given distances and higher throughput. Throughput means the volume
that is handled within a certain system over a certain period of time.
The electric pallet ground conveyor is a conveyor running on a system of tracks
in which the individual elements are propelled by electric engines. Pallets are not
only conveyed straight but crosswise transport also makes it possible to divert
certain pallets. Gravity can be utilized on sloping sections along the path in order to
forego the use of an electric engine. Changes in direction are realized by means of
turntables or transfer platforms (see Fig. 6.23).
In comparison to discontinuous conveyors, the advantage of pallet conveyor
systems is characterized by the possibility of transporting many pallets at the same
time without the need for large numbers of ground conveyors. Goods can be
transported to another floor more easily since a forklift truck, for example, would
have to change floors as well, or the goods would have to be passed on to another
truck. Changes in elevation can be overcome with the use of lifting tables. Inflexible
conveying paths and obstruction of working area constitute a disadvantage.
Electric pallet conveyors are frequently used for storage in high-rack
warehouses. In most cases, a discontinuous conveyor passes the pallets on to the
rail system at a transfer point, from where the pallets are conveyed automatically.
Fig. 6.22 Circular conveyor
114 6 Warehousing, Handling, and Picking Systems
Floor-mounted drag chain conveyors are propelled from one central point. At
certain intervals they are fitted with hooks and loops with which special carriers or
pallet trucks can be attached and conveyed. The system may be made up of one or
several main circles whose chains are propelled by several engines, depending on
how much power is needed. Carriers can be diverted or led in automatically at the
destination or departure point by using appropriate switch technologies.
This solid technology is especially common in transshipment terminals of
forwarders since their conveying paths are rather straightforward and need not be
changed. To preclude danger to crossing persons and to ensure easy attachment and
disengagement of carriers, the conveying speed is low.
Case Study 6.3: Floor-Mounted Drag Chain Conveyors in Logistics Centers
The logistics service provider Dachser GmbH & Co. KG. uses floor-mounted
drag chain conveyors in some of their logistics centers.
One of these conveying systems was implemented at the Dachser logistics
center Berlin/Brandenburg in Scho¨nefeld, where large volumes of orders are
processed. Quick handling and high throughput rates are among the main
objectives of the system. Furthermore, the operations can be carried out more
steadily and smoothly than it would be possible using discontinuous conveyors
(e.g. forklift trucks). As a consequence, the packages are not exposed to a high
risk of damage and staff members enjoy low accident risk. Finally, the independence of staff members operating the conveyor system from the order volume
leads to a degressive development of human resources with an increase in
throughput volume.
The operations in the warehouse are divided into a goods-incoming and
goods-outgoing process. The goods-incoming process comprises shipments
which are intended for distribution at the site in Berlin/Brandenburg and
which are usually received from other Dachser locations by 5 o’clock in the
Fig. 6.23 Electric pallet ground conveyor
6.3 Conveyors 115
morning. The goods-outgoing process deals with shipments which had been
posted in the afternoon in the greater Berlin area and in the bordering state of
Brandenburg by Dachser customers. These shipments have been taken to the
logistics center where they are transshipped in the evening for on-carriage to
national and international destinations. Transshipment in this process is carried
out using a floor-mounted drag chain conveyor system (see Fig. 6.24).
The shipments to be transshipped are largely made up of standardized pallets.
A pallet truck takes the pallets from the truck to the goods receiving zone, where
they are scanned and hooked into the floor-mounted drag chain. Information
about the pallet’s destination was attached to each pallet during the scanning
process. Once hooked into the conveyor system, the pallet is part of a continuously circulating system. As soon as the pallet reaches the loading gate of its
target relation, the pallet truck is disengaged from the chain and deposited at the
relation’s buffer space. The parking spaces at the destination (rails) serve as a
provisioning buffer until dispatch of the shipment on the same day. The empty
pallet truck is hooked into the chain system again. The intervals between the
pallet trucks as well as the conveying speed allow additional transshipment
processes with different conveyors (e.g. forklifts) to be carried out simultaneously. This renders the entire transshipment process highly flexible.
Fig. 6.24 Floor-mounted drag chain conveyor at the Dachser logistics center in Berlin/
Brandenburg
116 6 Warehousing, Handling, and Picking Systems
6.3.3 Sorters
Sorters are used to separate and sort goods while using conveying technologies.
Several sorter types are described in the following.
Push sorters are hydraulic push or pull arms which are installed above the
conveyor belt. They push or pull the conveying goods towards chutes that are
attached to the conveyor. The fact that they can also be added on to the conveyor at
a later point and their compatibility with roller as well as belt conveyors is
advantageous. The hydraulically operated pushers generate a high level of noise.
Only sturdy goods can be sorted using this technique due to the pusher’s high
impact speed onto the goods, which also depends on the conveying speed.
Tilt-tray sorters have platforms that can be tilted to one side. These platforms are
tilted so that the goods can slide down laterally under the force of gravity.
Conveying goods may be fed in or discharged at any point of the conveyor. To
this end, appropriate transfer or collecting equipment needs to be installed (see
Fig. 6.25).
Tilt tray sorters do not require a separate conveyor, it is an integrated conveyorsorter type. The goods are already separated on the conveyor or sorter before the
handling process and can thus be deposited at their destination in a quick and
accurate way. Problems may arise for goods with high friction since the tilting
mechanism works in the same way for all goods. Tilt-tray sorters are very well
suited for transporting robust, medium-weight goods. To achieve higher curve
speeds, the trays may be tilted in the curves or the side walls could be molded
into a bowl-shape.
