The downward pull of the earth on a body is called gravity. When gravity acts on a body, all the particles of the body are subjected to this downward pull. These particle forces are equivalent to a single force, equal to the weight of the body, acting through the Centre of Gravity. This is shown by the fact that a body can be supported by a single force, providing the force acts upwards through the centre of gravity. Supporting a weight on a rope is an example of this.
Since the weight of a body acts vertically downwards, the direction of the supporting force must also be vertical. Because of this, a small weight on a string (a plumb-line) always hangs vertically. This device provides a means of locating the centre of gravity of a body.
In designing a machine which has parts that are moved or rotated the weight of the part is taken to be concentrated at its centre of gravity, and it is important to know the positions of these centres of gravity. Very often the parts are cut from metal plates, so the centre of gravity lies somewhere between the edges of the plate. The exact location of the centre of gravity will depend on the shape of the plate and this can be found by experiment.
The object of this part of the experiment is to determine the position of the centre of gravity of various shaped plates.
When three forces in the same plane act in different directions on a stationary body their lines of action meet at a point. Because of this the forces can be represented by a force diagram called the Triangle of Forces. This can be used to find the size of two of the forces when the third force is known.
The object of this experiment is to test that three non-parallel forces in equilibrium can be represented by a triangle of forces, from which two of the forces can be found when the third force is known, provided that the direction or line of action of the forces is known.
When two forces act on a body in different directions in one plane, they are equivalent to a single force (the resultant) acting somewhere in between them. An example of this is when a sledge is pulled by two horizontal ropes spread at an angle. The sledge will move in a direction between the ropes along the line of their resultant force. Until the sledge moves, it will pull back against the ropes with a single horizontal force equal and opposite to the resultant of the two rope forces.
It can be shown that when three such forces are balanced (that is in equilibrium), their lines of action all meet at a point. Using this fact, the resultant of two forces in the same plane at an angle can be found by a graphical method called the Parallelogram of Forces.
The object of this experiment is to test that when three non-parallel forces in the same plane are in equilibrium, their lines of action meet at a point, and hence to show that the resultant of two forces can be found using the Parallelogram of Forces.
In the design of pin-jointed plane structures such as girders, bridges and roof trusses, it is necessary to find the forces acting in each member so that the frame can be made strong enough to withstand the maximum loads exerted upon it. The Polygon of Forces is frequently employed to find such forces and deals with each joint in turn. This experiment could be regarded as ONE such joint on a structure, and it will be shown that in a system containing four or more forces, two unknowns can be found in magnitude or direction if the remaining information is known. The Polygon of forces is an extension of the Triangle of Forces, and whereas Tri means three, Poly means many.
The object of this experiment is to test that when four or more forces are in equilibrium at a point, they can be represented by a Polygon of Forces from which unknown forces can be found.
When forces produce a turning effect, this turning effect can be measured by the product of the force and the perpendicular distance between the pivot and the line of the force. The product is called the TURNING MOMENT of the force.
If a body has several forces applied to it which have turning effects in opposite directions, the body will not turn if the total turning moments in each direction are equal. This is called the PRINCIPLE OF MOMENTS.
The Principle of Moments is frequently used in engineering and building work where forces have to be balanced to prevent any turning movement. It can be applied both to parallel forces and to oblique forces; but in all cases, when calculating the turning moment the length is the perpendicular distance from the pivot to the line of the force. A method of calculating the effect of turning forces to produce equilibrium is to say The Moments Clockwise = The Moments Anti-Clockwise.
The object of this experiment is to verify the Principle of Moments for parallel and non-parallel forces.
Experiment No 5 shows that if a pivoted bar has forces applied to it which have turning effects, the body will not turn if the turning moments in each direction are equal. A turning moment being the force multiplied by the perpendicular distance from the centre of the pivot.
The pivot (or beam) balance makes use of this principle for weighing. In the beam balance, the weight to be measured is placed in one pan and is balanced by known weights in the other, both pans being at the same distance from the pivot. In the slide balance, the arms are of unequal length; the weight being measured is placed in the pan on the short arm and balanced by a known weight which slides along the long arm. A scale marked on the long arm is calibrated to show the weight in the pan.
The object of this experiment is to demonstrate that the action of weighing with a beam balance or slide balance is based upon the Principle of Moments.
