What are The Parts of Transmission?- Function, Diagram

Transmission Parts

Transmission parts knowledge for people researching how transmissions work.

A transmission is a formidable puzzle for most drivers, made up of many intricate connections between a multitude of transmission parts. The complexity of these parts and how they fit together is intimidating, especially when faced with transmission repairs and failing to understand what is happening or why.

While we must trust our transmission mechanics to take care of the details, a little knowledge of the possibly faulty transmission parts can help put you at ease during the transmission repair process.

The first step in having a basic understanding of the transmission parts is understanding the purpose of the transmission as a whole. Your automatic transmission is responsible for transferring the power of your engine to the driveshaft and wheels so your car can move within its optimal speed range per minute (RPM). The transmission maintains this optimal range by shifting the transmission as you go faster or slower.

For more Information Check out our Article: What is Gear box(Transmission)?

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The main transmission parts that need to work together are:

  • Planetary Gear Sets
  • Hydraulic System
  • Torque Converter
  • Computer
Automatic Transmission

Planetary Gear Sets

Manual transmissions use a clutch to connect the engine and transmission. They require the driver to shift gears, which means that the gears are actually moved in a somewhat linear, sliding transmission to engage the coordinating gears necessary to maintain the correct speed ratio. Automatic transmissions hold the transmission gears in one place in a more circular structure. This is no different from a small solar system, hence the name Planetary Gear Sets.

Through the combination of an outer ring gear, a central “sun” gear and two or more smaller “planet” gears, all of which are constantly intermeshed, the gear shifts from the driver. Similar to the solar system, the sun gear is in the center and remains stationary, and the smaller planet gears mesh with the gear and ring gear to keep the car running smoothly.

The ring gear is connected to the input shaft that supplies power to the motor. The planetary gears are located in a housing or carrier that is connected to the output shaft that transmits the power to the drive train and the wheels. The planet gears are also connected to a clutch pack. The sun gear is connected to a drum which is connected to the other half of the clutch pack.

Transmission clutch packs consist of a series of washers, half of which are wedged on the outer edge and half with wedges on the inner edge. These alternating disks fit together to lock and rotate together. They do this with hydraulic functions.

Hydraulic System

All transmission parts are constantly immersed in gear oil. This fluid is manipulated to create pressure that compresses the transmission clutch pack at the right time. A complex system of pipes moves the fluid around the transmission and torque converter to create this pressure.

The transmission hydraulic system has three main purposes: to help control the shifting process of the transmission, lubrication of the transmission parts to prevent frictional damage, and cooling of the transmission. The fluid pressure in the transmission must be maintained at all times to avoid damage.

The tubes that carry the transmission fluid have two large outer seals at the front and rear. The seal on the front protects the connection to the torque converter and the rear seal contains the fluid where the transmission meets the output shaft.

Seals are made of neoprene. Within the transmission exists another type of seal, called a gasket, which connects and protects two stationary transmission parts. Gaskets may be made of any of a variety of materials, such as rubber or silicone. Seals and gaskets can harden over time, which might cause leaks and a drop in transmission fluid pressure, both of which can lead to damage to the transmission.

Torque Converter

When driving a manual transmission, the driver must engage the clutch or shift to neutral when the vehicle comes to a standstill, such as at a red light, or the engine stalls. The torque converter in an automatic transmission allows the engine to keep running when the vehicle is stopped and still in gear. Torque is defined as a force that causes rotation.

The torque converter uses the pressure of the transmission fluid to control the rotation of its parts. When stopped at this red light, one half of the torque converter is spinning while the other is stationary. As you accelerate, the fluid pressure forces the other half to spin in conjunction with the first half, causing the vehicle to move forward.


In most automobiles today, a computer controls the transmission function so that all of the vehicle systems can work together to achieve optimal fuel economy and performance. Up to 30 sensors read all of the various factors such as vehicle speed, engine temperature, engine RPM, etc. that control the shifting of the transmission to ensure that the optimal shift points are used.

The many transmission parts in your vehicle may remain a mystery, but understanding some of the basics can help you have a more informed conversation with your transmission mechanics before leaving it in their capable hands.