A crossbelt sorter is a conveyor belt with traverse belt-trays, which are moved
by an electric engine upon passing the discharge position, thus discharging the
conveying good (see Fig. 6.26).
To avoid tipping of the goods, their bottom side should be flat due to the traction
impacting on it. The conveyor’s construction is not suitable for the transport of
heavy goods. It is, however, supremely suited to be used in e.g. mail order
companies, owing to its low noise generation and positioning accuracy if little
traction is impacting on the goods.
Sliding sorters or slat sorters offer high-performance sorting of a variety of
goods. This type of sorter consists of a slat conveyor – mostly made of extruded
Fig. 6.25 Tilt-tray sorter
6.3 Conveyors 117
aluminum carriers – on which the sliding blocks move. These push the goods
towards the discharge exits (chutes) (see Fig. 6.27).
This system can be used to sort goods either to one side or to two sides. With this
sorter, the number of the sliding blocks can be varied and thus be adapted to the
requirements of the conveying good.
Case Study 6.4: Tilt-Tray Sorters in Parcel Centers
Deutsche Post AG uses tilt-tray sorters in their parcel centers to sort those parcels
which are suitable for automated parcel handling, i.e. mostly parcels which do
not exceed or fall below a certain maximum size or minimum size, respectively.
Each carrier is equipped with two trays. If small parcels are being handled, each
of the trays can carry one parcel. Bigger parcels take up the space on both of the
carrier’s trays.
Shipments turned in by Deutsche Post customers at, for example, post offices
or post-office substations in the Leipzig area for delivery all over Germany, are
collected by vans or trucks at these offices and transported to the parcel center in
Leipzig. The shipments are unloaded there and individually put on a so-called
feeding belt, as can be seen in Fig. 6.28. The feeding belt is located laterally to
the sorter.
From there the parcels are fed consecutively onto the trays of the pre-sorter.
In doing so, the system automatically recognizes free and occupied trays. Each
pre-sorter is fitted with an address scanning device through which the parcel’s ID
and barcode as well as its address in plain text are identified. A picture is taken of
each of the parcel’s sides. The scanned barcodes are sent to the material flow
Fig. 6.26 Crossbelt sorter
Fig. 6.27 Sliding sorter or
slat sorter
118 6 Warehousing, Handling, and Picking Systems
computer (controller) while the delivery address, i.e. ZIP code, street, and house
number are identified using the plain text. As soon as the material flow computer
has identified the destination, it sends a feedback to the address scanner, which
then terminates the process and deletes the picture. On the pre-sorter, the tray ID
is then associated with the destination and discharge onto one of the connecting
belts is determined.
After discharge onto one of the connecting belts, the parcels are conveyed
onto a main sorter where they are scanned. The ID code and tray number are then
sent to the material flow computer which confirms the destination. Subsequently,
the computer decides which control system will trigger which tray to tilt the
parcel into the exit chute. The tilting mechanism is triggered by an electronic
contact prompting a discharge ramp to fold by which the tray is then tilted. The
main sorters supply all terminal points in the parcel center. Shipments to other
parcel centers are loaded onto swap bodies for on-carriage. Shipments for the
center’s own delivery district are loaded onto roller containers, which are then
transported to the delivery bases.
Similar sorters are used at the destination centers with which the parcels are
distributed to smaller delivery regions of the area.
6.4 Packages, Loading Units, and Load Carriers
The packaging of goods and the formation of packages serve more than purely
logistical functions.11 The packaging primarily has a protective function to preserve
the goods in a saleable state and to prevent damage or spoilage. The storage and
transport function enables the movement of goods within the logistics process.
The identification and informational function of the packaging supports the
controlling of goods. This kind of data may be attached to the packaging in plain
Fig. 6.28 Transition from feeding belt to tilt-tray sorter
11 Cf. Jansen (1989), p. 79.
6.4 Packages, Loading Units, and Load Carriers 119
text, as barcodes, or using a RFID data carrier. Barcodes, which are used for
identification, are made up of an opto-electronic script of lines that vary in thickness. RFID is a data transmission and storage technique based on radio technology
and is also used for identification purposes. The data can, for example, contain
information about the type of good, the content quantity, sender and receiver,
weight, volume and handling instructions (fragility, cooling, information for installation etc.).
The sales function is especially relevant for consumer goods. Thus, it should
contain buying stimulation to encourage customers to pick up the packaged item
from the shelf. This includes advertisements, price, colorful packaging, or interesting shapes.
Finally, the packing sometimes also fulfills a usage function. This is mainly the
case in the consumer goods sector, where e.g. food is consumed directly out of the
packaging. The packaging also serves a usage function if goods are directly taken
out of their packaging for assembly in production processes or if they are produced
into the packaging.
In order to efficiently handle goods in general and packages or general cargo in
particular, they are consolidated into loading units. Load carriers, such as pallets,
wire-mesh boxes, or containers are generally used to form loading units.
Packaging falls into the broad categories of disposable packaging and reusable
packaging. Disposable packaging is mostly made from cardboard and is designed
for one-time protection during transport since the goods are only in some cases
stored in it as well. Reusable packaging consists of more solid materials, such as
plastics or metal. This type of packaging is characterized by the fact that the goods
need not be taken out of the box during the entire supply chain to the installation
location because the packaging fulfills all of the above mentioned packaging
functions. Another advantage is the significant reduction in waste disposal costs
compared to disposable packing and the environmental aspects associated with it.