A lever is a bar with a pivot or fulcrum at a selected position along its length so that a force acting at one point can be converted into another force at a different point along its length. Levers are frequently used to move heavy loads or to operate mechanisms. In these applications, a force turns the lever about a pivot or fulcrum to overcome a resistance. The positions of the force, fulcrum and resistance vary in different levers, but broadly there are two types, one where the fulcrum is between force and the resistance, and the other where the force and the resistance are both on the same side of the fulcrum.
When used to move a load, the lever is a simple form of machine. Usually the force is smaller than the resistance, as with the crowbar, and a Mechanical Advantage is gained.
The object of this experiment is to determine the Mechanical Advantage of various types of levers.
A beam is a horizontal member of a structure which rests on supports (often walls or columns) and spans an open space. The beams over the ground floor of a house not only form the ceiling of the ground floor but, more important, support the floor and all the contents of rooms on the first floor. If a beam rests on two supports without any "fixing down" devices, it is said to be SIMPLY SUPPORTED. If a load is placed on the beam and covers a very short length of the beam, it is called a POINT or CONCENTRATED load, but if the load is spread over an appreciable length of the beam it is called a DISTRIBUTED load.
If the supports are placed at each end of a beam and the beam is symmetrically loaded, the weight carried at each support (called the REACTIONS) must be half the total weight on the beam. Beams which are not symmetrically loaded must still carry the total load at the supports, but the proportion of the total weight carried by each support will depend on the weight of each individual load and the position which it occupies along the beam.
The object of this experiment is to show that:
1.A distributed load may be considered as an equivalent concentrated load acting on the beam at the centre of gravity of the distributed load.
2.The reactions at the supports may be calculated by applying the Principle of Moments no matter where the supports are positioned along the beam.
A pulley is a wheel used to guide ropes or belts in selected directions so that a force transmitted in one direction can be changed to another direction which is more convenient. The pulley spindle is made small compared with the diameter of the wheel so that the pulley is easily turned. There is then little loss of effort in turning and the pull or tension transmitted by the rope is practically the same on both sides of the pulley.
Pulleys can be "fixed" (they do not move bodily relative to the earth) or they can be "movable". Movable pulleys are supported by the rope passing under the pulley, thus forming a cradle. The pulley block then moves as the rope moves. By combining fixed and movable pulleys it is possible to lift heavy loads with reduced effort and so gain a Mechanical Advantage.
The object of this experiment is:
1.To test whether the tension in a pulley cord is affected by a change in direction of the cord as it passes over a pulley.
2.To determine the Mechanical Advantage of a simple combination of fixed and movable pulleys.
Pulley blocks are widely used to lift heavy loads when only "manpower" is available and are sometimes used in conjunction with motorised lifting tackle. Experiment No 9 shows that by using "fixed" and "movable" pulleys it is possible to gain a Mechanical Advantage. To develop this principle further, this experiment shows that by using multiple pulley blocks the Mechanical Advantage can be increased.
The object of this experiment is to investigate the mechanical characteristics of a set of pulley blocks which has three sheaves in the upper block and two sheaves in the lower block.
The simple wheel and axle is used for hoisting or hauling a load through a considerable distance, and is usually called a winch windlass. It consists of a wheel fixed to an axle or drum of smaller radius.
The load is attached to a rope or wire which is wound round the drum when the wheel is turned. The wheel may be replaced by a handle, as in the windlass used for hoisting water from a well, or it may be turned by projecting bars as in a ship's capstan. For heavy duty winches the wheel is usually power driven and may have gearing.
The wheel and axle is basically a lever. The applied force acts at the end of a lever arm equal to the radius of the wheel and lifts a resisting load acting at the end of a short lever arm equal to the radius of the drum. Neglecting friction, the Mechanical Advantage of the wheel and axle is equal to the Velocity Ratio.
Although the theoretical Mechanical Advantage is equal to the Velocity Ratio the actual Mechanical Advantage is reduced because of friction in the spindle which increases with the load. The applied force, the load and the friction losses gives rise to the relationship called the LAW OF THE MACHINE.
The object of the experiment is:
1.To determine the Law of the Machine for a simple wheel and axle.
2.To investigate the variation of Mechanical Advantage and Efficiency with load.
In Experiment 11 Part 1, it is shown that with a Simple Wheel and Axle the mechanical advantage could be increased by making the axle smaller compared with the wheel. A more practical way is to use the DIFFERENTIAL WHEEL AND AXLE. This has the axle made in two parts of different diameter. The axle rope is wound in opposite directions round the two parts of the axle and passes under a movable pulley to which the load is attached. When the wheel is turned the rope winds onto the larger axle but off the smaller axle. For one turn of the wheel the length of the pulley rope taken up will be equal to the difference of the circumference of the two parts of the axle.
