The transmission of an automobile consists of those parts that transmit to the driving wheels the power developed by the engine.

THE CLUTCH

Because a gasoline engine must be in operation before it can deliver power, a clutch is provided by means of which it may run free or be so connected that it drives the car, one of its two chief parts being attached to the engine and the other to the transmission. When the two parts are in contact, the transmission is driven, and when separated, the engine and transmission are independent of each other, and may be stationary or in motion. A clutch must be of such a nature that it does not apply the power of the engine instantly, but gradually, so that the car starts slowly and without jerking. If the power were to be applied suddenly, the effort of starting the stationary car would either overcome the momentum of the engine and stop it, or would jerk the car into such sudden motion that it might be badly wrenched. By making the clutch so that it is permitted to slip when first applied, the part that is driven is gradually brought to the speed of the part that drives, when slipping ceases and the two make firm contact.

The most usual form of clutch is the friction cone, in which the fly wheel of the engine is utilized as the driving part, the rim being broad and thick, with its inner side funnel-shaped, or beveled. A metal cone that fits the bevel is carried on the end of a shaft of the transmission, the shaft at this point being square, to fit a square hole in the hub of the cone. This arrangement permits the cone to slide along the shaft while always revolving with it. When the cone is pressed to a seat in the fly wheel, which is accomplished by means of a heavy coil spring, the friction between its leather-covered surface and the surface of the fly wheel causes the two parts to revolve together, and by its fit on the square shaft the transmission is set in motion, and through that the car. The clutch is thrown out of contact by means of a foot pedal that acts on a ring fitting in a groove around the hub of the cone; when the pedal is released, the spring forces the cone to its seat (Fig. 31). In order to support it, the end of the shaft carrying the cone projects into the hub of the fly wheel, where it rests in a bearing, this arrangement in no manner preventing the two parts from acting independently of each other.

In the reversed type of friction-cone clutch a funnel-shaped ring is bolted to the rim of the fly wheel and forms the seat, the cone fitting inside of it (Fig. 31). Depressing the pedal moves the cone toward the fly wheel instead of away from it, as in the regular type, and while there is no difference in the effect of one as against the other, the reversed type is more compact.

The multiple-disk clutch, which is rapidly coming into use, depends on the friction between the flat surfaces of metal when pressed together. An experiment illustrating this is to place a silver dollar between two half dollars, and to press them together between the thumb and finger. It will be found that a light pressure is sufficient to produce friction that will make it difficult to revolve the large coin between the smaller ones.

The parts of a simple form of multiple-disk clutch, as shown in Fig. 32, are a flange on the engine shaft, a smaller flange with a square-hole, square-shaft arrangement on the transmission shaft, and large and small rings placed alternately. The large rings are driven by the large flange, fitting loosely on pins, or studs, projecting from it, and the small rings are similarly attached to the transmission shaft. The openings in the large or driving rings are large enough to contain the studs carrying the small rings, so that when the parts are assembled the outer surfaces of the small or driven rings are in contact with the inner surfaces of the driving rings. A heavy coil spring is arranged to press the small flange toward the flange on the engine shaft, binding the rings that it carries between the driving rings, and the latter are often faced with leather to increase the friction between them. When the small flange is released from the pressure of the spring by depressing the pedal, the driving and driven flanges with their rings are independent of each other, and the engine may run free while the transmission shaft is stationary or revolving. A clutch of this type is incased and runs in oil, which prevents the rings from gripping suddenly; when the pressure of the spring is applied, the oil is gradually squeezed out from between them, and the slipping of the driving and driven rings is reduced as they are forced into contact.

An internal-expanding clutch consists of a broad ring, or drum, against the inner surface of which bear two pieces of metal shaped to fit. The pieces of metal, or shoes, are pivoted together at one end so that they may be moved in or out, after the manner of the handles of a pair of scissors; when open they bear against the inside surface of the drum, and when closed they are free from it. The drum is attached to the engine shaft and the shoes to the transmission shaft, the friction between them being so great that the transmission shaft is carried around as the drum revolves. The shoes are kept in contact with the drum by a coil spring, the depression of a pedal releasing them from its pressure.