Return transport of empty packaging constitutes a disadvantage.
Packaging and loading units are further distinguished as follows.12 Packaging is
produced of packaging materials and serves to wrap up or hold together the
packaged goods in order to render it able to be dispatched, storable and salable.
Loading units are goods which are compounded for handling purposes by a load
carrier. The formation of loading units requires additional effort. The advantages
thus achievable, however, include efficient handling within the transport chain by
forming greater pieces of cargo, cost-efficient usability of means of storage,
conveyance, traffic and handling, as well as a general reduction of costs in the
flow of materials and enhanced delivery service.
As mentioned before, one way of forming loading units is the use of pallets.
Load-carrying pallets come in different forms, measurements and materials. Within
Europe, the most common types of pallets are the euro pallet measuring
0.8
1.2 m and the industrial pallet, which measures 1
1.2 m (see Fig. 6.29).
12 Cf. DIN 55405.
120 6 Warehousing, Handling, and Picking Systems
How many pallets with goods can be stacked on top of each other is dependent on
the resilience of the good. Straps may be used to secure the load. Alternatively, a
stretch or shrink-wrap procedure may be employed. With the stretch procedure, a
flexible film is wrapped around the goods on the pallet so as to hold them together.
If the goods are shrink-wrapped a film is pulled over the goods which is subsequently heated up. This makes the film shrink and hold the goods together.
Wire-mesh pallets have a higher stackability. At the same time, the need for repackaging is reduced. Wire-mesh pallets are made up of three fixed wire-mesh
walls and one two-part front wall, which is detachable. They are usually used for
storage of small parts that cannot be stacked on pallets. They have the same
measurements as euro pallets or industrial pallets. Collapsible wire-mesh boxes
on which all walls can be collapsed are sometimes used as well in order to save
space on the return transport of the empty pallet (see Fig. 6.30).
If the loading unit is supposed to be as small as possible, small parts carriers or
boxes can be used to transport small parts.13 These loading units are suitable for the
formation of small batch sizes, which also increases flexibility. During transport the
small parts carriers are consolidated into loading units on the pallets. Small parts
carriers are available in different sizes which can all be combined on the pallet.
Fig. 6.29 Euro pallet
Fig. 6.30 Wire-mesh pallet
13 Cf. DIN 30820.
6.4 Packages, Loading Units, and Load Carriers 121
Further examples of small parts carriers include:
¢ Stackable boxes (one side wall is slanted and only half-closed so that goods are
accessible if boxes are stacked on top of each other)
¢ Rotary stacking container (stackable and can be turned 180 to nest inside one
other)
¢ Folding container (foldable, mostly for light goods)
Closed load carriers or containers can be utilized to transport goods at sea, on
the road, on rail, or in the air. Depending on capacity we distinguish between small,
medium-sized, and large containers. Large containers can hold up to 14 euro pallets
per layer. Inland containers have a rear door or several side doors and are
standardized as 10-, 20-, 30-, and 40-foot14 containers.15 ISO-containers are also
available in these four main versions.16 They only have one rear door which makes
them more difficult to load but delivers greater protection for the freight especially
on overseas transport. ISO-containers are used world-wide, as opposed to inland
containers, which are not used outside Europe (see Sect. 5.4.2).
Case Study 6.5: Usage Function of Packaging
In the consumer goods sector, packaging is adapted according to its use by the
consumer. A famous example is the 5-Minuten-Terrine by Maggi GmbH, which
only needs to be filled with hot water so that the content can be directly eaten out
of the packaging. Other examples include the Joghurt mit der Ecke (Mu¨ller
Corner range) by Alois Mu¨ller GmbH & Co. KG (Mu¨ller Dairy Ltd.), yogurt in
general, or drinking bottles.
It is also in the consumer goods sector that shipping units are formed out of
the load carriers and the packaging containing the product. The packaging (e.g.
stretch film) is removed and the product is made available for sale at the store on
its load carrier (e.g. euro pallet).
In the manufacturing industry and especially in the automobile industry, the
packaging fulfills a usage function in the production process. Manufacturers of
components (supplying industry) which are needed during the car assembly
process package the components at the end of production. The components are
provided at the assembly in their packaging (largely reusable packaging) and are
directly taken out of the packaging by an assembly line worker. The way the
packaging is constructed must ensure an optimal position of the components for
removal. This means, for example, that the workers should not be forced to bend
down and that they should be able to remove the component without meeting any
resistance.
14 One foot corresponds to a length of 0.3048 m.
15 Cf. DIN 15190.
16 Cf. DIN ISO 668.
122 6 Warehousing, Handling, and Picking Systems
6.5 Picking and Handling
The process of taking goods out of the warehouse for compilation of a customer
shipment is called picking. The receiver may be an external customer of the
company or an internal accepting instance (e.g. production). The consumer information can be a sales-oriented order (customer order) or a production-oriented one
(internal order). The process of unloading goods from a transport mode into the
goods receiving area of a warehouse, from the goods issuing area of a warehouse
into a transport mode, or from one transport mode into another is termed
transshipment. Transshipment areas, which can be outdoors or indoors, are frequently used to effect transshipment between modes of transport. Additional
picking processes may be carried out during transshipment if the shipments
(packages or load carriers) need to be compiled into amended, customer-specific
shipments. Such facilities are termed transshipment points or cross-docking points.
