Motor-car principles; the gasoline automobile

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.

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.

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.

FINAL DRIVE

From the change-speed mechanism the power is passed to the driving wheels by the final drive.

In the most usual construction the engine is so placed that the crank shaft is at right angles to the axle, and it is therefore necessary to change the direction in which the power acts, which is done by means of bevel gears. In ordinary spur gears the teeth are parallel to the shaft, and the two shafts that carry them are parallel, while in bevel gears the teeth are at an angle, and the shafts may be at right angles to each other. In Fig. 37 the diagram of the single-chain drive illustrates a car in which the engine is in the center of the frame, and as the crank shaft is parallel to the axle, the power may be directly applied. In the illustration of the propeller or driving-shaft drive the crank shaft is at right angles to the axle, and the power is turned by means of the bevel gears at the rear axle.

The single-chain drive can only be used for light cars, and is usually applied in connection with a change-speed mechanism of the planetary type.

The propeller-shaft drive requires the use of universal joints, which are devices that permit one shaft to drive another, even though they are at an angle with each other. A typical universal joint is illustrated in Fig. 38. The ends of the shafts bear yokes, the ends of which are pivoted to a block of metal of + shape. When the two shafts are in line, the joint will force one to rotate with the other, and this will not be prevented if the two are out of line, for then the pivots will act, the + swinging on its pivots in the yokes.

The change-speed mechanism is carried on the frame of the car, and is therefore supported by the springs, but the axle end of the driving shaft follows the axle as that follows the inequalities of the road. One end of the propeller shaft is therefore comparatively stationary, while the other is in constant motion, and if the shaft were inflexible it would be jammed in its bearings and twisted out of line. This is prevented by the universal joints with which the shaft is provided, there being one and often two in the shaft, and usually one between the clutch and change-speed mechanism.

The single-chain and driving-shaft drives require the use of a live axle, which is an axle that revolves with the wheels. The simple type of live axle consists of the shaft to which the wheels are attached, and the housing that contains and supports it (Fig. 39). This axle is continuous, and usually has square ends that fit into the square hubs of the wheels so there may be no slipping. The second diagram in Fig. 40 shows a live axle of the floating type, in which the revolving part serves only to turn the wheels. The housing is extended, and the wheels run on its ends, the driving part projecting beyond the housing and having square ends that are secured to the outside of the hubs by square caps. The wheels thus run on the housing, which takes the weight of the car from the driving part. A live axle must be divided into two parts in order that a differential gear may be fitted, and the housing must therefore be strong enough to support the weight and prevent sagging. The efficiency of a bevel gear is greatly reduced if the teeth are not in their exact mesh, and sagging of the axle will throw them out to such an extent that they will be noisy, and wear rapidly. The floating type of live axle, in relieving the driving part of the weight, has a great advantage over the simple type, and is in general use.

With the driving-shaft drive it is necessary to use a torsion rod, which extends from the gear case, or a crosspiece of the frame, to the rear axle. The necessity for this is the tendency of the driving bevel gear to roll around on the driven bevel gear rather than to revolve it. If it were not for the torsion rod, there would be a continual strain on the parts because of the tendency of the axle housing to revolve around the axle, instead of the axle being revolved inside of the housing. The torsion rod has a flexible joint at one end, that permits it to give as the axle follows an uneven road surface, but it retains the housing in the correct position, preventing the bevel gears from getting out of line (Fig. 41).

In Fig. 42 is shown a car with double-chain drive, in which the bevel gears that change the direction in which the power is applied are contained within the gear case that incloses the change-speed mechanism. As will be seen from Fig. 34, the bevel gears connect the square shaft with the jack shaft, which is a shaft passing across the car, and bearing on its ends the sprockets by which the wheels are driven. This type of drive requires the use of a dead axle, which is stationary with the wheels running loose on its ends, like the axle and wheels of a coach. An axle of this type may have great strength with light weight, and is usually a manganese-bronze or steel forging. The sprockets on the rear wheels are bolted to the spokes, and should be, of course, exactly in line with the sprockets on the jack shaft in order that the chains may run true.

DIFFERENTIAL

When a car takes a corner, the outside wheels make a larger curve than the inside, and cover a longer distance. As the front wheels are loose on the axle, they accommodate themselves to this; but as both rear wheels are driven by the engine, it is necessary to apply a device that will permit them to rotate at different speeds without interfering with their driving the car. This is accomplished by means of a compensating and differential gear.

To understand the necessity for a differential, stand behind a wagon, with one hand on each tire; push, and if the vehicle is steered straight ahead, the hands will move ahead equally; but if the vehicle turns, the hand on the outside wheel will move ahead faster than the other. Now take a stick, and run it through the rear wheels so that it bears against the spokes; press it forward from its center, and if the vehicle moves straight ahead, the stick will go forward equally; but if the vehicle turns, the outside end of the stick will go ahead faster and farther than the other, although the pressure is being applied to its center.

