The parts of a tractor by which the power of the engine is applied to the driving wheels are called the transmission, and include the clutch, the change speed gear, the differential and the drive.

It has been shown that a gas engine delivers power only when it is running at speed; it cannot run until some outside power drives it through the inlet and compression strokes.

The tractor cannot move until the engine is running and delivering power, and it follows, therefore, that it must be possible to disconnect the engine from the driving mechanism in order that it may run independently. This is done by means of a clutch, which is a device that connects two shafts, or disconnects them.

A clutch must be so made that when it is engaged it takes hold, not suddenly, but gradually. If it took hold suddenly, the tractor would be required to jump at once into full motion; this would cause a severe straining of the parts and probable breakage. The alternative would be the abrupt stopping of the engine, and this would also strain things.

By making the clutch in such a way that it slips, and takes hold little by little, the tractor starts slowly, and gradually comes up to speed; the slipping of the clutch then ceases, and it takes hold firmly.

All clutches operate by the friction of one surface against another; in some, the surfaces are curved and in others flat, while in still others the clutch is a band around a wheel, or drum. A clutch is operated by a hand lever or by a foot pedal.

Figure 60 shows a type of clutch that operates inside a drum, which is often the overhanging rim of the flywheel. The shaft in the center is independent of the flywheel, and it is the purpose of the clutch, which is attached to the shaft, to lock the shaft and flywheel together when the tractor is to be started.

The brake shoes, which bear against the drum, form the ends of pivoted levers, and are lined with an asbestos material that resists the heat caused by the friction against the drum.

A cone-shaped block of steel slides lengthways on the shaft; when it is pushed into position, it forces out the yokes, and thus presses the brake shoes against the drum.

A plate clutch, or disk clutch, is shown in Figure 61. The principle of a plate clutch may be illustrated by placing a half-dollar between two quarters and pinching them with the thumb and forefinger. If they are held loosely, the half-dollar may be turned between the quarters, but if they are pinched tightly, the friction between the coins will be so great that one cannot be turned without turning the others.

Attached to the flywheel are studs, which support a disk, or plate; this plate revolves with the flywheel, and is practically a part of it. On either side of this plate are other plates that are supported on the drive shaft; they revolve with it, but can slide along it. The end of the shaft is square and fits a square hole in a collar, so that while the collar may slide along the shaft, the two must turn together. Cams are mounted on the hub of one of the plates in such a position that they can press the outside plates together and pinch the flywheel plate between. The cams are operated by pressing the collar against them.

The first drawing shows the clutch out, or released; the flywheel may then turn without turning the shaft, for the plates are not in contact. The second drawing shows the clutch in, or engaged. The collar is pressed against the cams, and the plates in turn are drawn together, pinching the flywheel plate between them. The flywheel and the drive shaft then revolve together.

Plate clutches are often made with more than three plates; some makes run in a bath of oil, and some are intended to work dry.

In a cone clutch, the overhanging rim of the flywheel is funnel-shaped, and into it fits a cone-shaped disk carried on the end of the drive shaft. To engage the clutch, the disk is slid along the shaft against the flywheel, the friction between the two being sufficient to drive the shaft.

When a clutch is thrown in it should take hold gradually, slipping at first, but finally having a firm grip. When it is thrown out, it should release instantly and completely.

The power delivered by an engine depends on the bore and stroke of the cylinder, and on the speed. The greater the bore, or diameter of the cylinder, and the greater the stroke, or distance the piston moves in a half-revolution of the crank shaft, the larger will be the combustion space, and the larger will be the charge of mixture that it can take in; the larger the charge, the greater will be the power produced when the charge burns.

Each cylinder produces power once during every two revolutions of the crank shaft; if the engine runs at 1,000 revolutions per minute there will be twice as many power strokes as there would be if it ran at 500 revolutions per minute, and during that minute it will produce twice as much power.

A traction engine is intended to run at a certain speed, at which it will produce its greatest power without overstraining its parts. This normal speed for any particular engine depends on the number of cylinders, their size and design, and other details established by the manufacturer. To get the best from the engine, this is the speed at which it should always be run.