CHANGE-SPEED MECHANISM

The change-speed mechanism, to which the clutch transmits the power of the engine, is to the engine what a block and tackle is to a man who lifts a heavy weight, and is necessary because of the varying resistance to the movement of the car in traversing steep, rough hills and smooth avenues. A change-speed mechanism may be defined as an arrangement by which the relative number of revolutions of the crank shaft and driving wheels may be altered to suit conditions. If the driving wheels revolve but once while the crank shaft makes twelve revolutions, the car will move at one sixth the speed that it would have if the wheels revolved once to every two revolutions of the crank shaft, but it will have six times the ability to overcome the resistance presented by a hill or sandy road.

If a gasoline engine were so connected that the relative number of revolutions of the crank shaft and wheels could not be changed, a slowing down of the car through the resistance presented by a rough hill would slow the engine to correspond, and as speed is an important factor in the power that the engine delivers, it would be prevented from doing the work of which it is capable at the time when it was most necessary. By means of the change-speed mechanisms in most general use, the relative number of revolutions of the crank shaft to one of the wheels may be varied to from two to eighteen, the former giving the car high speed over a smooth road, and the latter slow speed, but greater ability to overcome hills and heavy roads.

To attain this result, gears are used. If two gears having the same number of teeth are in mesh, they will make the same number of revolutions, and the force with which the driven gear will revolve will be the same as that of the driving gear, less the friction of the teeth. If the driven gear has twice the number of teeth of the driving gear, it will revolve at half the speed, but with twice the force.

The forms of change-speed mechanism most largely used are based on this principle, which is so applied that the gear driven by the engine may be in mesh with a gear that has many more teeth and revolves much slower in consequence, or a gear that has possibly one and a half times the number of teeth, or a gear that has the same number of teeth, and therefore revolves at the same speed. The sliding-gear mechanism takes its name from the arrangement by which the changes in the combination of gears is effected by sliding them along a shaft, to mesh with other gears on a shaft driven by the engine.

The driver changes the gears by moving a lever that in the progressive type moves forward by degrees to move the car on the slow speed, the intermediate speeds, and the high. A typical arrangement of the progressive type of sliding change-speed mechanism is shown in Fig. 33. The power of the engine is transmitted to a short, hollow shaft, called a sleeve (A), which carries a gear (B) that is in permanent mesh with a gear (C) on the end of the countershaft. Parallel to the countershaft is another shaft, one end of which is held in a bearing in the hollow sleeve; while the sleeve supports this shaft, the two may revolve independently of each other. The second shaft is square, or of such a construction that while the two gears that it carries may slide along, they revolve with it. The gears on the square shaft are of different sizes, and in sliding on it come successively into mesh with gears carried on the countershaft. Because of the gears between them, the countershaft revolves when the engine revolves the sleeve; but the speed of the square shaft depends on the combination of gears in mesh between it and the countershaft. When the sliding gears are in such a position that they are not in mesh with the countershaft gears, the square shaft is independent of the countershaft, and may revolve or be stationary, the gears then being in the neutral position. When the sliding part is moved so that its largest gear is in mesh with the smallest of the countershaft gears (D), the square shaft will revolve at a slower speed than the countershaft, because its gear is larger than the one driving it. Again sliding the moving part will separate these gears, and bring the next pair (E) into mesh, the square shaft then moving at a higher speed, but still slower than the countershaft because of the difference in the size of the gears. Sliding the moving part still farther along the shaft will disengage the second-speed gears and engage the high speed, in which the square shaft revolves at the speed of the sleeve and crank shaft, this being effected by locking the moving part to the sleeve by means of a clutch (G). This clutch consists of several fingers projecting from the moving part, corresponding to the spaces between similar fingers on the end of the sleeve. The locking together of the square shaft and sleeve gives what is known as the direct drive, which is of comparatively recent development; many designs of sliding gears still use a third pair of gears which, being of the same size, give the square shaft the speed of the crank shaft. By the use of the direct drive, the power of the engine is directly applied to the square shaft, avoiding the loss that occurs through the friction of the teeth of the gears.

The revolution of the square shaft is transmitted to the driving wheels, the speed of the car therefore corresponding to the speed at which the square shaft is driven by the gear combinations between it and the countershaft. To obtain the reverse, which enables the car to be backed without reversing the engine, a third gear is introduced between the low-speed gears of the square shaft and countershaft. When two gears are in mesh, they revolve in opposite directions, but when one of them is in addition meshed with a third gear, the first and third will revolve in the same direction, and opposite to the direction in which the middle gear revolves. When the car is going forward, the square shaft and countershaft revolve in opposite directions, but when the reverse gear is introduced between them, the square shaft is revolved in the same direction as the countershaft, reversing the rotation of the driving wheels.