In applying a simple form of differential the axle is divided into two parts, to the inner ends of which are fitted bevel gears, these being held at a fixed distance apart by the construction of the housing (Fig. 43). Between the bevel gears and in mesh with both of them are small bevel gears, or pinions, which may revolve on short studs carried on a ring so that they are a fixed distance apart. When the ring is revolved it carries with it the studs and pinions. The ring forms a housing that incloses the differential, and is driven by the single chain or driving shaft. To understand the action of the differential, imagine the rear wheels of a car to be jacked up clear of the ground so that they are free to revolve, and the housing to be revolved by hand. As it turns, the driving wheels will turn also, for the resistance to each is the same, and the pinions, being in mesh with both bevel gears, cannot revolve on their studs. If one of the wheels is now held stationary and the housing revolved, the bevel pinions will revolve on their studs, and roll around on the stationary gear; this will drive the gear of the free wheel at twice the speed of the housing. The revolving of the housing in the first instance caused the wheels to turn equally at the speed of the ring, and in the second permitted one to remain stationary while the other turned at twice the speed of the housing, the speed of the latter being unchanged. The first is the effect when the car moves straight ahead, and the second the result if the car could make so short a turn that it would pivot on one wheel.

With the wheels jacked up, hold one hand lightly against one of the wheels, so that while it may turn, there is more resistance to it than to the other. If the housing is now revolved, the bevel pinions will revolve on their studs, and roll slowly around the gear of the wheel that presents the resistance, the free wheel being revolved at a higher speed than the housing. This is the condition when the car takes a corner, for there is then more resistance to the inside than to the outside wheel, and it slows down; this will start the pinions revolving on their studs, and they will drive the outside wheel correspondingly faster.

With the housing revolving at a fixed speed, the outside wheel will revolve as much faster as the inner wheel is revolving slower; for an illustration, if the housing makes fifty revolutions a minute, and the inner wheel is slowed to forty, the outer will be driven at sixty revolutions.

If one wheel of a jacked-up car is revolved by hand, the other wheel will revolve in the opposite direction. This is caused by the housing remaining stationary and the pinions being revolved on their studs by the turning of the wheel, the movement being transmitted to the free bevel gear and wheel in the reverse direction.

The bevel gear differential described was the early type, but a more recent design employs spur gears. The axle ends carry spur instead of bevel gears, these being in mesh with other spur gears that are long, but of small diameter. These small gears are in pairs, as shown in Fig. 43, being in mesh with each other at their inner ends, and each member of a pair meshing with one of the axle gears. The small gears revolve on studs supported by the housing that is revolved by the drive, the studs in this case being parallel with the axle instead of at right angles to it, as are the studs in the bevel-gear type.

If the small gears meshed only with the axle gears, and not with each other, revolving the housing would cause them to roll around the axle gears, all rotating on their studs in the same direction, and the axle gears remaining stationary. Being in mesh with each other, they cannot revolve in the same direction, for when two gears are in mesh they must revolve in opposite directions. Thus the small gears cannot roll around on the axle gears when the housing is revolved, and if there is equal resistance to the turning of the wheels, the small gears will not revolve on their studs, but will carry the axle gears with them.

If the car is turning a corner, the greater resistance to the inner wheel will cause the small gears to revolve on their studs, rolling around the resisting gear and driving the other correspondingly faster.

On cars with double-chain drive, the differential is fitted to the jack shaft, and of course receives the drive from the change-speed mechanism through its housing.

Both the driving-shaft and double-chain drive have points of advantage and of weakness, and each type has its advocates. For the double chain, great strength can be claimed with light weight, as the axle is in one piece, and perfectly adapted to support the car. Against it is the difficulty of keeping the chains properly lubricated, and their consequent wear and stretching. The driving-shaft type has the advantage of the perfect lubrication of the parts, for all may be inclosed and running in oil or grease; the rear axle must be divided, however, which requires it to be heavily braced in order that the weight imposed on it may not bend or spring it out of line. Where a bent dead axle can be straightened by a blacksmith, a similar condition in a live axle requires the services of an expert mechanic; on the other hand, bevel gears make less noise than chains.

DRIVING-GEAR RATIOS

Even when on the direct drive, the crank shaft makes more revolutions than the rear wheels, in order that the momentum of the moving parts of the engine may be sufficient to keep the car in motion. On shaft-driven cars, the bevel on the driving shaft has fewer teeth than that on the axle, so that it revolves more than once to one revolution of the axle. On chain-driven cars, the driving sprockets are smaller than those driven. This driving-gear ratio, as it is called, varies from one and a half to three and a half, or, in other words, the wheels revolve once while the driving shaft or sprocket makes from one and a half to three and a half revolutions. Other conditions remaining equal, a higher driving gear gives the car lower speed, but greater ability in hill-climbing and the traversing of heavy roads.