Although a large theoretical Mechanical Advantage can be obtained with a Differential Wheel and Axle, the practical Mechanical Advantage is smaller due to friction and also because the movable pulley has to be lifted as well as the load.
The object of the experiment is:
1.To determine the Law of the Machine for a differential wheel and axle.
2.To verify that Mechanical Advantage and Efficiency increase with load up to a limiting maximum.
The Weston Differential Chain Blocks are used for hoisting heavy loads. This type of hoist is frequently used on gantry for loading and off-loading heavy equipment because of its simple and effective design. The Weston Chain Blocks are a variation on the Differential Wheel and Axle, dealt with in Experiment No 11.
Whereas other types of hoists are usually made up of ropes and pulleys, the Weston Blocks consist of an endless chain and chain wheels. These are very strong and the engagement of the chains and notched chainwheels prevent slipping.
The top block consists of two wheels which are fixed to rotate together, one wheel being slightly larger than the other. The bottom block contains one wheel only and acts as a snatch block. The chain is endless and passes round the larger top wheel, then round the snatch block wheel, then round the smaller top wheel and finally the two ends are connected and allowed to hang loose.
By making a small difference between the size of wheels in the top block a large Velocity Ratio and a theoretical Mechanical Advantage can be obtained. In practice the Mechanical Advantage is reduced mainly due to friction in the system and as a result the Efficiency is low. This, however, is not a disadvantage in that if Efficiency is less than 50% the machine will not "overhaul". That is to say the system will support a load on the snatch block without applying a force on the effort chain. This feature makes it a "safe" hoisting machine.
The object is to carry out tests on Weston Differential Chain Blocks and show:
1.Load is directly proportional to effort.
2.Mechanical Advantage increases with load up to a limiting value.
3. The method of finding the Velocity Ratio.
4.The Efficiency is such that "overhauling" will not occur.
By fixing pulleys to two parallel shafts and connecting the pulleys with a belt, one shaft can be driven by the other. This is called a BELT DRIVE. Using a belt drive, power can be transmitted from a driving shaft to other shafts at a distance. A familiar example of this is the belt drive used in motor cars for driving the fan and alternator from the engine shaft. In this drive the belt is V-shaped and runs in V-shaped pulleys. For longer belt drives the belt is flat and runs on pulleys with flat or slightly rounded faces.
In open belt drives, the driving and driven shafts rotate in the same direction. With a flat belt, the belt can be crossed between pulleys and the two shafts will rotate in opposite directions. A combination of open and crossed belt drives is used to reverse the direction of rotation of belt driven machinery.
If the pulleys are the same size, the shafts speeds will be the same. If the driving pulley is larger than the driven pulley, the driven shaft will rotate at a higher speed and vice versa. Providing there is no slip between the belt and the pulleys it can be shown that the ratio of the speeds will be inversely proportional to the ratio of the pulley diameters.
The object of this part of the experiment is to verify:
1.The direction of rotation of open and crossed belt drives.
2.That the speed of rotation of the two pulleys is inversely proportional to their diameters.
If two weights are attached to a belt passing over a pulley, the belt is pressed on to the face of the pulley by the weights, setting up friction. If one weight is larger than the other, a turning force will be applied to the pulley. Providing this does not exceed the force of friction between the belt and the pulley, the belt will turn the pulley without slipping.
A similar action takes place in the belt drive. The belt is fitted over the pulleys with sufficient INITIAL TENSION to provide a friction force between the belt and pulleys. When a drive is transmitted between the pulleys, the tension in the belt is increased on the driving side (the tight side) of the belt, and reduced to on the other (slack) side. The difference in tensions on either side is the effective turning force providing the drive. The greater this difference in tension, the greater the torque transmitted, but the difference in tension cannot be too great, otherwise the friction force between the belt and the pulleys will be overcome, and the belt will slip.
The torque provided by the driving shaft is not all transmitted to the driven shaft as some of it is used to overcome friction in shaft journal bearings and in moving the belt. The work output at the driven end is therefore less than the work input at the driving end. The ratio of the work output to the work input can be used to measure the Efficiency of a belt drive.
The object of this part of the experiment is:
1.To measure the difference in tension between the two sides of a belt drive.
2.To determine the Efficiency of drive transmission.
