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Automobile Power Transmission System

Updated on August 24, 2010


The clutch is a mechanism for connecting and disconnecting the engine and the attached manual transmission in cars equipped with a hand-operated gearshift. This operation is necessary when shifting gears from one ratio to another, when starting the car from rest, or when the car is to be stopped with the engine running. Pressure on a pedal disengages the clutch, allowing the engine to run free of the transmission. The single dry-plate friction clutch is used almost universally for passenger cars equipped with manual transmissions. This clutch consists of a disk, which is keyed to the transmission shaft. By means of a spring-loaded pressure plate, the disk (which is faced on both sides with a friction material) is sandwiched between the smooth flywheel face and the pressure plate. By applying pressure gradually, power is transmitted smoothly from the flywheel to the disk and thence to the transmission.

The development of the hydraulic coupling, or "fluid flywheel," led to fully automatic transmission. In the hydraulic coupling, power is transmitted through a fluid rather than a friction disk. It is essentially an efficient centrifugal pump consisting of two members: a driving member attached to the engine and a driven member attached to the transmission shaft. Its operation can be compared to two electric fans facing each other at close range. Air from the driving fan turns the driven fan, making the combination, in effect, a coupling. In the fluid flywheel mechanism, oil is the medium of power transfer instead of air, and a great deal less slippage takes place.

Manual Transmission

The manual transmission, or gearbox, provides three or four forward speeds and one reverse. In the United States, 3-speed transmissions are standard on passenger cars, with 4-speed transmissions offered as an option. Low gear (first speed) gives an average gear ratio of approximately 2.6:1 between the engine and the propeller shaft; intermediate gear, or second, approximately 1.6:1; and high gear, or third, a direct 1:1 coupling. Reverse gear has a ratio of approximately 3.4:1. Actual engine-to-wheels ratio will vary with the rear-axle ratio selected.

The forward gears are used in sequence for starting the car from a standstill and are selected according to driving conditions. Highest forward torque is available in first gear; third or fourth gear offers highest road speed and best economy for level driving.

Synchronous meshing permits an unskilled driver to shift through the forward speeds of a manual transmission, either up or down, with no clashing of gears. The 3- and 4-speed manual transmissions used in American passenger cars are of the synchronized constant-mesh type. In most cases, these transmissions are constructed so that all forward gears are continuously in mesh.

An overdrive is offered in conjunction with standard transmissions, adding a higher gear ratio for better economy and quieter engine operation at high road speeds. The overdrive usually is set between the transmission and the drive shaft. It is controlled automatically by a governor-operated solenoid switch that causes the overdrive unit to engage at a predetermined speed. It also can be disconnected manually.

Automatic Transmissions

Introduced in 1939, fully automatic transmissions have become optional or standard equipment on every American passenger car. The driver merely has to depress the accelerator pedal, and the hydraulically controlled transmission automatically will shift through its entire low to high range as the automobile gathers speed. Conversely, it will automatically provide the proper gear ratio as the car loses speed. Automatic transmissions are of two general types: step-ratio shifter and torque converter.

Step-Ratio Shifter

In this type the changing of gear ratios is controlled by governors within the transmission. Responding to throttle-position and vehicle-speed changes, the governors cause hydraulically operated brake bands or clutches to lock the appropriate section of a planetary-gear set. Planetary gears are employed in step-ratio shifter transmissions, as this type of gear arrangement permits shifting under power without disengaging the transmission. A fluid coupling is used between transmission and engine to cushion the shock of changing gear ratios. The automatic step-ratio shifter transmission provides three forward gear ratios, reverse, and neutral. Another model of this same transmission offers a fourth forward gear ratio. When the transmission has shifted through its complete forward-speed range, the engine is connected directly to the drive shaft through the fluid coupling.

