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Module Title: Machines and Mechanisms

Title of Assignment: Effect of Balance Weights on Crank Mechanisms

To be marke

Module Number: AS1

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ZERO SPRING

CRANK DISK

ROD CRANK DISK

CRANK MECHANISM BASE PLATE

ONE_SPRING

CRANK DISK

Aim

My main aims of this assignment is firstly to do the researching about single and multi-cylinder crank mechanism where I will cover the areas like what produces the unbalance / vibrations & how to avoid the vibrations or try to balance the crank mechanism. Then after that I need to create a dynamic model of crank shaft with piston to see what unbalanced vibrations it produces by using accelerometers where I can measurement the magnitude of vibrations and the frequencies spectrum. To do the investigation, diagnostics and modelling of the crank mechanism I will use a powerful tool like MSC Adams software where I will focus on strength & rigidity during the modelling process. Also the aim of the modelling the crank assembly virtually is to see if the dynamic modelling can simulate a complex balancing problem.

Theory (10 marks)

Introduction:

The resultant of all the forces acting on the body of the engine duo to inertia forces is known as unbalanced force or shaking force. • If the resultant of all the inertia forces is zero, then there will be no unbalanced force, but even then an unbalanced couple or shaking couple may be present. • Balancing of a mechanism includes counterbalancing both the inertia forces and the inertia couples. In some cases we can eliminate this effects almost completely. However, in general it is too difficult to prepare a device to balance a mechanism totally. • In most of cases we can reduce the shaking forces and the shaking couples by adding appropriate balancing masses. In such cases, the mechanism is said to be partially balanced.

The crankshaft is supported by the bearing, which is pressed inside the bearing support. A twin will be faster. The smaller pistons and valves–and lower vibration–allow a higher red-line, and thus more power. It would have MORE top end than a single, but less low end torque.

A single cylinder engine has its torque in low RPMs. Multi-cylinder engines don’t develop their torque until higher RPMs. Multi-cylinder engines need stronger clutch springs and lighter clutch weights to prevent engine bogging. This higher RPM plus heavier engine weight goes against the machine performing well from a standing start in deep powder. The main drawback to the single cylinder machines is lack of hill climbing power.

A single-cylinder engine is a basic piston engine configuration of an internal combustion engine. It is often seen on motorcycles, auto rickshaws, motor scooters, mopeds, dirt bikes, go-karts, radio-controlled models, and has many uses in portable tools and garden machinery. It has been used in automobiles and tractors.

Single-cylinder engines are simple and compact, and will often deliver the maximum power possible within a given envelope. Cooling is simpler than with multiple cylinders, potentially saving further weight, especially if air cooling can be used.

Single-cylinder engines require more flywheel effect than multi-cylinder engines, and the rotating mass is relatively large, restricting acceleration and sharp changes of speed. In the basic arrangement they are prone to vibration – though in some cases it may be possible to control this with balance shafts.

A variation known as the split-single makes use of two pistons which share a single combustion chamber.

Single-cylinder engines are simple and economical in construction. The vibration they generate is acceptable in many applications, while less acceptable in others. Counterbalance shafts and counterweights can be fitted but such complexities tend to counter the previously listed advantages.

Components such as the crankshaft of a single-cylinder engine have to be nearly as strong as that in a multi-cylinder engine of the same capacity per cylinder, meaning that some parts are effectively four times heavier than they need to be for the total displacement of the engine. The single-cylinder engine will almost inevitably develop a lower power-to-weight ratio than a multi-cylinder engine of similar technology. This can be a disadvantage in mobile operations, although it is of little significance in others and in most stationary applications.