The ends of the teeth of the gears are chisel-shaped, instead of being flat, as in ordinary gears, so that they will go into mesh easily.

The greatest economy in the operation of a gasoline engine results from its running at as nearly constant a speed as possible, and the gear is therefore changed when the resistance of the road to the movement of the car decreases the speed of the engine, or permits it to increase.

The selective type of sliding change-speed mechanism as shown on Fig. 34 is in use on many of the high-grade cars, and its control differs from that of the progressive type described in that the lever has only a short movement forward and back, and in addition may slide sideways. To the lower end of the lever is attached a shaft that rocks in its bearings as the lever is moved forward or back, and slides lengthways when the lever is moved toward or away from the car.

The arrangement of the countershaft and square shaft is the same as in the progressive type, but there are two sets of sliding gears instead of one, and these are moved by means of arms that extend from rods sliding in bearings at the side of the gear case. When these rods are slid endways, the gears attached to their arms slide on the square shaft to correspond, and go in or out of mesh with the countershaft gears. Across the ends of these rods are grooves, which when the gears are in the neutral position are in line with the rocking shaft attached to the control lever. From the inward end of the rocking shaft a finger (L) projects downward into the groove; when the grooves are in line, the rocking shaft may be slid endways, the finger passing from one groove to the next without affecting the rods. When the shaft is rocked, however, the finger in engaging one of the grooves slides the rod endways, shifting the gears controlled by its arm. Moving the control lever into such a position that it may enter the middle slots of the guide plate (M) slides the rocking shaft so that its finger projects into the groove of the central sliding rod (J), and if the control lever is then pushed forward so that it enters the front half of the slot, the sliding rod will be moved by the finger in the opposite direction, and the low-speed gears (D) will be brought into mesh. Bringing the lever back to the central position will separate the gears, and moving it to the back half of the slot will slide the same gears in the opposite direction, meshing the second-speed combination (E). Moving the control lever outward so that it is in line with the outside slot brings the finger into the groove of the sliding rod (H) that moves the third and high speeds (F and G), the latter being direct drive, and the reverse is obtained through the movement of the sliding rod (K) that is engaged by the finger when the control lever is in the inside slot. The movement of this rod brings a third gear (O) into mesh with the low-speed gears on the square shaft and countershaft, and the rotation of the countershaft is reversed.

While this type is in general use, the gears are often so arranged that the direct drive combination is reached when the control lever is in the third-speed position, the gears meshed by the fourth-speed position driving the square shaft at a still higher speed. This high speed can only be used for running under the best road conditions.

The advantages of the selective type over the progressive are the shorter movements of the control lever and the ability to pass from one speed to any other without the necessity of first meshing and unmeshing those between, or, in other words, there is a neutral position between every combination of gears, and from neutral any desired combination may be obtained directly without reference to the others.

In starting up a car fitted with either of these types of change-speed mechanism, it is necessary to withdraw the clutch before sliding the gears, and this is also necessary in changing from one combination to another. The square or corresponding shaft of a change-speed mechanism is always connected with the driving wheels, and is at rest when the car is standing. With the engine running, the countershaft will be revolving, and it will obviously be difficult to slide the stationary gear into mesh with a gear that is revolving. When the clutch is withdrawn, the countershaft moves only through momentum, and will be brought to a stop by the contact of the teeth of the sliding gear as that is moved against it, the two then easily going into mesh. The clutch is then thrown in slowly, and will bring the speed of the countershaft to the speed of the crank shaft.

When the car is moving, sliding the gears without first withdrawing the clutch will bring together two gears that are revolving at different speeds, and as it is necessary for them to be rotating equally in order that they may mesh, either the speed of the car must be changed to bring the speed of the gear on the square shaft to that of the countershaft gear, or the speed of the engine must be changed to bring the countershaft gear to the speed of the gear on the square shaft. If the change is from a low to a higher speed, the countershaft will be moving much faster than the square shaft, and their gears being brought into contact will result in the slowing of one and speeding up of the other until the speeds are the same, but in so doing the ends of the teeth will grind against each other, resulting in the wear of the chisel-pointed ends, if not in the breaking of the teeth. Withdrawing the clutch obviates this difficulty, for it frees the countershaft, permitting its gear to take the speed of the square-shaft gear without wear or damage, and when the change is made, the slow engagement of the clutch brings the speed of both to that required by the crank shaft.