In Experiment No 13 Part 2 it is shown that the turning force on a pulley is equal to difference in the tensions in the belt on the tight side and on the slack side of the pulley. The difference in tension is produced by the friction between the belt and the pulley.
This experiment will show that friction force and therefore the turning force may be increased by increasing the angle of lap of the belt around the pulley. At first this may appear to be due to the increased area of contact, but as shown in Experiment No 23, friction is substantially independent of area and the assumption that area alone is responsible for the increased driving force is therefore FALSE.
The object of this experiment is to show that the driving force of a belt drive increases as the angle of lap increases.
When power is transmitted between two shafts by a belt drive, the belt is liable to slip on the pulleys under heavy loads. In a Chain Drive, the chain consists of a number of pivoted links which fit over suitably shaped teeth or "sprockets" on the chain wheels. This ensures that no slip takes place, so that the chain drive gives a "positive' drive". A common example of the chain drive is that used on bicycles.
Since the chain has to fit on both sprocket wheels, the "pitch" of the teeth must be the same on each wheel, and the number of teeth on each wheel must be directly proportional to their diameters. The speed ratio of a chain drive can therefore be found by counting the number of teeth on each wheel.
The most common type of chain used in a chain drive is the "roller" chain. The links of this are held together with pins, and a hardened steel sleeve is fitted over each pin to make a roller which forms the surface in contact with the teeth. There is, however, some friction set up in the chain and between the chain and the sprocket wheel. The ratio of the work output to the work input can be used to measure the efficiency of the chain drive.
The object of this experiment is:
1.To check the Speed Ratio of a chain drive.
2.To measure the Efficiency of drive transmission.
In the Chain Drive, Experiment No 15, one wheel is made to drive another at a distance, by linking them with a chain. If the wheels are placed close together and the teeth on the wheels are suitably shaped, they will provide a direct drive. The wheels are then called GEAR WHEELS. To mesh properly the teeth must be the same size and must be shaped so that the engaging surfaces "roll" rather than "slide" upon each other.
With the chain drive, the sprocket wheels rotate in the SAME direction. In the gear drive, the gear wheels rotate in OPPOSITE directions, but the speed ratio will still be the ratio of the number of teeth on the wheels.
The object of the experiment is:
1.To compare the Velocity Ratios of a single-stage and double-stage geared winch.
2.To measure the Mechanical Advantages and Efficiencies under varying loads.
In many machines and power transmission assemblies it is necessary to have shafts at 90 degrees to each other and the common method is to use BEVEL WHEELS. Bevel gear teeth are difficult to cut as the size of the tooth changes from the inner pitch diameter to the outer pitch diameter. For efficient transmission it is very important to set up the gears very accurately.
To ascertain the Efficiency, Velocity-Ratio and Mechanical Advantage of a Bevel Gear Unit under different loads with the gear teeth set correctly and incorrectly.
In many rotating machines the input shaft is driven by a high speed electric motor or internal combustion engine. The output shaft, however, is often required to run at a much lower speed. A method of speed reduction must therefore be provided and this is usually some form of gearing.
In Simple Gear Trains as in Experiment No 15, it is shown that a pair of gear wheels could be used to provide a speed reduction in order to gain a Mechanical Advantage. With a single pair of gear wheels the reduction in speed is produced by making the small wheel drive the large wheel, the speed reduction being determined by the number of teeth in the gear wheels. When a very large reduction of speed is required, a single pair of gear wheels would not be practical and additional gear wheels are needed. An alternative method is to use a WORM AND WHEEL.
The worm is a screw with "threads" which engage in correspondingly shaped (helical) teeth on the wheel. As the worm revolves, its threads slide along the teeth on the wheel and push them in the direction of the worm axis, making the wheel turn.
A worm gear provides means of obtaining a large speed ratio and a large Mechanical Advantage. It also enables the driver and driven shafts to be at 90 degrees, unlike the ordinary gear drive where the shafts are parallel. In the ordinary gear drive, the gear wheels have "straight teeth" parallel to the shaft. These engaging teeth have a rolling action, whereas with the worm and wheel, the worm thread has a sliding action along the wheel teeth and the friction loss is greater. The efficiency of the drive is therefore correspondingly reduced.
A modified form of worm and wheel is used in the steering mechanism of some motor cars to enable the movement of the steering wheel to turn the front wheels of the car. The worm and wheel is also widely used in industry for high speed reduction gears.
The object of the experiment is:
1.To verify the Speed Ratio of a simple worm and wheel.
2.To measure the Efficiency of the drive under various loads.