A multi-cylinder engine is a reciprocating internal combustion engine with multiple cylinders. It can be either a 2-stroke or 4-stroke engine, and can be either Diesel or spark-ignition. The cylinders and the crankshaft which is driven by and co-ordinates the motion of the pistons can be configured in a wide variety of ways. Multi-cylinder engines offer a number of advantages over single-cylinder engines, chiefly with their ability to neutralize imbalances by having corresponding mechanisms moving in opposing directions during the operation of the engine.[1] A multiple cylinder engine is also capable of delivering higher revolutions per minute (RPM) than a single cylinder engine of equal displacement, because the stroke of the pistons is reduced, decreasing the distance necessary for a piston to travel back and forth per each rotation of the crankshaft, and thus limiting the piston speed for a given RPM. Typically, the more cylinders an engine has, the higher the RPM’s it can attain for a given displacement and technology level, at a cost of increased friction losses and complexity. Peak torque is also reduced, but the total horsepower is increased due to the higher RPM’s attained.

Although there are 1, 3 and 5-cylinder engines, almost all other inline engines are built with even numbers of cylinders, as it’s easier to balance out the mechanical vibrations. Another form of multiple cylinder internal combustion engine is the radial engine, with cylinders arranged in a star pattern around a central crankshaft. Radial engines are most commonly used as aircraft engines, and in basic single-row configuration are typically built with odd numbers of cylinders (from 3 to 9), as odd numbers are easier to balance in this configuration. “Twin-row” or “multi-row” radials are also built, which is basically two or more single-row radials connected front-to-back and driving a common crankshaft. In this “twin row”, or “multi-row” configuration, the total number of cylinders will be an even number, although each row still has an odd number. For example, an typical single row radial such as the Wright Cyclone has 9 cylinders. The twin row Wright Twin Cyclone is based on this engine and thus has two banks of 9 cylinders, for a total of 18, an even number.

The inline-four engine or straight-four engine is a type of inline internal combustion four cylinder engine with all four cylinders mounted in a straight line, or plane along the crankcase. The single bank of cylinders may be oriented in either a vertical or an inclined plane with all the pistons driving a common crankshaft. Where it is inclined, it is sometimes called a slant-four. In a specification chart or when an abbreviation is used, an inline-four engine is listed either as I4 or L4 (for longitudinal, to avoid confusion between the digit 1 and the letter I).

The inline-four layout is in perfect primary balance and confers a degree of mechanical simplicity which makes it popular for economy cars.[1] However, despite its simplicity, it suffers from a secondary imbalance which causes minor vibrations in smaller engines. These vibrations become more powerful as engine size and power increase, so the more powerful engines used in larger cars generally are more complex designs with more than four cylinders.

Today almost all manufacturers of four-cylinder engines for automobiles produce the inline-four layout, with Subaru’s flat-four engine being a notable exception, and so four-cylinder is synonymous with and a more widely used term than inline-four. The inline-four is the most common engine configuration in modern cars, while the V6 engine is the second most popular.[2] In the late 2000s, with auto manufacturers making efforts to reduce emissions and increase fuel efficiency due to the high price of oil and the economic recession, the proportion of new vehicles sold in the U.S. with four-cylinder engines (largely of the inline-four type) rose from 30 percent to 47 percent between 2005 and 2008, particularly in mid-size vehicles where a decreasing number of buyers have chosen the V6 performance option.[3][

Balance and smoothness: The inline-four engine is much smoother than one or two cylinder engines, and this has resulted in it becoming the engine of choice for most economy cars for many years. Its prominent advantage is the lack of rocking vibration, and the lack of need for heavy counterweights makes it easier to be sporty (quick revving up and down). However, it tends to show secondary imbalance at high rpm because two pistons always move together, making the imbalance twice as strong as other configurations without them.

Piston speed: An even-firing inline-four engine is in primary balance because the pistons are moving in pairs, and one pair of pistons is always moving up at the same time as the other pair is moving down. However, piston acceleration and deceleration is greater in the top half of the crankshaft rotation than in the bottom half, because the connecting rods are not infinitely long, resulting in a non-sinusoidal motion. As a result, two pistons are always accelerating faster in one direction, while the other two are accelerating more slowly in the other direction, which leads to a secondary dynamic imbalance that causes an up-and-down vibration at twice crankshaft speed. This imbalance is common among all piston engines, but the effect is particularly strong on inline-four because of the two pistons always moving together.