If goods are also purposefully stored at these transshipment points over a certain
period we speak of distribution warehouses, as described above.
6.5.1 Picking
The process of picking is divided into the sub-systems of organization, materials
flow, and data flow. The organization of sections division can be one-zone or multizone. Articles with different properties, e.g. large and small or light and heavy,
should be stored in separate sections to ensure optimal operation by the picking
person. The order may be processed in a single-stage or multi-stage manner. Singlestage order processing leaves the entire picking process to be implemented by one
picking person while with multi-stage order processing, only one stage is allocated
to one person and the article is passed on for further sorting. Since multi-stage
picking necessitates temporary storage and sorting, it is more suited to be used for
order structures with long throughput times and which are not characterized by
urgent or prioritized orders.
The materials flow is divided into provisioning, movement, withdrawal, and
handing over. Provisioning is either static if the picking person moves towards the
article (man-to-goods) or it is dynamic if the article is brought to the picking person
at a picking workplace (goods-to-man). Movement is effected either on one level or
on several levels using either manual or automated withdrawal techniques. The
article is handed over either at one location (centralized) or at several locations
(decentralized) in the warehouse.
The data flow is characterized by data preparation and data transmission. The
data is prepared either according to order sequence (Order Picking) or in batches
(Batch Picking). In the case of order picking the orders are processed synchronously
to the order inflow. The processing may be carried out simultaneously or in series
(consecutively). Batch Picking is a way of picking in which the orders are first
collected. Individual items of several orders are subsequently compiled to one
aggregated item. These orders are then allocated to the individual picking sections
6.5 Picking and Handling 123
and processed simultaneously or in series. As a last step, the aggregated orders are
then distributed back to the original orders. This procedure is also called two-step
picking.
Traditionally, we distinguish between electronic data transmission and data
transmission on paper. The use of handheld devices, displays on picking vehicles,
Pick by Voice or Pick by Light renders paperwork unnecessary. With Pick by Voice
the order is transmitted to the picking person via headphones, while Pick by Light
technologies activate small lights at the respective point of withdrawal. Confirmation of the picking order is done actively by the picking person or automatically, for
example by means of an automatic withdrawal confirmation system on the shelf.
Using one picking system or another mainly depends on the range of articles and
the volume of articles to be picked. In general, it seems recommendable to use
single-stage picking systems for a small and homogeneous range of articles or in
cases of little daily throughput. Multi-stage order picking or batch picking is more
suitable for a more heterogeneous article range and higher throughput volumes.
The following outlines several common combinations of picking procedures
resulting from Table 6.1.17 Manual systems with static provisioning, onedimensional movement, centralized handover, and manual withdrawal are suited
for small numbers of picking items per day. Using a picking vehicle, the picking
person drives through the aisles and brings the readily picked order back to the
central picking station (see Fig. 6.31).
An extended range of articles would increase the transit times with this picking
procedure. Growing throughput would soon lead to picking persons hindering each
other in the aisles. In order to decrease transit times and to be able to distribute more
picking persons to the aisles, it is more suitable to employ decentralized withdrawal
in conjunction with a conveyor belt instead of picking vehicles (see Fig. 6.32).
The picking person puts the required number of articles into the passing carriers
move e.g. by conveyors. The goods in the carriers are sorted at another location.
Table 6.1 Basic principles of picking systems

17 Cf. Gudehus (2010), p. 679 et seq.
124 6 Warehousing, Handling, and Picking Systems
This two-stage commissioning procedure increases the throughput time of the order
since the goods are on-carried and transshipped more often than with single-stage
picking. This type of picking can also be implemented in a pallet rack warehouse. In
this case, platforms are mounted along the aisles with approx. 2 m space from each
other in which the picking persons are positioned. Loading with full pallets is
implemented through special aisles by e.g. narrow aisle trucks. This type of storage
is called gallery storage. Two-dimensional movement of the picking person is most
suitably implemented if the range of articles makes the use of one-dimensional
storage impossible due to its space requirements and if the number of picking items
per day is low at the same time.
The two-dimensional movement better utilizes the height of the warehouse (see
Fig. 6.33). In comparison to the use of one-dimensional picking with picking vehicles,
using shelving operating devices or narrow aisles trucks reduces the aisle width.
The two-dimensional picking utilizes the same approach as one-dimensional
picking. A multi-stage strategy is pursued to pick more orders within a certain
timeframe from a larger range of articles. The picking persons are able to pick more
Fig. 6.31 Single-stage, one-dimensional picking with manual withdrawal and centralized
handover
Fig. 6.32 Static provisioning, one-dimensional movement, manual withdrawal and decentralized
handover
6.5 Picking and Handling 125
articles from one shelving bay and they can operate more shelving bays than with a
single-stage approach since the articles are only allocated to the orders in the next
stage.
This approach may be automated with the use of automated storage and retrieval
systems. The withdrawal device must be fitted with a collecting magazine so as to
avoid return journeys to the transfer station after each withdrawal. This is why this
type of withdrawal is more suitable for small goods.
If the picking and storage areas are separated – as is the case in high rack
warehouses, for example – the loading units need to be retrieved and picked outside
the warehouse before they are provisioned dynamically (see Fig. 6.34).
The goods to be commissioned are taken from the pallet, which is subsequently
taken back to the warehouse, or open pallets are pooled. This is done in a
decentralized way in the example presented and on-carriage is carried out using
unmanned transport systems.