The Screw Jack is a simple device for raising a heavy load with comparatively little effort and when only a short lift is required. The motor car jack is a typical example. It consists essentially of a nut and screw in which the nut is held stationary while the screw is turned and lifts the weight. The same principle is used in the bench vice and screw press.
A long lever such as a crowbar will also lift a heavy load through a short distance with only a small effort. With the screw jack the friction of the screw is sufficient to hold the load in the raised position when the effort is removed so that it will not run back or "overhaul". This is an important feature of the screw jack.
The object of this experiment is:
1.To measure the effort required to raise various loads using a simple form of screw jack and to determine how the Mechanical Advantage and Efficiency varies with load.
2.To test whether the screw jack "overhauls".
When a coiled spring is stretched or compressed, it stores energy. Because of this, coiled springs can be used to cushion the effects of sudden loads, as in shock-absorbers and buffers, and they provide the spring loading of engine valves etc. Coiled springs are also used to measure weights by recording the amount of movement of the spring which is needed to balance the weight. When used in this way they are called SPRING BALANCES. These can be designed with very stiff springs to measure heavy loads, or with light springs which will measure smaller weights or forces. It is the lighter type of spring balance which is dealt with in this experiment.
When a spring is stretched it can be shown that the increase in length of the spring (the extension) is always proportional to the stretching force, providing the spring is not over-stretched. The same applies to a spring which is compressed. Because of this, a spring balance has a uniform (evenly divided) scale which is graduated to show units of weight. The range of the scale will depend on the stiffness of the spring and a spring balance must be properly calibrated so that it registers the actual weight or force on its hook.
The object of this experiment is to verify that the extension of a coiled spring is proportional to the load applied and to demonstrate the principle of a spring balance.
A weight suspended by a cord forms a simple pendulum. When the pendulum is set swinging, the time of the swing is found to be constant for a given length of pendulum and is not affected by the weight of the bob or (within limits) by the arc of swing. This constant time of swing of a simple pendulum forms the basis of time-keeping by some clocks.
A pendulum swings under the action of gravity. The force of gravity acting on a freely falling body will give it a steadily increasing speed, or acceleration, which is the same for all bodies, whatever their weight. This acceleration can be calculated using the time of swing of a simple pendulum.
The object of this experiment is to show that the time of a simple pendulum depends only on the length of the pendulum and to determine the value of the force of gravity using a simple pendulum.
The energy of body is a measure of its capability for doing work. Energy exists in a variety of forms but it cannot be created or destroyed. Energy can only be transformed. When an engineer refers to "losses" in energy he is only implying that it is not doing useful work.
Friction is by far the most common form of "lost" energy in machines. There are others such as air resistance or unwanted heat and sound but in any event no energy is destroyed. Because there are "losses" in any machine the USEFUL energy given out is always less than the energy put in. Engineers are concerned with various types of energy, two of which are "POTENTIAL" and "KINETIC" energy.
The object of this experiment is to investigate some aspects of Potential Energy and Kinetic Energy and to show that:
1.Energy exists.
2.That it may be transformed.
3.That it may be "stored" and "given back".
Every reciprocating engine produces fluctuations of energy and speed at the crankshaft due to the fact that the turning moment at the crankpin varies throughout each revolution. This is inherent in a crank motion and is dealt with in detail in Experiment No 35. In the case of a steam engine, the turning moment reaches a maximum twice during each revolution (assuming the steam pressure on the piston is substantially constant) and twice during each revolution the turning moment is zero. It is therefore necessary to control the energy so that it can be distributed in a more uniform manner throughout each revolution. The flywheel which is attached to the crankshaft is capable of receiving and storing energy and then giving it back. This reduces the fluctuations at the crankshaft to a minimum.
To explain the way in which this happens, consider a heavy object on wheels such as a railway goods wagon which is to be set in motion. Because the heavy wagon has a reluctance to start moving, the demand for energy to start the movement is greater than the energy necessary to maintain movement once it has started. Therefore the wagon stores a quantity of energy which must be "used up" before the wagon is brought to rest. This reluctance to start and stop is called INERTIA. A flywheel with a heavy rim has inertia and is capable of storing energy when the turning moment is at the maximum. This energy is released and given back as the engine tries to slow down during each revolution.
The object of this experiment is to find the energy stored in a flywheel by supplying a known quantity of energy. The energy stored in a flywheel, called Kinetic (or moving) Energy, will be shown to be constant for a given flywheel at 1 rev/min and the energy at this speed may be calculated from the dimensions of the flywheel. It may also be shown that the kinetic energy is proportional to the square of the speed of rotation.