Most inline-four engines below 2.0 L in displacement rely on the damping effect of their engine mounts to reduce the vibrations to acceptable levels. Above 2.0 L, most modern inline-four engines now use balance shafts to eliminate the secondary vibrations. In a system invented by Dr. Frederick W. Lanchester in 1911, an inline-four engine uses two balance shafts, rotating in opposite directions at twice the crankshaft’s speed, to offset the differences in piston speed.

Single & Multi-cylinder crank mechanisms unbalance:

The slider-crank mechanism is a particular four-bar linkage configuration that converts linear motion to rotational, or vice versa. Internal combustion engines are a common example of this mechanism, where combustion in a cylinder creates pressure which drives a piston. The piston’s linear motion is converted into rotational motion at the crank through a mutual link, referred to as the connecting rod. As the geometry of the crank forces the conversion of linear motion to rotational, shaking forces are generated and applied to the crank’s housing. These shaking forces result in vibrations which impede the operation of the engine.

The engine’s crosshead slides along two parallel shafts of length 4.75” and diameter 0.25”. The rotational nature of the crank’s motion produces a component of force orthogonal to the direction of the crosshead’s translation. As such, the shafts will deflect to some extent during operation. Large deflections would result in misalignment, increasing friction and possibly causing the crosshead to bind or chatter as it travels along the guides. The magnitude of deflection must be evaluated.

Compression type dampers will be utilized to isolate vibrations in both the x and y directions. Four dampers will be mounted along the x axis, isolating the x component of the shaking force. Four additional dampers will be mounted along the y axis, isolating the y component of the shaking force.

Single & Multi-cylinder crank mechanisms balance:

In order to partially balance the shaking force, a balancing mass is added to system. If the balancing mass is selected properly, it can reduce the shaking force considerably. The primary unbalanced force may be considered as the component of centrifugal force produced by rotating mass m placed at crank radius r along the line of stroke. Hence, balancing of primary force may be considered as equivalent to the balancing of mass m rotating at the crank radius r. This is balanced by having a mass B at a radius b, placed diametrically opposite to the crank pin C.

But the centrifugal force produced by revolving mass B has a vertical component also. • It is thus obvious, that effect of above method is to change the direction of unbalanced force from the line of stroke to the perpendicular of line of stroke.

Primary Balancing of Multi-cylinder In-line engines: • The multi-cylinder engines with the cylinder center lines in the same plane and on the same side of the center line of the crank-shaft, are known as in-line engines. • Two conditions must be satisfied in order to give the primary balance of the reciprocating parts of a multicylinder engine: 1. The algebraic sum of the primary forces must be equal to zero. 2. The algebraic sum of the couples about any point in the plane of primary forces must be equal to zero.

In order to give the primary balance of a multi-cylinder engine, it is convenient to imagine the reciprocating masses to be transferred to their respective crankpins and to treat the problem as one of the revolving masses. For a two cylinder engine with cranks at 180o , the first condition may be satisfied, but this will result in an unbalanced couple. Thus, the above method of balancing can not be applied in this case. • For a three cylinder engine with cranks at 120o , the first condition may be satisfied. However, by taking a reference plane through one of cylinder center lines, two couples with non-parallel axes will remain. Hence, the above method of balancing fails in this case also.

For a four cylinder engine, similar reasoning will show that complete primary balance is possible and it follows that: • For a multi-cylinder engine, the primary forces may be completely balanced by suitably arranging the crank angles, provided that the number of cranks are not less than four.

Secondary Balancing of Multi-cylinder In-line engines • When the connecting rod is not too long (comparing to crank), then the secondary inertia force due to reciprocating mass arises. As in the case of primary forces, the secondary forces may be considered as equivalent to the component, parallel to the line of stoke, of the centrifugal force produced by an equal mass placed at the imaginary crank of length R/4n and revolving at twice the speed of actual crank.