Other picking systems designed to better adapt to article characteristics and to
achieve certain inventory and performance data can be formed by combining
various types of stage systems, provisioning, withdrawal, and handover. The
manifold possibilities of designing picking systems highlight the complexity of
planning them. The picking system needs to meet service levels requirements of the
processed order while at the same time keeping costs low.18 The logistical service
level is significantly influenced by an extended product range, decreased order
sizes, and an increase in order frequency. This leads to an increase in the complexity and costs of picking. Improved picking performance while ensuring consistent
Fig. 6.33 Static provisioning, two-dimensional movement, manual withdrawal and centralized
handover
18 Cf. Bode and Preuß (2004), p. 325.
126 6 Warehousing, Handling, and Picking Systems
quality standards can only be achieved by making adaptations to the picking system
(more complex design).
6.5.2 Handling
Handling of goods means that a change of load carrier, conveyor, or storage
position takes place. Since goods are passive objects, active handling techniques
must be employed. This task is in the simplest case fulfilled by a warehouse worker.
Warehouse workers use handling equipment if they cannot handle the goods under
their own power or within the required timeframe. Even a fully-automated handling
process without any warehouse personnel is viable. This type of handling equipment can be made up of conveyors and load carriers. Conveyors pick up the goods
and overcome vertical and horizontal distances. Load carriers are used to consolidate goods and facilitate or enable their being picked up.
This section only covers handling between external and internal materials flow
systems. The need for handling to take place between different means of transport is
described in Chap. 5.
Handling between warehouses and external means of transport is most commonly carried out to establish a connection to road traffic. Which kind of handling
equipment needs to be used is dependent on the way the goods are transported on
the truck. In most cases the goods are on pallets and are not stacked. Loading and
un-loading of the truck can be carried out using manual lift trucks if a loading
bridge is available. If the pallets are stacked or have to be stacked, a conveyor with
lift function, such as manual lift pallet forklift trucks or forklift trucks, is required.
The goods may also be loaded loosely, as is the case with parcel transport in the
mail-order business or transport of tires in the automobile industry. To facilitate
loading and un-loading, conveyor belts can be led into the truck. The loading person
Fig. 6.34 Dynamic provisioning, one-dimensional movement, manual withdrawal and decentralized
handover
6.5 Picking and Handling 127
can drag the conveyor to the position of the current workplace and thus avoids
moving in and out of the truck.19
The handling of goods between means of rail transport or inland waterway
transport and internal materials flow systems is of minor importance for the supply
of most warehouses.
Railroad wagons are always loaded and un-loaded from the side to avoid
decoupling.
Goods that are being transported on inland waterway vessels are usually stored
in containers. At the port, these containers are either loaded onto trucks first, which
enables regular handling in the goods receiving and goods issuing areas, or they are
taken to a loading zone using conveyors.
6.6 Warehouse Organization
Warehouse organization distinguishes between operational warehouse organization and structural warehouse organization. Operational warehouse organization
defines the temporal and spatial organization of sequences which are also reflected
in the order of warehouse sections that certain goods or parts of goods pass through.
Structural warehouse organization defines work contents and competencies in a
hierarchical structure.
The most important warehouse areas are the goods receiving area, the warehouse
itself, and the goods issuing or outgoing area. The distances leading from the goods
receiving area through the warehouse to the goods issuing area are called circulation area. The aisles between storage locations belong to the circulation area as
well. It is also in this order that goods pass through the warehouse. Areas located in
between these areas may fulfill additional functions. These include picking and
packing areas, quality inspection zones, and provisioning areas in general. In
addition, sanitary and administration areas for staff members must be taken into
account.
The goods receiving and goods outgoing areas constitute an interface between
internal and external traffic and are termed loading zone.
The goods receiving area mainly serves to accept the incoming goods and to
forward them to the following warehouse areas (see Sect. 7.3.4). The goods are
either stored or directly forwarded to the goods issuing area (transit area) to leave
the warehouse shortly. Goods which do not meet the quality criteria after inspection
in the goods receiving area are prepared for return shipment and also passed on to
the goods issuing area. Stored articles first need to be compiled into a shipment
(picking) upon receipt of a shipping order before they are taken to the goods issuing
area.
19 Cf. Ju¨nemann and Schmidt (2000), p. 290.
128 6 Warehousing, Handling, and Picking Systems
The goods outgoing area provisions those goods that are ready for dispatch in
their respective loading units and effects the loading process onto the vehicle
(mainly trucks or trains).
Loading zones are designed according to organizational and constructional
criteria. Routine tasks allocated to staff members on a daily basis have to be
considered in the organization. Planning is further based on the availability of
technological facilities (conveyors) and on space requirements. The size of the
loading zone is determined by different performance variables, such as arrival times
of vehicles, their loading capacity, and load factors throughout the day and over
periods of seasonal fluctuations. The peak load describes a percentage margin with
which temporary peak performance is expressed. In addition to the performance
characteristics, the properties of the goods to be loaded have to be taken into
account when designing the facilities for handling between loading zone and truck.
The way in which the goods are handed over should be adapted to the vehicle.
