The transmission or transaxle is a vital link in the power train of any modern vehicle. The purpose of the transmission or transaxle is to use gears of various sizes to give the engine a mechanical advantage over the driving wheels. During normal operating conditions, power from the engine is transferred through the engaged clutch to the input shaft of the transmission or transaxle. Gears in the transmission or transaxle housing alter the torque and speed of this power input before passing it on to other components in the power train. Without the mechanical advantage the gearing provides, an engine can generate only limited torque at low speeds. Without sufficient torque, moving a vehicle from a standing start would be impossible.

In any engine, the crankshaft always rotates in the same direction. If the engine transmitted its power directly to the drive axles, the wheels could be driven only one direction. Instead, the transmission or transaxle provides the gearing needed to reverse direction so the vehicle can be driven backward.

TRANSMISSION VERSUS TRANSAXLE

Vehicles are propelled in one of three ways: by the rear wheels, by the front wheels, or by all four wheels. The type of drive system used determines whether a conventional transmission or a transaxle is used.

Vehicles propelled by the rear wheels normally use a transmission. Transmission gearing is located within an aluminum or iron casting called the transmission case assembly. The transmission case assembly is attached to the rear of the engine, which is normally located in the front of the vehicles. A drive shaft links the output shaft of the transmission with differential and drive axles located in a separate housing at the rear of the vehicles. The differential splits the driveline power and redirects it to the two rear drive axles, which then pass it on to the wheels. For many years, rear-wheel-drive systems were the conventional method of propelling a vehicle.

Front-wheel-drive vehicles are propelled by the front wheels. For this reason, they must use a drive design different from that of a rear-wheel-drive vehicle. The transaxle is the special power transfer unit commonly used on front-wheel-drive vehicles. A transaxle combines the transmission gearing, differential, and drive axle connections into a single case aluminum housing located in front of the vehicle. This design offers many advantages. One major advantage is the good traction on slippery roads due to the weight of the power train components being directly over the driving axles of the vehicles. It is also more compact and lighter than the transmission of a RWD vehicle.

Four-wheel-drive vehicles typically use a transmission and transfer case. The transfer case mounts on the side or back of the transmission. A chain or gear drive inside the transfer case receives power from the transmission and transfers it to two separate drive shafts. One drive shaft connects to a differential on the front drive axle. The other drive shaft connects to a differential on the rear drive axle.

Most manual transmissions and transaxles are constant mesh, fully synchronized units. Constant mesh means that whether or not the gars is locked to the output shaft, it is in mesh with its counter gear. All gears rotate in the transmission as long as the clutch is engaged. Fully synchronized means the unit uses a mechanism of brass rings and clutches to bring rotating shafts and gears to the same speed before shifts occur. This promotes smooth shifting. In a vehicle equipped with a four-speed manual shift transmission or transaxle, all four forward gears are synchronized. Reverse gearing may or may not be synchronized, depending on the type of transmission/transaxle.

Gears

The purpose of a gear in a manual transmission or transaxle is to transmit rotating motion. Gears are normally mounted on a shaft, and they transmit rotating motion from one parallel shaft to another.

Gears and shafts can interact in one of three ways: the shafts can drive the gear; the gear can drive the shaft; or the gear can be free to turn on the shaft. In this last case, the gear acts as an idler gear.