The following two conditions must be satisfied in order to give a complete secondary balance of an engine: 1. The algebraic sum of the secondary forces must be equal to zero. In the other word the secondary force polygon must be closed. 2. The algebraic sum of the couples about any point in the plane of primary forces must be equal to zero. In the other word the secondary couple polygon must be closed. • The closing side of the secondary force polygon gives the maximum unbalanced secondary force an the closing side of the secondary couple polygon gives the maximum unbalanced secondary couple.

BALANCING OF THE SHAKING FORCE: Shaking force is being balanced by adding a rotating counter mass at radius ‘r’ directly opposite the crank. This provides only a partial balance. This counter mass is in addition to the mass used to balance the rotating unbalance due to the mass at the crank pin.

BALANCING OF INLINE ENGINES: An in-line engine is one wherein all the cylinders are arranged in a single line, one behind the other.

Engine balance is the design, construction and tuning of an engine to run smoothly. Engine balance reduces vibration and other stresses, and may improve the performance, efficiency, cost of ownership and reliability of the engine, as well as reducing the stress on other machinery and people near the engine.

These benefits are produced by:

Reduced need for a heavy flywheel or similar devices.

Reduced wear.

The opportunity to reduce the size and weight of components (other than the obvious one of the flywheel) as a result of reduced stress and wear.

Reduced vibration transmitted to the surroundings of the engine.

The opportunity to extract more power from a given engine by:

Higher maximum operating speeds made possible by reduced stress.

Spreading loads equally over multiple components, for example if multiple carburetors are poorly balanced, the maximum available throttle will be reduced.

Even a single cylinder engine can be balanced in many aspects. Multiple cylinder engines offer far more opportunities for balancing, with each cylinder configuration offering its own advantages and disadvantages so far as balance is concerned.

Primary and secondary balance

Historically, engine designers have spoken of primary balance and secondary balance. These terms came about because primary balance is concerned with vibrations at one times crank speed, and secondary balance at two times crank speed. These excitations can produce both couples and forces.

Primary balance is the balance achieved by compensating for the eccentricities of the masses in the rotating system, including the connecting rods. Primary balance is controlled by adding or removing mass to or from the crankshaft, typically at each end, at the required radius and angle, which varies both due to design and manufacturing tolerances. In theory any conventional engine design can be balanced perfectly for primary balance.

Secondary balance can include compensating (or being unable to compensate) for:

The kinetic energy of the pistons.

The non-sinusoidal motion of the pistons.

The sideways motion of balance shaft weights.

The second of these is the main consideration for secondary balance. There are two main control mechanisms for secondary balance – matching the phasing of pistons along the crank, so that their second order contributions cancel, and the use of Lanchester balance shafts, which run at twice engine speed, and so can provide a counteracting force.

No widely used engine configuration is perfectly balanced for secondary excitation. However by adopting particular definitions for secondary balance, particular configurations can be correctly claimed to be reasonably balanced in these restricted senses. In particular, the straight six, the flat six, and the V12 configurations offer exceptional inherent mechanical balance. Boxer eights with an appropriate configuration can eliminate all primary and secondary balance problems, without the use of balancer shafts.

Vibrations not normally included in either primary or secondary balance include the uneven firing patterns inherent in some configurations.

The above definitions exclude the dynamic effects due to flexure of the crankshaft and block, and ignores the loads in the bearings, which are one of the main considerations when designing a crankshaft.

Single cylinder engines

A single cylinder engine produces three main vibrations. In describing them we will assume that the cylinder is vertical.

Firstly, in an engine with no balancing counterweights, there would be an enormous vibration produced by the change in momentum of the piston, connecting rod and crankshaft once every revolution. Nearly all single-cylinder crankshafts incorporate balancing weights to reduce this.