Today, height-adjustable ramps that are attached to the building and which the
conveyor crosses to directly reach the vehicle’s loading platform are most commonly used. As the following figure shows, the ramps (also termed loading bridges)
are designed for rear discharge of the trucks (head ramp). The relatively high space
requirements for maneuvering constitute a disadvantage (see Sect. 4.2.6). The socalled saw-tooth ramp, which aligns the trucks in a zigzag pattern, is a good
alternative to this. If no ramps are used, the height difference between vehicle
and warehouse must be overcome using a lift truck. In cases where the conveyors
carry out loading operations outdoors, the suitability of the goods and conveyors for
certain weather conditions must be taken into account. Conveyors need not access
the loading platform of the trucks, aside from the fact that this is not even always
possible (see Fig. 6.35).
The right choice of loading gates depends on the intended closing and opening
times – which are important, for example, when loading chilled goods – and on the
space required. Hinged and folding doors open relatively slowly and take up space
inside or outside the building. Since sliding doors are slid to the right or left, they
Fig. 6.35 Loading and un-loading of trucks with and without the use of ramps
6.6 Warehouse Organization 129
preclude a dense arrangement of several doors next to each other. Shorter distances
can be realized with roll-up doors or sectional doors as they are pulled overhead
when opened. These door types are used preferentially because they open and close
quickly and do not take up much space.
A complete gate system consists of a loading bridge, a shelter (optional), and of
the door itself. The main purpose of a gate is to enable loading and un-loading of
trucks. Additionally, improved conditions for the protection of the goods can be
achieved with outdoor shelters or temperature and air locks inside the building.
What is more, drafts can be avoided this way, which leads to an improvement of the
working conditions for the loading personnel (see Fig. 6.36).
Gate houses offer the best insulation. They are built in front of the actual gate
opening and contain a loading bridge and a second gate, which creates a temperature lock.
The most common layouts for goods receiving and goods issuing areas are
shown in the figure below (see Fig. 6.37).
Fig. 6.36 Gate system
Fig. 6.37 Possible goods receiving (ingoing) – goods issuing (outgoing) layouts
130 6 Warehousing, Handling, and Picking Systems
By virtue of their shape, the letters I, U and L imitate the flow of goods. The
I-layout provides for all goods to travel through the entire warehouse. This is also
the layout’s disadvantage since the frequently handled goods have to travel the
same distance as the less frequently handled ones. The straightforward flow of
goods can be seen as an advantage. Both the U-layout and the L-layout enable
frequently handled goods to be stored close the goods receiving and goods outgoing
areas to avoid travels through the entire warehouse. Goods that are supposed to be
moved directly to the goods issuing area without intermediate storage do not have
to pass through the entire warehouse as is the case with an I-layout. The U-layout is
greatly advantageous in that all gates are situated on the same side of the warehouse. Thus, the space needed for the maneuvering of the trucks is located on one
side and smaller than with the other layouts. By-passes for trucks are therefore not
necessary and the warehouse premises can be kept smaller. Since the traffic takes
place on one side, however, obstructions and confusions of goods receiving and
goods outgoing areas are more likely to occur.
The following describes the organizational design of the actual area where the
goods are stored – the so-called storage zone. Storage zones can be classified
according to various characteristics (also see Sect. 6.2):
¢ Storage temperature (temperature-controlled – heated or chilled – or not temperature-controlled)
¢ Weight and volume of articles
¢ Turnover rate (fast or slow-moving consumer goods, ABC articles)
¢ Property relations of articles (customer, consignment, or customs warehouse)
¢ Degree of hazardousness of article (hazardous goods classification)
¢ Value and fading frequency of articles
Storage zones should not be planned too small to take advantage of synergies.
Neither should they be too large so that the area can be taken in at a glance.
Storage zone management aims at minimizing distances, producing even utilization of existing storage capacity and at avoiding an overaging of the stored goods.
To this end, various strategies for storage space allocation and storage and withdrawal are available.
With Fixed storage space allocation, each article is allocated a fixed storage
space while the space remains empty if the article is out of stock. Fixed storage
space allocation can theoretically be practiced without the use of IT systems.
Free storage space allocation within fixed areas or storage zones allocates
storage spaces to the article according to its turnover frequency. Articles with a
high turnover rate are located within short reach. In chaotic storage systems, each
article is allocated the nearest available storage space or the nearest free storage
space which is best suited for the article.
These strategies may be combined with permission or prohibition of additional
storing in storage spaces that are already partly occupied. If additional storing is
permitted, the age of the articles may be of no relevance. Quantity adjustments upon
withdrawal may be permitted or prohibited. This means that loading units are
opened to comply with the order quantity.
6.6 Warehouse Organization 131
The practicability of withdrawal strategies depends on the storage type used and
on the degree of complexity of the storage management. Withdrawal of those
articles that had been stored first is called First In – First Out (FiFo). Live storage
shelves, for example, automatically realize a FiFo strategy. Using appropriate
control devices or marking, however, a FiFo strategy can be implemented for any
storage technique. If the most recently stored article is taken out first, we speak of a
Last In – First Out (LiFo) strategy. This strategy is utilized if goods are stored
whose age is irrelevant, e.g. goods that are not subject to a sell-by date.
Apart from FiFo and LiFo, other strategies can be adopted and combined. Thus,
withdrawal may be organized according to the shortest distance to the point of
withdrawal and then be combined into orders with optimized distance coverage.
Furthermore, withdrawal can follow a certain order or is implemented according to
quantity adjustments. For example, withdrawal may be organized according to size
or weight of the articles. In the case of perishable goods it is best to combine the
FiFo principle with the FeFo principle. The FeFo (First Expired – First Out)
principle provides for goods whose sell-by date will be reached next to be
taken out first. Quantity adjustment means that only full or open loading units are
taken out.
