Transmission came into use with the application or adoption of the internal combustion engine as a factor in motor car propulsion.

As this type of engine develops its power by a rapid succession of explosions in the combustion chambers, each explosion delivers an impulse or power stroke to the piston, which in turn sets the crank shaft and flywheel to revolving. The momentum gathered by the crank shaft and flywheel may therefore be termed the power for duty, or in other words, unless there is momentum or carrying motion at this point, there will be little or no power for duty.

This brings us up to a point where it is easy to see that a rapid series of explosions are necessary to gain carrying momentum or power to move a dead weight load. As this motional power could not be applied to the load without serious damage to the gears and bearings, it was necessary to invent a device to gradually transmit or apply the power to the movable load by graduating the leverage. This resulted in the development of the automobile transmission. The natural way of doing this at first seemed to be by applying the power to the load by frictional slippage. Many ingenious devices of this sort were tried out without much success until the driving and driven disc type made its appearance.

Fig. 109 shows the driving and driven disc type of friction transmission. This type of transmission is not being used by any of the present day manufacturers of automobiles, but may still be found on some of the three and four-year-old models still in operation.

A, the drive shaft, is squared and slides backward a distance of three inches through a squared sleeve extending from the hub of the flywheel. The action of this shaft is controlled by a leverage arrangement to a foot pedal. B, the steel plate driving disc, is attached to the end of shaft A, and drives C, when held back against it by pressure on the foot pedal. Disc C can be slid in any position on the jack or cross shaft D, and is controlled by a leverage arrangement connected to a hand lever. The various speeds are obtained by sliding disc C into different positions and contacts on the left side of disc B. Reverse speeds are obtained by sliding disc C over center where it forms contact on the right side of B and is driven in an opposite direction.

The Planetary Type of Transmission.—The planetary type of transmission made its appearance along about the same time as the friction type. The power is transmitted to the load through a set of reduction gears arranged in a drum. A king gear on the engine shaft operates a set of small gears in the drum. These small gears reduce the leverage speed and transmit the power to the drive shaft, a band similar to that used on brakes is fitted to the face of the drum. When this drum containing the reduction gears is not in use it turns at crank shaft speed. The speed is used by pressing a foot pedal which tightens the brake band and holds the drum stationary, thereby forcing the smaller gears into action.

Planetary transmissions are shown and fully explained in a later chapter. (See Model T Ford Supplement.)

The Sliding Gear Transmission.—This type of transmission has proved very successful, and is used by 98 per cent of the present day automobile manufacturers. This type of transmission made its first appearance with a progressive gear shift, that is, it was necessary to proceed through one speed or set of gears to engage the next. This arrangement caused considerable confusion at times, as it was necessary to reshift the gears back through these speeds to attain neutral, when the car was brought to a stand still.

The control lever operated on a straight forward and backward direction on a quadrant, having a notch for each speed change. This gear shifting arrangement has also been abandoned by manufacturers in favor of the selective gear shift which is arranged so that the driver may choose any speed at will. Fig. 110 shows the control lever which operates in a frame resembling the block letter H and the ball and socket shift which operates in the same manner. Fig. 111 shows the complete assembly of the selective sliding gear transmission. The sliding gears are arranged on a separate core and are operated by an individual throw fork, which seats in a groove on the shoulder of the gear. Low and reverse are always opposite each other on the same core. High and intermediate are located on another core, and are controlled by another individual shifting fork. The gear box arrangement (Fig. A) shows the cast gear box which contains the gears, shafts, and bearings, and a roomy compartment below the gears in which grease is carried, as the gears in this type of transmission always operate in an oil bath which prevents excessive wear and causes them to operate noiselessly. Fig. B, the gear case cover, contains the slotted sliding shafts, to which the gear in shifting forks are attached. Fig. C shows the arrangement of the gears in the case and explains their operation. Gear No. 1 is attached to the extreme end of the engine shaft, and is continually engaged with gear No. 4, which causes the counter shaft No. 11, containing the stationery gears, to revolve whenever the engine shaft No. 9 is in operation. The drive shaft No. 8 does not run straight through and connect with No. 9, the engine shaft, but ends and takes its bearing in the core of gear No. 1. Consequently, when the gears on the drive shaft are slid into mesh with the gears on the counter shaft, variable speeds are attained. Low speed is obtained by sliding gear No. 3 into mesh with gear No. 6; second or intermediate is obtained by meshing gears No. 2 and gear No. 5.

High, or engine speed, is obtained by sliding gear No. 2 which is cored and shouldered over the end of gear No. 1, making a direct connection of the drive shaft No. 8, and the engine shaft No. 9, at this point. Reverse is obtained by meshing gear No. 3 on the drive shaft with gear No. 10, which is an extra or idle gear mounted on a stub shaft on the rear of the gear case. Idle gear No. 10 is always in mesh with gear No. 7, on the counter shaft.

Functional operation engine shafts always turn to the right or clockwise, which causes the counter shaft to turn to the left or anti-clockwise. This causes the drive shaft to turn to the right when low or intermediate speed gears are engaged, driving the car forward. Reverse, is obtained by the use of an extra gear in this way. Counter shaft turning to the left turns idle gear to the right, and this gear turning to the right, turns gear on the drive shaft to the left, and causes the car to be driven in a backward direction. In the unit power plant shown in Fig. 112, the operation and gear shifting are identical with that of the separate gear case. The crank case of the motor is either extended or another case attached to the motor which has a compartment arranged to contain the clutch and transmission gears. This arrangement results in compactness, and does away with the supports required to carry the transmission separately.

Transmission Care.—The transmission should be thoroughly cleaned and refilled with fresh grease or heavy oil once in every thousand miles that the car is driven to prevent excessive wear and much noise. To clean, remove the plug at the bottom of the case, and the cover from the top. After the old oil has drained out, replace the plug, fill the case half full of kerosene, replace the cover, and let the motor run for a few minutes with the gears in neutral. Drain the kerosene off, and wash the case and gears off with a paint brush which has been dipped into fresh kerosene. Then examine the gears for blunt burrs and the bearings for looseness. If the gears are burred or chipped, file, or grind them down to level. If the bearings are loose they will have to be replaced, as the bearings used to carry both the counter and drive shaft are seldom provided with means of adjusting. These bearings, however, will not show wear for years if properly cared for. Next, see that the gear case is free from grit and filings, replace the drain plug, and fill the gear case to within one half inch from the drive or propeller shaft with a light graphite grease or heavy oil, and replace the cover using a new gasket.