When two rough surfaces are made to slide over one another, the minute, uneven surface particles resist the sliding and are sometimes torn away. This resistance to sliding is called friction. Even so-called "smooth" surfaces have microscopic roughness which cause friction and the friction force must be overcome before sliding can take place.
Friction is usually regarded as wasteful, as in machines where it absorbs power and causes wear, but it can be useful, for example in friction brakes. In designing machines where sliding takes place, the effect of friction must be taken into account and for this the LAWS OF FRICTION are used. The laws are only approximately true, but they form a useful and practical basis for dealing with friction problems.
Friction opposes sliding and depends on the roughness of surfaces in contact. In practice it is found that the friction force is a fixed proportion of the force pressing the surfaces together. This proportion is called the COEFFICIENT OF FRICTION. Friction which opposes movement from rest is called STATIC FRICTION. As soon as sliding takes place it is found that less force is required and this is called KINETIC FRICTION.
To verify the Laws of Friction and to measure the Coefficient of Friction for different materials.
A weight force always acts vertically downwards towards the centre of the earth. By various means a force can be applied to an object at any angle we desire but a weight force always acts downwards.
This experiment shows that when a body is on an inclined plane it exerts a force down the plane and a force at 90 degrees (or normal) to the plane and that both are caused by the weight acting vertically downwards.
If a garden roller rests on a horizontal plane ALL its weight acts downwards and there is no tendency for the roller to move unless we pull or push the handle. At the line of contact between the roller and plane, the force pressing the roller and the plane together will be equal to the weight.
If the plane is vertical the roller tries to roll down the plane and to prevent it from doing so we must support the entire weight. In this condition the force pressing the roller and plane together is zero because ALL the weight acts downwards.
Therefore, on a plane inclined at an angle between the horizontal and vertical, the roller will still try to roll down the plane, but the force required to prevent it will be LESS than the roller weight and the force pressing the roller and the plane together will also be LESS than the roller weight. A typical example of the incline plane is the roller conveyor where a load is transported on "frictionless" rollers due to its own weight.
To investigate the forces acting on an inclined plane due to a weighted roller supported on the plane.
When a block is placed on an incline the tendency is for the block to slide down the plane. If the angle of inclination is small the block is prevented from slipping by the friction between the surfaces. As the angle is increased, the force exerted down the plane due to the weight of the block also increases, but the force pressing the surfaces together decreases. At the Angle of Friction, the force acting down the plane just overcomes the friction and sliding takes place.
To investigate friction on the inclined plane and to investigate the relationship between the required force (applied parallel to the plane) to slide a block up the plane.
When a block rests on a horizontal plane its WHOLE weight presses on the plane and the pressure between the surfaces sets up a resistance to movement which is called friction. If the plane is vertical no pressure takes place between the surfaces because the whole weight is acting downwards parallel to the plane and the block will slide down the plane. Therefore, when the plane is inclined at an angle between the horizontal and vertical, PART of the block weight acts parallel to the plane and PART of the weight produces pressure between the surfaces.
As the angle of inclination increases, the force acting along the plane increases but the force pressing the surfaces together decreases and so the friction force decreases. At a certain angle the force acting down the plane will overcome the frictional resistance between the surfaces and sliding will take place. The angle at which sliding begins to take place is called the ANGLE OF FRICTION and this experiment will show the relationship which exists between this angle and the Coefficient of Friction.
1.To measure the Angle of Friction and from it find the Coefficient of Friction.
2.To show that the Coefficient of Friction is equal to Tangent of the Angle of Friction.
If we have two objects which are to be moved along a floor, one being a roller and the other a rectangular block, we know that the round object will move more freely than the rectangular one. The block scuffs across the floor with much resistance called friction while the round object ROLLS over the floor. Even the roller encounters some resistance, due to the interaction between the surfaces and the softer the surface the greater the resistance. If the block is on top of rollers the same rolling action which takes place between the rollers and the floor also takes place between the rollers and the block. The block will move more freely. Primitive man used this idea to move heavy blocks of stone. The only disadvantage with this method is that the rear roller has to be moved in turn to the front in order to keep the movement going. To compensate for this difficulty the wheel was invented, where the axle pulls the "roller" along as it goes.