Case Study 6.6: Storage Space Requirements
At the goods distribution center of Amazon.de GmbH in Leipzig, articles are reordered according to the following principle. Whenever the stock reaches the
reorder level of three loading units, six loading units are ordered. The loading
unit of one article requires two storage spaces. One loading unit always contains
ten articles. This means that the loading units have to be depleted one after the
other so that 30 articles are left if three loading units are in stock. The following
question needs to be answered in order to assess how the storage space must be
allocated in order to take up the least amount of storage capacity:
How many storage spaces must be made available if
(a) Fixed storage space allocation is implemented and
(b) A chaotic storage strategy has been chosen?
Case Study 6.7: Warehousing Processes
The trading company Gebru¨der Heinemann KG a distributor of international
brand articles on the travel market and, amongst other things, operates duty-free
shops on airports. Both consolidation and flexibilization of their logistical
activities to international market requirements and their own positive business
development have led to a turnkey-ready logistics center being built in Hamburg
Allermo¨he with SSI Scha¨fer as the general contractor. The logistics center is one
of the most modern and high-performing ones world-wide. It keeps 35,000
articles in stock with an outgoing tonnage of about 1,000 t per day, which is
equivalent to 85,000 picked boxes (see Fig. 6.38).
132 6 Warehousing, Handling, and Picking Systems
The warehouse is divided into a gallery storage warehouse for pallets, a
container storage warehouse with picking zone, a bay shelving warehouse, and
a tray warehouse.
In the goods receiving area the goods are completely transshipped from the
truck onto conveyors and separated for the different warehouse sections.
Cigarettes, coffee, candy, and spirits are stored on whole pallets in the gallery
storage warehouse. IT-supported quality inspection and labeling of the pallets
are carried out in the goods receiving area. Accurate weighing is especially
important since all inspections of the outgoing goods will be based on
comparisons of weight later on.
The gallery storage warehouse consists of eight aisles, each furnished with
one shelving operating device which stores and retrieves the pallets in the
location allocated to them by the warehouse management system. To achieve
the highest possible storage capacity, the pallets are stored double-deep. The
gallery storage warehouse supplies the various storage and picking areas with
goods. The tray storage warehouse serves to provide whole cartons for direct
withdrawal by the dispatch area or to re-stock the picking area.
High-priced articles, such as perfume, cosmetics, cigars, or articles for
individual picking are transported from the goods receiving area to the second
warehouse level. The articles are separated into system containers at 16
repacking stations. The packed containers undergo a shape control and are
1: Good receiving area
2: Tray warehouse
3: High rack warehouse
4: Container warehouse
5: Bay shelving warehouse
6: Sorter
7: Order consolidation
8: Dispatch area
Fig. 6.38 Warehouse layout Gebr. Heinemann, KG (SSI Scha¨fer (2007))
6.6 Warehouse Organization 133
electronically recorded using a barcode. Conveyors turn the containers in the
right direction and transport them to one of the storage locations allocated by
the warehouse management system within one of the 36 picking zones of the
container storage warehouse. The containers are stored double-deep as well.
Accessories, such as suitcase sets, toys, and designer articles are likewise
separated in the goods receiving area and – analogous to the picking articles –
taken to the second warehouse floor using conveyor technologies and vertical
conveyors. The articles are then distributed manually to the storage locations of
the storage shelves. Goods and storage location are associated with each other
using a barcode reader to enable the warehouse management system to retrieve
the manually stored articles at any time.
The distribution center is designed for approximately 1,600 outgoing pallets
per day. Apart from the dispatch of whole pallets from the gallery storage
warehouse, goods are picked in the individual storage areas, analogous to
storage procedures. The routes of the picking vehicles in the manual storage
areas are optimized by the warehouse management system.
Picking systems have been integrated into four levels of the gallery storage
warehouse to carry out retrieval. Goods pallets are provided for whole-box
picking. The shelving operating devices automatically supply the picking station
according to the stipulated minimum stock levels. These picking stations are
partly dynamic picking stations for slow-moving articles in order to realize a
greater range of articles. The warehouse management system provides the
respective pallets for picking. Once the goods have been picked, the pallets are
re-stored into the gallery store. Double deep pallets with fast-moving articles are
continuously provided at the static picking stations. Whole boxes are picked
manually in the gallery store on the pallets located on the picking vehicles. Staff
members receive all data via a radio data transmission terminal which is attached
to the picking vehicle. Thus, the picking staff can be directed through the
warehouse on optimized routes which take article weights into account. By
means of a control weighing and for the purpose of accurate documentation of
withdrawals and inventory, the pallets undergo a quantity check by weight
comparison both at the beginning and at the end of picking. The picked pallets
are temporarily stored in buffer zones and are combined with the rest of the order
in the dispatch area.
Articles – especially perfume and cosmetics – are picked purely by individual
picking in the bin storage warehouse. The articles are supplied from the 36 silos
of the warehouse by a dynamic picking system. In order to pick the articles, the
warehouse management system requests a box that is adapted to the picking
volume. An order label is attached to the box, which associates the box with the
transport tray through a barcode. The tray with the box on it then travels through
the picking warehouse fully automatically. Depending on the goods to be picked
the tray is diverted at the respective picking station where the articles are stored
and provisioned in a time-efficient way. The tray’s diversion prompts the
shelving operating device to trigger the immediate transport of the storage
container with the requested articles from the storage zone to the provisioning
134 6 Warehousing, Handling, and Picking Systems
zone. Static and dynamic picking stations provide the respective articles for
picking. Picking can now take place at 77 picking points. The box is sorted out
by means of a radio scanner. Subsequently, all picking items and picking
quantities are indicated by a pick-to-light installation at the point of picking.
