Motor-car principles; the gasoline automobile

by Roger B. Whitman

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.

Fig. 33.—Sliding Gear—Progressive Type. A, Sleeve driven by engine; B, gear on sleeve; C, gear on countershaft; D, low-speed gears; E, second-speed gears; F, idler for reverse; G, clutch for high speed; H, connected to rear wheels; J, gears sliding on square shaft.

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.

Fig. 34.—Selective Type. A, Sleeve driven by engine; B, fixed gear on sleeve; C, fixed gear on countershaft; D, low-speed gears; E, second-speed gears; F, third-speed gears; G, clutch for direct drive; H, rod and arm for third-speed and direct drive; J, rod and arm for low and second speeds; K, rod and arm for reverse, L, rocking-shaft finger in groove; M, guide plate and control lever; N, bevel gears on square shaft and jack shaft; O, idler gear for reverse.

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.

CHAPTER IX
TRANSMISSION—(Continued)

While the planetary type of change-speed mechanism, which is in extensive use for runabouts and light commercial wagons, also employs gears, their arrangement is along different lines. The first three diagrams in Fig. 35 serve to illustrate the principle.

Fig. 35.—Planetary Type.

The gear A in these diagrams is attached directly to the crank shaft, and in mesh with it are four other gears (B) of the same size. Surrounding them is an internal gear (C), this being a ring with teeth cut on its inner face, the four gears meshing with it. The shafts, or studs, on which the four gears revolve are supported by a metal ring (D), which maintains the gears at equal distances from each other. The first diagram shows the mechanism in the reverse position, for driving the car backward, the car being driven by the internal gear. To have the internal gear revolve in the direction opposite to that of the crank shaft, as is necessary, the ring supporting the four gears is held stationary, with the result that as the crank-shaft gear revolves the four gears are revolved on their studs. As these gears are in mesh with the internal gear, that is revolved, and moves in the same direction as the four gears and in the opposite direction to the crank shaft.

For the low-speed forward, the ring is released and the internal gear held stationary, the car now being driven by the ring instead of by the internal gear. If the four gears were free from the internal gear, they and their ring would revolve with the crank-shaft gear without rotating on their studs, but being in mesh with the internal gear, they roll around it as a wheel rolls along the ground, rotating on their studs. A simple experiment that will illustrate this motion is to crook the forefinger around a napkin ring or similar object, placing a pencil between it and the finger, and revolving the ring with the other hand. The finger being stationary, the pencil, which is revolved in the opposite direction to the ring, will roll along it. In this the napkin ring represents the crank-shaft gear, the pencil one of the four gears, and the finger the internal gear. As the four gears roll around, the ring moves also, for it is carried by the studs on which the four gears revolve. If each of the four gears has fifty teeth, and the internal gear two hundred teeth, each gear must make four revolutions in order to roll around the internal gear to the point where it started. The crank-shaft gear also having fifty teeth, it revolves at the same speed, and as four revolutions of the four gears are necessary in order that they may roll completely around the internal gear, the crank-shaft gear will make four revolutions in the same time. The ring moves with the four gears, and revolves once around the crank shaft in the same time. As the car moves according to the rotation of this ring, it will go at one quarter the speed that it would make if the wheels were directly connected with the crank shaft instead of with the ring.

For the high speed, the internal gear and the ring are locked to the crank shaft so that all revolve together, the wheels being driven by either the ring or the internal gear.

In these diagrams the drive of the wheels is supposed to be shifted from the internal gear to the ring, which is not a practical arrangement, and the planetary change-speed mechanism as applied to an automobile is shown in the lower diagram in Fig. 35.

In this there are two sets of crank-shaft gears, gears and rings, and internal gears, one set being for the reverse and the other for low and high speeds. Between the two crank-shaft gears is a loose sleeve, one end of which forms the internal gear for the reverse, and the other end the ring supporting the studs on which revolve the four gears for the low speed. The sprocket for the chain drive to the rear axle is carried on this sleeve. Two more loose sleeves are on the shaft, one forming the ring on which revolve the four gears for the reverse, and being extended to form a brake drum outside of the internal gear, and the other carrying the internal gear for the low-speed combination, its outside face serving as a brake drum.