The power required to move the tractor depends on various things; the hardness and smoothness of the ground, the grade, the load it is pulling, and so on. The tractor might be running on level ground, pulling so great a load that the engine is called on for all of the power that it can deliver.

On coming to a hill, still more power will be required, for now the tractor and its load must be lifted as well as moved forward. The engine, already working at its limit, cannot deliver the extra power needed, and will slow down and stop unless something is done to aid it. In such a case, the change speed gear is used to give the engine a greater leverage on its work, just as a block and tackle gives a greater leverage or purchase to a man who must lift a heavy weight.

Let us say that the normal speed of the engine is 1,000 revolutions per minute, and that it is so connected that it makes 40 revolutions while the driving wheels make 1, the speed of the tractor being 3 miles per hour. If it is a 4-cylinder engine there will thus be 80 power strokes to every revolution of the driving wheels. The engine is delivering its full power and cannot do more should the tractor be called on for an extra exertion, such as climbing a hill or crossing rough ground.

By changing the connections between the engine and the driving wheels, the engine can be made to run twice as many revolutions to one turn of the driving wheels, which will give double the number of power strokes; the wheels will thus be turned with twice the force. As no change is made in the speed of the engine, the wheels will now turn at half their former speed, and the tractor will run at 1½ miles per hour. It will, however, have twice the ability to overcome obstacles.

This change in the connections between the engine and the drive is performed by the change speed gear, which is driven by the engine and which in turn drives the wheels.

There are many varieties of change speed gears, but the main principle in them all is the same, for they depend on the action of cog-wheels, or gears.

When two gears running together, or in mesh, have the same number of teeth, they will revolve at the same speed. If one has half as many teeth as the other—10 teeth and 20, let us say—the 10-tooth gear will make two revolutions while the 20-tooth gear is making one.

There are two shafts in a change speed gear, one driven by the engine and the other driving the wheels; each carries gears that mesh with gears on the other shaft. These pairs of gears are of different sizes, and any pair may be used; the shaft driven by the engine runs as the engine runs, while the speed of the other shaft depends on the pair of gears that is being used.

By changing from one pair of gears to another, the driven shaft, and, consequently, the wheels, may be run at a greater or less number of revolutions, while the speed of the engine and the driving shaft do not change. The number of power strokes that occur during one revolution of the wheels is thus changed, and they turn with more force or with less.

High speed, or high gear, means the combination of gears that gives the greatest speed to the wheels, but the fewest power strokes to each revolution. The combination that gives the slowest speed to the wheels, but the greatest number of power strokes, is called low speed, or low gear.

Many tractors have but two speeds, a low and a high; but others have an intermediate combination for conditions too severe for running on high gear but too easy for low.

The change speed gear mechanism also provides for reversing or backing the tractor. Two gears running together turn in opposite directions, while in a train of three gears the outside gears turn in the same direction. The usual combination in a change speed gear uses two gears for going ahead; to run the driven shaft the other way, which will make the tractor back, a third gear is meshed between the two.

The differences between various makes of change speed gears are in the methods used to put into action the desired pair of gears.

Two general plans are used. In one of them, a gear of each pair can slide endways on its shaft, but must revolve with it; thus it can be slid into mesh or out. In the other, the gears of a pair are always in mesh, but one of them is loose on its shaft, so that shaft and gear can revolve independently. To make the pair of gears operate, the loose gear is locked to its shaft.

Figure 62 shows the principle of the sliding gear type. One part of the shaft driven by the engine is square, and fits into square holes in its gears, which may thus slide along it, but must revolve with it. Each sliding gear is moved by a shifter block, which is operated by a shift lever. There is a shifter block for each gear, and the shift lever may be moved sideways to operate either one of them.

Figure 63 shows the jaw clutch type of change speed gear, in which the gears are in mesh all of the time, but run loose on their shaft when they are not working. The drawing shows bevel gears, which are used when the driving and driven shafts are at a right angle. The same principle is used for spur gears on shafts that are parallel, as in Figure 62.

