Automatic Transmission

Automatic Transmission

An automatic transmission does not need a clutch pedal and shifts through the forward gears without the control of the driver. Instead of a clutch, it uses a torque converter to transfer power from the engine's flywheel to the transmission input shaft. The torque converter allows for smooth transfer of power at all engine speeds Shifting in an automatic transmission is controlled by a hydraulic and/or electronic control system. In a hydraulic system, an intricate network of valves and other components uses hydraulic pressure to control the operation of planetary gearsets. These gearsets provide the three or four forward speeds, neutral, park, and reverse gears normally found in automatic transmissions. Newer electronic shifting systems use electric solenoids to control shifting mechanisms. Electronic shifting is precise and can be varied to suit certain operating conditions.

Drivelines

Drivelines are used on RWD vehicles and 4WD vehicles. They connect the output shaft of the transmission to the gearing in the rear axle housing. They are also used to connect the output shaft to the front and rear drive axles on a 4WD vehicle. A driveline consists of a hollow drive or propeller shaft that is connected to the transmission and drive axle differential by universal joints (U-joints). These U-joints allow the drive shaft to move up and down with the rear suspension, preventing damage to the shaft. Differential on RWD vehicles, the drive shaft turns perpendicular to he forward motion of the vehicle. The differential gearing in the rear axle housing is designed to turn the direction of the power so that it can be used to drive the wheels of-the vehicle. The power flows into the differential, where it changes direction, then flows to the rear axles and wheels. The gearing in the differential also multiplies the torque it receives from the drive shaft by providing a final gear reduction. Also, it divides the torque between the left and right driving axles and wheels so that a differential wheel speed is possible. This means one wheel can turn faster than the other when going around turns.

Driving Axles

Driving axles are solid steel shafts that transfer the torque from the differential to the driving wheels. A separate axle shaft is used for each driving wheel. In a RWD vehicle, the driving axles and part of the differential are enclosed in an axle housing that protects and supports these parts. Each drive axle is connected to the side gears in the differential. The inner ends of the axles are splined to fit into the side gears. As the side gears are turned, the axles to which they are splined turn at the same speed. The drive wheels are attached to the outer ends of the axles. The outer end of each axle has a flange mounted to it. A flange is a rim for attaching one part to another part. The flange, fitted with studs, at the end of an axle holds the wheel in place. Studs are threaded shafts, resembling bolts without heads. One end of the stud is screwed or pressed into the flange. The wheel fits over the studs and a nut, called the lug nut, is tightened over the open end of the stud. This holds the wheel in place. The differential carrier supports the inner end of each axle. A bearing inside the axle housing supports the outer end of the axle shaft. This bearing, called the axle bearing, allows the axle to rotate smoothly inside the axle housing.

Transaxle

A transaxle is used on FWD vehicles. It is made up of a transmission and differential housed in a single unit. The gearsets in the transaxle provide the required gear ratios and direct the power flow into the differential. The differential gearing provides the final gear reduction and splits the power flow between the left and right drive axles. The drive axles extend from the sides of the transaxle. The outer ends of the axles are fitted to the hubs of the drive wheels. constant velocity (cv) joints mounted on each end of the drive axles allow for steering and suspension (up-and-down) motion without affecting the power flow to the wheels.

Four-wheel-Drive-System

4wD or AWD vehicles combine the features of rear-wheel-drive transmissions and front-wheel-drive trans-axles. Additional transfer case gearing splits the power flow between a differential driving the rear wheels and a rear differential that drives the rear wheels. This transfer case can be a housing bolted directly to the transmission/transaxle, or it can be a separate housing mounted somewhere in the driveline. Most RWD-based four-wheel-drive vehicles have a drive shaft connecting the output of the transmission to the rear axle and another connecting the output of the transfer case to the front drive axle. Typically, AWD cars have a center differential that splits the torque between the front and rear drive axles.