While these weights can balance the crankshaft completely, they cannot completely balance the motion of the piston, for two reasons. The first reason is that the balancing weights have horizontal motion as well as vertical motion, so balancing the purely vertical motion of the piston by a crankshaft weight adds a horizontal vibration. The second reason is that, considering now the vertical motion only, the smaller piston end of the connecting rod is closer to the larger crankshaft end of the connecting rod in mid-stroke than it is at the top or bottom of the stroke, because of the connecting rod’s angle. So during the 180° rotation from mid-stroke through top-dead-center and back to mid-stroke the minor contribution to the piston’s up/down movement from the connecting rod’s change of angle has the same direction as the major contribution to the piston’s up/down movement from the up/down movement of the crank pin. By contrast, during the 180° rotation from mid-stroke through bottom-dead-center and back to mid-stroke the minor contribution to the piston’s up/down movement from the connecting rod’s change of angle has the opposite direction of the major contribution to the piston’s up/down movement from the up/down movement of the crank pin. The piston therefore travels faster in the top half of the cylinder than it does in the bottom half, while the motion of the crankshaft weights is sinusoidal. The vertical motion of the piston is therefore not quite the same as that of the balancing weight, so they can’t be made to cancel out completely.

Secondly, there is a vibration produced by the change in speed and therefore kinetic energy of the piston. The crankshaft will tend to slow down as the piston speeds up and absorbs energy, and to speed up again as the piston gives up energy in slowing down at the top and bottom of the stroke. This vibration has twice the frequency of the first vibration, and absorbing it is one function of the flywheel.

Thirdly, there is a vibration produced by the fact that the engine is only producing power during the power stroke. In a four-stroke engine this vibration will have half the frequency of the first vibration, as the cylinder fires once every two revolutions. In a two-stroke engine, it will have the same frequency as the first vibration. This vibration is also absorbed by the flywheel.

Dynamic Modelling (30 marks)

Power supply for crankshaft is based on literature review

Must use the following components: Bearing, belt (or chain) and motor

Predict dynamic horizontal & vertical loads at 300RPM

Prediction must be for two conditions: No balance weights & with balance weights

From above I need to calculate the change in horizontal and vertical loads from adding the balance weights as a percentage change in load

Carry out analysis, simulation & optimization in MSC Adams software

Measurements (10 marks)

Using the same PT500.16 crank mechanism on the GUNT test rig you are to set up the experiment with accelerometers in the horizontal and vertical planes.

Obtain frequency spectrum traces and vibration magnitude from both horizontal and vertical accelerometers when running at 300 RPM for two conditions: Without balance weights & with the balance weights

Must show the change in horizontal and vertical acceleration magnitude at 5Hz from the balance weights as a percentage change in acceleration magnitude

Adequate strength and rigidity are essential, so these would all be key design aims.

Results (20 marks)

Tabulate results from (1) & (2) in order to compare theoretically modelled results with measured results

Compare the predicted reductions in horizontal and vertical loads from balancing weights with reductions in vibration magnitudes

The aim is to see if there is a correlation between predicted loads and the measured vibration levels.

Discussion (20 marks)

Comment and discuss any correlations found between predicted loads and vibration levels measured for the different states of the mechanism with and without balancing weights

Comment on problems you encountered with the ADAMS model, and what you can show are the advantages and limitations

Comment on any difficulties or limitations of the measurements with the accelerometers, and on the analysis software

Discuss any links with theory from your literature search

Conclusion (10 marks)

Show whether dynamic modelling has a use in predicting dynamic loads, and whether there is also the ability to predict vibration levels and frequencies.

Show whether dynamic modelling is a viable design tool for optimising design so that costs and time can be reduced in prototype build.

(Minimum 1000 words)

Reference

Appendix of reference information.

Attach any reference tables, data, graphs and diagrams.

Attach any pictures of systems or information that need to support your design and not included earlier.

Attach any other information supporting work you have done, correctly annotated.

Your submission must be made in the form of a well-structured written report with a clear introduction, main body and conclusions. Your work and findings should be supported by references to relevant good quality sources of information (such as journal articles, conference papers, books, legislation, reputable website sources) with a list of references provided at the end of the report.