The center of the shaft is square, and fits a block that can slide endways, but that must revolve with it. The ends of the block have heavy teeth that can mesh with teeth on the hubs of the loose gears; meshing the block with one of the gears forces that gear to revolve with the shaft.

The drawing shows only one speed forward; the reverse is obtained by a second gear on the same shaft, which is placed on the opposite side of the center of the driven gear, and turns it in the opposite direction.

When a tractor turns, the outside wheel makes a larger circle than the inside wheel, and has a longer path to travel. Both wheels travel their paths in the same time, so it follows that the outside wheel must move faster than the inside wheel, although both are being driven by the engine. This is allowed for by the differential, which is driven by the change speed gear, and which in turn drives the wheels; it operates automatically by the difference in the resistance to the rolling of the wheels.

The action of the differential is illustrated by an experiment that requires a pair of wheels on an axle, like buggy wheels, and a stick long enough to reach from one to the other. With the wheels on smooth ground, put the ends of the stick through the wheels at the top, each end pressing against a spoke. Hold the stick at its center and push it forward; the stick will transmit the pressure to the spokes, and the wheels will turn. The wheels being on smooth ground, there is equal resistance to their movement, and they will run straight forward.

Now repeat the experiment with the wheels so placed that one is on a smooth roadway and the other on sand; as the wheel on the smooth surface meets with less resistance than the other does, it moves faster, and the pair of wheels circles, although the stick applies equal pressure to both.

The power developed by the engine is transmitted by the differential to both rear wheels; when the wheels meet with equal resistance, they turn equally, but when one wheel meets with greater resistance than the other, it slows down, while the other speeds up to correspond.

A tractor with two driving wheels must use a differential in order to make turns easily. Without a differential, the wheels would run always at equal speed, and in making a turn one would be obliged to slip.

The use of a differential has a disadvantage, however. If one wheel is in a mudhole and the other is on hard ground, the wheel in the mud meets with little resistance, and all of the power of the engine goes to it; it spins without moving the tractor, while the other wheel remains stationary. In such a case all of the power should be applied to the wheel that has traction in order to move the tractor, but this the differential fails to allow.

In some tractors the differential is so made that the parts may be locked together. This lock is used when one wheel is in a mud hole, and as by its use power is transmitted equally to both wheels, the tractor moves.

Great care must be taken to unlock the differential as soon as the need for the lock has passed, for otherwise the wheels would slip on a turn, and the parts of the transmission might be strained or broken.

A differential is usually made with two bevel gears placed face to face; between them is a frame holding three or more small bevel gears that are in mesh with them both. The engine revolves the frame with its small gears; each of the large bevel gears revolves a driving wheel.

When the tractor moves straight ahead the differential turns as if it were one solid piece. When there is less resistance to one driving wheel than to the other, the small bevel gears, in addition to revolving with the frame that carries them, turn on their shafts. This transmits the power of the engine to one wheel more than the other, according to the resistance of the wheels.

Figure 64 shows one of the large bevel gears of a differential, with the three small gears, the other large bevel gear being removed. A differential in section is shown in Figure 65.

A tractor with only one driving wheel has no differential. Such tractors usually have two wheels, but one of them runs loose on the axle, and serves only to support the tractor. The rear axle construction of a tractor with a 1-wheel drive is shown in Figure 66, which should be compared with the 2-wheel rear construction shown in Figure 65.

There are a number of methods used for transmitting power to the driving wheels. In Figure 64 a chain is used; there are tractors with but one chain, and others with a chain for each driving wheel.

The most usual method is by a master gear, or bull gear, which is a large and heavy gear attached to the driving wheel, as shown in Figures 65 and 66. In some tractors this gear is nearly the size of the wheel, and is fully exposed; in others it is smaller, and enclosed in an oil-tight housing.

The small gears that drive the bull gears are on the ends of the cross shaft, called the jack shaft, that carries the differential.

In the Fordson tractor the differential is built into the axle, as it is in an automobile, and power is applied by a worm. The worm is driven by the change speed gear, and is a screw meshing with a gear on the differential, whose teeth are cut at the proper angle to make them fit the threads of the worm. A worm, which is shown in Figure 67, is always enclosed, and runs in oil.