Automatic Transmission

What is an Automatic Transmission & How It Works?


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About The Automatic Transmission

An automatic transmission is a type of transmission or gearbox that can automatically change the gears while the vehicle is in motion. Manufacturers also term it as just the Auto or AT, the self-shifting transmission. It is also known as the 'n-speed' automatic, where n is the number of forward-gear ratios. It is a type of motor vehicle transmission that frees the drivers from shifting the gears manually or by themselves. In vehicles with automatic transmission, there is no clutch pedal. The driver can control the car only thru’ the accelerator and brake.

How an Automatic Transmission works?

The most popular type of automatic transmission in automobiles is the hydraulic type. However, the heavy-duty commercial and industrial vehicles use similar but larger devices for the purpose. The automatic transmission uses a fluid coupling in place of a friction clutch. It carries out the gear changes by hydraulically locking and unlocking the planetary gears.

Automatic Transmission
Automatic Transmission

So, an automatic transmission has a definite set of gear ranges. It often comes with a parking brake that locks the output shaft of the transmission. Thus, it keeps the vehicle from going either forward or backward when parked. Some vehicles have limited speed ranges or stable engine speeds, such as some forklifts and lawn-mowers. Such vehicles use only a torque converter to provide the variable speed of the engine to the wheels.

Hydramatic Transmission:

It is a fully automatic type of transmission. The hydra-matic drive combines the automatic four-speed forward and reverse geared transmissions. It uses a fluid flywheel such as the fluid coupling or the torque converter for the purpose. The geared transmission consists of three sets of constant-mesh helical planetary gears in series. Two sets of the planetary gears provide four forward gear ratios while using the other set for reverse. The fluid flywheel cushions the impact of automatic gear shifting and reduces the engine's torque reactions.

Hydramatic Transmission
Hydramatic Transmission

Components of an Automatic Transmission system:

The main type of automatic transmission operates hydraulically. It uses a fluid coupling or torque converter and a set of planetary gears to provide a range of gear ratios. The Hydraulic automatic transmissions consist of three major components. They are - Torque converter, planetary gears, and hydraulic controls.

Torque Converter:

The Torque converter is a type of fluid coupling. It connects/disconnects the engine hydraulically to the transmission. The torque converter replaces the friction clutch in a manual transmission. It connects and disconnects the engine power to the planetary gears. Thus, it allows the vehicle to come to a stop without stalling the engine. It offers a variable amount of torque multiplication at low engine speeds. So, it increases the breakaway acceleration. A fluid coupling works well when both the impeller and turbine are rotating at the same speeds. However, it is very inefficient at the initial acceleration, where rotational speeds are very different.

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