Hydraulic Torque Converter

Unlike the Step-ratio shifter type of automatic transmission, the hydraulic torque-converter transmission provides what amounts to an infinite number of gear ratios by continuously and smoothly changing the torque at the drive shaft through the entire range. The torque converter resembles a fluid coupling, but with one important difference: the fluid coupling is the same as a friction clutch in that the transmission receives no more torque than is available at the engine crankshaft, but the torque converter multiplies engine torque in much the same fashion as a gear transmission does. The addition of a stationary element, or stator, makes the difference. Oil is thrown out from the driving member by centrifugal force and directed against the vanes of the driven member, or turbine, to which the drive shaft is attached. The turbine is so constructed that the oil not only imparts torque to the turbine but also leaves the turbine faster than it entered. The stator then catches the oil and redirects it with little loss of energy, and it reenters the pump at a speed greater than that at which the pump is being driven. This swiftly moving oil is given a further increase in velocity by the pump, thus multiplying the speed of the oil and hence the torque on the turbine. As the turbine begins to turn, the multiplication factor gradually becomes smaller because of the centrifugal reaction from the turbine and because the oil now enters the stator vanes at a more and more disadvantageous angle. Finally, at the cruising speed of the automobile, the unit is nothing more than a fluid coupling with the turbine rotating at the same speed as the engine, minus slippage.

Torque converters are equipped with planetary-gear sets to multiply torque further and to provide a reversing gear.

Propeller Shaft and Universal Joints

The propeller shaft, or drive shaft, carries the power from the transmission back to the final-drive gears and rear axles. The propeller shaft may be either exposed or enclosed in a stationary torque tube. Inasmuch as an automobile's rear axles are suspended from springs, the rear end of the propeller shaft will move up and down, and the distance from transmission to rear axles will change slightly. This angular motion of the drive shaft is accommodated by one or more universal joints. In addition, a splined (keyed) slip joint takes care of the changes that occur in drive-shaft length.

The most commonly used universal joint is the double yoke and cross joint, consisting of two U-shaped members placed at right angles to each other and connected by a cross, an arrangement that permits relative motion along both the horizontal and vertical axes. This type of joint will transmit power through an angle of 6° to 8° with little loss in efficiency. The slip joint, which must be used with the universal joint, normally consists of external splines (keys) on one shaft meshed with internal splines on the other. This allows axial movement both toward and away from each member, while at the same time the splines cause the shafts to rotate together.

With torque-tube drive, only one universal joint is required, and this is located at the transmission end of the propeller shaft. With the Hotchkiss drive, in which an exposed tubular drive shaft is used and the rear springs are designed to absorb both braking and driving torque, a universal joint must be used at each end of the drive shaft.

Final Drive and Differential

The final drive provides a right-angle transfer of power from the propeller shaft to the rear axles and wheels. It consists of a ring-gear-and-pinion set in which the center line of the pinion is offset from the center line of the ring gear. The pinion meshes with the lower half of the ring gear so that the automobile chassis can be lowered without interfering with the movement of the drive shaft. The relatively large amount of sliding friction between the ring and pinion gear teeth requires a special high-pressure lubricant. A further reduction of the engine-to-rear-axle ratio is accomplished by keeping the pinion gear much smaller than the ring gear. The standard rear axle ratio of American passenger cars varies from approximately 2.8:1 to 3.6:1.

When an automobile turns a corner, the outside wheels travel farther than the inside wheels; consequently, it is necessary to turn the outside rear wheel faster if the torque transmitted from the engine is to be divided equally between the two wheels. In order to make this possible, two differential gears are splined to the inner ends of the axle shafts. Meshed between these are small bevel gears called differential pinions, which are carried in the differential case. The differential case is riveted to the ring gear and also serves to hold the differential gears in mesh with the pinions.

Power is transmitted from the propeller shaft through the final-drive pinion and ring gear to the differential case, from which it is transmitted through the differential pinions to the differential gears splined to the rear axles. When power is applied and the two rear wheels are turning at equal speeds, the differential pinions will not revolve. However, if a corner is being turned, there is movement of the differential pinions about their own axes, causing one axle shaft to revolve faster than the other. As the speed of one axle shaft accelerates, the other one is slowed by an equal amount. (Offered as optional equipment, the limited-slip differential directs power to the rear wheel that offers the most resistance to torque. As long as one tire is resting on a surface that provides good traction, the car can be moved.)

The final-drive gears and the differential case are assembled in the differential carrier, which in turn is mounted in the rear-axle housing. The differential carrier also holds the bearings that support the differential case and the drive shaft. Two tubular extensions of the differential housing enclose the rear-axle shafts. Sufficient high-pressure lubricant is carried in the housing to lubricate the entire final-driving unit.

Rear Axle

Power from the differential is transmitted to the wheels through the rear axles. Each axle is splined to a differential gear at one end and secured to a wheel at the other. Most cars use what is known as the semifloating axle, in which the weight of the car is carried on two bearings between the axle housing and the axle itself. Bending stresses are distributed along the axle between the differential and the outer axle bearing.


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