To obtain the reverse, a brake band is tightened on the drum of the reverse combination, which holds stationary the ring supporting the four gears, giving the result shown on the first diagram of the four gears revolving on their studs, and rotating the internal gear in the direction opposite to that of the crank shaft. The sleeve bearing the sprocket is thus revolved, and the car backs.

For the low speed, the reverse brake band is loosened, and the internal gear of the low-speed combination held stationary by the tightening of the brake band surrounding its drum. The revolution of the crank-shaft gear causes the four gears to revolve on their studs and to roll around the internal gear, revolving the ring and the sleeve bearing the sprocket, which now turns in the direction opposite to that resulting to the application of the reverse, or in the same direction as the crank shaft.

For the high speed, a clutch is engaged that locks the internal gear to the crank shaft, and the four gears then being held between these two are carried around with them, and the sprocket rotates accordingly. When this combination is used, none of the gears are in motion, all revolving with the crank shaft but not on their studs.

The planetary change-speed mechanism gives excellent results for light work, but having only two speeds forward is not adapted to high-powered cars. As the speeds result from the tightening of brake bands on the drums, there is no danger of damaging the gears by mishandling, for the brakes will slip before the teeth will give way. The brakes, which are leather-lined strips of steel, require attention from the wearing of the leather, and the slipping that results from oil working in between them and their drums. No foot clutch is necessary, for the tightening and loosening of the brake bands is controlled by a lever; in some designs, the reverse is applied by means of a foot pedal, and this may be used in braking the car.

Fig. 36.—Individual Clutch and Friction Drive.

The individual-clutch type of change-speed mechanism consists of two shafts, one being an extension of the crank shaft, and the other parallel to it (Fig. 36). On the crank shaft is a sleeve bearing the sprocket and a gear, this sleeve being so arranged that it may revolve loosely, or be locked to the crank shaft and made to revolve with it by a clutch. The crank shaft in addition bears two fixed gears, one being for the low speed and the other for the reverse. On the countershaft is a fixed gear in mesh with the gear carried on the sleeve on the crank shaft, and two loose sleeves bearing gears that are in mesh with the fixed low-speed and reverse gears on the crank shaft. These sleeves are provided with clutches by which they may be locked to the countershaft to revolve with it, or disconnected from it. When the three clutches are disengaged, the crank shaft in revolving carries with it the fixed low-speed and reverse gears, the sleeve bearing the sprocket and gear remaining stationary. The sleeves on the countershaft revolve because their gears are in mesh with the fixed crank-shaft gears, but the countershaft remains stationary. In engaging the low speed, the clutch is thrown in, forcing the countershaft to revolve with the fixed gear, the fixed gear on the countershaft then revolving the gear and driving sprocket carried on the sleeve on the crank shaft. Because of the difference in the size of the gears, the countershaft will revolve at a slower speed than the crank shaft, and the driving sprocket will make but one revolution while the crank shaft makes several. This it is free to do, for the sleeve carrying the sprocket is in no way connected with the crank shaft. For the high speed, the low-speed clutch is withdrawn, and the driving-sprocket clutch engaged, causing the sprocket to revolve with the crank shaft.

The reverse is caused by the introduction of an idler gear between the gears of the crank shaft and countershaft, by which the movement of the latter is reversed.

The application of the friction type of change-speed mechanism to automobiles is recent, and is giving good results for light work. It consists in its simplest form of a heavy disk carried on the engine shaft, on the face of which runs a wheel sliding on a square shaft, so that the two may be in contact at any point from the edge to the center of the disk (Fig. 36). When the wheel is at the center of the disk it is not moved, and the number of speeds at which the square shaft may be driven in relation to that of the disk varies from nothing to the limit, which is obtained when the wheel is in contact with the outer edge of the disk. For the reverse, the wheel is moved across the center of the disk, where it is revolved in the opposite direction.

Another form of friction drive provides two driving disks, the wheel bearing against both, so that its movement is more positive, and there is less chance for slipping.