Using these indications and the data from the radio terminal, the picking person
can now compose the articles. The picking process is confirmed and triggers a
control weight mechanism along with a booking in the system. By means of a
variance analysis, the system determines the contents of the box. After picking is
finished, the staff member pushes the tray with the box back onto the conveyor.
The tray is then transported to the next picking station where picking continues.
If the picking order is completed, the box is discharged from the container
storage warehouse. A continuous scale checks the order once again during this
process. All picked orders are transported to a control terminal. If any
discrepancies are detected, the staff member at the control station is informed
and performs a manual check. Incorrect pickings are fed back into picking cycle
through the conveyor for completion. The conveyor system carries the inspected
boxes on to the tray collection point. The trays are collected and led back into the
system. The picked boxes go through a strapping machine, after which the
conveyor transports them to the sorter.
Picking from the assortment of accessories and slow-moving articles takes
place in the bay shelving warehouse. By means of multi-order picking, up to
eight orders can be processed at the same time. The picking list to do so is given
to the staff members through mobile hand terminals. Depending on the article
and order, whole boxes may be retrieved in the tray warehouse. The goods are
then repacked from the boxes into cartons. The content is recorded and checked
by scanning the EAN code. After attachment of a dispatch label, the cartons are
transported to the sorter.
Cartons from the automated picking warehouse and from the bay shelving
warehouse are conveyed to a sorting machine for initial order combination in the
dispatch area. They are sorted there according to orders and customers or
destinations and then discharged through the respective chute. Staff members
at the chutes compile the cartons on a pallet. Upon completion of the palletization, the warehouse management system checks whether the pallet weight is
correct, in another control weighing. Pallets from the perfumes and accessories
area are then transported to the goods issuing area. The warehouse management
system determines whether a pallet goes directly through the shrinking machine
to the allocated loading gate or if it should be collected at a loading station in the
loading area, together with the pallets from the gallery storage warehouse. Staff
members from the loading zone decide how they would like to load the goods
onto the matching pallets and communicate their decision to the warehouse
management system via radio scanner. Subsequently, another control weighing
takes place to avoid loading mistakes. On the way to the dispatch area the pallets
are secured by an automated pallet shrinking station. Within a live storage
system, four transfer carriages then distribute the pallets to gravity roller
conveyors for dispatch. From there, the shipments are loaded onto the trucks.
This way, up to 100 trucks are sent off with goods on a daily basis.
6.6 Warehouse Organization 135
The extremely complex movements of goods are managed and comprehensively monitored by SSI Scha¨fer’s warehouse management system named ant®.
ant® controls all processes from the goods receiving area to the goods issuing
area and manages the data transfer between shelving operating devices, picking
persons and mobile radio terminals. Its main objective is exact trip management
of the trucks to the gates as well as just-in-time actuation of retrieval processes
so that loading times can strictly be adhered by and the loading zones do not
become blocked as a consequence of early order provisioning. Ongoing checks
and inventory management through numerous weighings and the resulting
reduction of shortages to under 1 % only become possible through the use of
an advanced management system. Additionally, the use of state-of-the-art visualization technologies makes it possible to clearly visualize the processes and
movements of materials.
136 6 Warehousing, Handling, and Picking Systems
Review Questions
1. Explain the basic functions of a warehouse.
2. Name the areas in which continuous and discontinuous conveyors can each
be applied.
3. What steps would you take in designing a system for reusable packing and
what must be taken into consideration?
4. Assuming a throughput of 1,000 items per day, is single-stage manual
picking or multi-stage automated picking more suitably implemented?
5. Allowances for a possible expansion of storage capacities should always be
made. Which provisions can be taken in the choice of the warehouse location
and storage technology?
6. Name possible ways to secure palletized loading units.
7. Name the advantages and disadvantages of unmanned transport systems and
the resulting task areas.
8. According to what criteria would you determine the use of stackers?
9. What needs to be considered when opting for a type of sorter?
10. What possibilities do we have to design and equip a warehouse as flexibly as
possible?
Additional Literature
Ackerman, K. B. (1997). Practical handbook of warehousing (4th ed.). Alphen aan den Rijn:
Kluwer Academic.
Arnold, D., Kuhn, A., Isermann, H., & Tempelmeier, H. (2002). Logistik. Heidelberg: Springer.
Bode, W., & Preuß, R. (2004). Intralogistik in der Praxis: Komplettanbieter der Intralogistik.
Suhl. Wirtschaftsverlag.
Fischer, W., & Dittrich, L. (2003). Materialfluss und Logistik. Optimierungspotenziale im Transport- und Lagerwesen. Berlin: Springer.
Frazelle, E. (2002). World-class warehousing and materials handling. New York: McGraw-Hill.
Gudehus, T. (2010). Logistik (3. Edition). Berlin: Springer.
Ju¨nemann, R., & Schmidt, T. (2000). Materialflusssysteme: Systemtechnische Grundlagen
(2. Edition). Heidelberg: Springer.
Additional Literature 137