Whilst rolling friction still takes place between the wheel and ground, sliding friction occurs where the wheel revolves around the axle. The effect of this resistance to motion is minimised by making the outside diameter of the wheel large compared with the diameter of the axle. The very low frictional losses due to rolling friction has led to the development of ball and roller bearings where very hard metals are used. In contrast the motor car, with its relatively flexible rubber tyres, relies on rolling friction to maintain its "grip" on the road surface both for driving and braking.
To show the extent to which friction is reduced by using wheels and rollers and to compare the effects of different bearing surfaces.
When using a wedge to split a log, the side "splitting" force is greater than the force exerted by the blow of the hammer and thereby a Mechanical Advantage is obtained. The wedge, therefore may be regarded as a "simple machine", since it is capable of receiving work from an external source and delivering it in a more suitable form for the purpose required.
A bolt may be compared with a wedge formed in a spiral. As a nut and bolt are tightened on a "job", the nut turns and moves up the spiral (or helical) thread and thereby produces a Mechanical Advantage as the clamping force is greater than the force required to turn the spanner. Also, the nut will not come undone accidentally because it "wedges" itself between the thread on the bolt and the surface of the job being bolted. The ability of a wedge to hold without slipping backwards depends on the angle of the wedge and the friction between the surfaces.
If the angle is small, such as the angle of the helix on a bolt thread, the nut will not "slip back" or, to use the technical term, it will not OVERHAUL. As the angle of the wedge is increased, the tendency for it to overhaul also increases.
The object of this experiment is to determine the Mechanical Advantage and Efficiency obtained using two different wedges and to show that overhauling may be prevented if the angle of inclination of a wedge is small.
Practically all machines, ranging from watches to locomotives, have shafts or spindles which rotate in bearings. Through the history of engineering there has been continuous development of bearing design using such materials as wood, steel, brass, metal alloys and nylon, to mention but a few. Broadly speaking, there are two types of bearings, the "Plain" bearing which produces SLIDING friction between the cylindrical surfaces of the shaft and the bearing, and Ball and Roller bearings which produce ROLLING friction between the hard steel balls and the surface in contact.
As is shown in Experiments No 23 and No 26, sliding friction is much more severe than rolling friction and hence Plain bearings are not nearly as free running as Ball or Roller bearings. All bearings absorb power due to friction forces. Lubrication plays an important part in reducing power losses to a minimum on Plain bearings in particular, but lubrication by oil or grease is not always convenient.
A popular type of Plain bearing which overcomes difficulties of supplying lubrication is the "self-lubricating" bearing which is manufactured impregnated with oil and sometimes with "dry" lubricants such as graphite and molybdenum. Ball and Roller bearings require little lubrication and attention but are obviously more expensive due to the high quality and precision of manufacture.
When designing a machine it is up to the engineer to choose the right kind of bearing which will be free running, have long life, require little attention and yet still be economical.
The object of this experiment is:
1.To compare the resistance to turning due to friction in cantilevered bearings of different materials.
2.To show something of the progress made in bearing development.
Cams are irregular shaped pieces of material which move past a follower and thus cause the follower to move in a predetermined manner. Generally speaking cams are shaped like discs with an irregular periphery. They are mounted upon a shaft which rotates and the follower usually consists of a pivoted lever with a roller at one end and the "operated equipment" at the other, together with a spring to keep the follower pressed against the periphery of the cam. As the cam rotates the follower will move to and from the centre of the cam shaft and the lever will transmit this movement to the equipment.
The object of this experiment is to study the following aspects of cam design:
1.To show the relationship between the angular movement of the cam and its follower.
2.To see how this can be applied to a constant rise cam with a small and large displacement.
3.To measure the turning force required on one of the above cams.
4.To note the dangers of "overhauling" in cams which have a large displacement.
The GENEVA MOTION is an ingenious and effective mechanism designed to convert uniform circular motion into intermittent indexed motion with accurately locked location of the driven member. It is particularly useful on packing and wrapping machines where a product is subjected to several operations in succession. It is also used extensively for cine film feeding equipment on projectors.
The object of the experiment is to show how the circular motion of the drive unit is transformed into the intermittent motion of the star wheel and how the latter is caused to accelerate and decelerate during the transmission process. A further object is to see how the star wheel is rigidly locked while the crank pin moves through its free orbit.
ln certain types of equipment such as a hand operated winch or crane there is a need to rotate a shaft against a load and to prevent the shaft from running backwards. This can be done by using a RATCHET MECHANISM. The components of a ratchet mechanism are the Pawl and the Ratchet.
The application of the Pawl and Ratchet which is most widely used is the bicycle free-wheel. This enables the drive to be overtaken when the bicycle is going downhill, but re-engaged when the cyclist starts to pedal. The gentle click-click of a freewheeling bicycle is due to the pawls running over the teeth of the ratchet.
To examine the working parts and operation of the Ratchet Mechanism.
The SCOTCH YOKE is a means for converting circular motion to straight line reciprocating motion in a simple manner, with the advantage of providing acceleration and deceleration at each end of the stroke. It is easy to make and is generally applied to light duty jobs. The circular movement is always the DRIVING element of the mechanism.
To record and study the movement of the driving crank and to show how this is related to the reciprocating element of the movement.
Shafts are used to transmit rotary motion and in most cases these shafts rotate in bearings set in a straight line. There are, however, cases where the shaft cannot be straight but where it has to operate through an angle. A good example in common use is the shaft which transmits power from a motor car engine through its gearbox to the back axle to drive the rear wheels. Here the shaft is generally at an angle and the operation is further complicated by the fact that the angle varies when the motor car runs over bumps in the road. Such conditions can be satisfied by the use of universal couplings. For uniform power transmission the design and assembly of the two couplings must be carefully considered.
The object of the experiment is:
1.To investigate the effect of introducing universal couplings to a simple drive shaft and to check the uniformity of angular movement between the driving and the driven end of the shaft in a straight line assembly and then again with an angular transmission.
2.To repeat these tests with the couplings set up at incorrect positions on assembly to see how this interferes with uniform angular transmission.
When a mass moves it always tries to travel in a straight line. Therefore when a mass is rotated about a "fixed" centre, it is reluctant to travel in a circular path and is continually trying to escape. To restrain the mass and hold it in orbit a force must be exerted from the centre of rotation. The vector force which is tending to pull the mass away from the centre is called the CENTRIFUGAL FORCE.
We have all experienced centrifugal force at some time or another. The most obvious example is rotating a weight attached to the end of a string. The faster the rotation and the longer the string, the greater the pull on the string. If the string is suddenly released, the weight flies off at high speed in a straight line tangential to the circular path. Centrifugal force is used in a variety of ways, one of which is in Governor design used to keep the speed of an engine within fairly close limits. In the test apparatus the central driving clutch automatically disengages when the centrifugal force produced by the rotating mass is equal and opposite to the force exerted by the tension springs.
The object of this experiment is to investigate Centrifugal Force produced by a rotating mass.
In certain types of machines it is necessary to convert straight line (or linear) motion into circular motion. The most common example is the reciprocating engine, whether it be a steam or internal combustion engine. Energy is produced in the cylinder and the piston moves backwards and forwards (or up and down). The piston transmits its motion via the crosshead and connecting rod, to a point called the crankpin which is fixed to an arm on the crankshaft.
The crankshaft is free to revolve about a "fixed" centre so that the crankpin rotates at a radius, this radius being equal to half the stroke of the piston. By this link mechanism (called the crank mechanism) the linear movement of the piston is converted into circular motion at the crankshaft. The driving force, called the Turning Moment is continually changing during each revolution of the crank. The effect of these extreme variations produces fluctuations of energy and speed at the crankshaft, but these can be smoothed out to some extent by the use of a flywheel attached to the crankshaft. This aspect is dealt with in detail in Experiment No 27.
The object of this experiment is to investigate the characteristics of a crank mechanism by constructing a Turning Moment diagram from experimental results and comparing the diagram with another constructed from theoretical values.
Experiment No 35 deals with crank motion and it is shown that as the piston approaches "dead centre" the turning moment approaches zero, even though the force on the piston remains constant. It will be shown in this experiment that if a constant turning moment is APPLIED to the crank in the region approaching dead centre, the resulting force at the piston will increase until its final value at dead centre approaches infinity. This feature of a link mechanism is called the TOGGLE ACTION and is used in the design of presses and clamps where a large force can be produced by a comparatively small effort.
The object of this experiment is to investigate the "toggle" action by using a crank mechanism and to show how the force increases as the links "flatten out".
Engineers often use crank movements to reciprocate parts of a machine where the inward and outward strokes are identical but there are occasions when it is necessary to have a slow outward movement followed by a quick return movement. A good example is the Shaping Machine where the slide which carries the cutting tool moves slowly forward and then returns quickly on the non-cutting stroke.
The object of the experiment is to show a Quick Return Mechanism at work and to record the relationship between the rotation of the crank and the movement of the slide.