Manual transmissions use a clutch to connect the engine and transmission. They require the driver to shift gears, which in effect means moving the gears in a somewhat linear, sliding gear, to engage the coordinating gears needed to maintain the correct rpm ratio. Automatic transmissions keep the transmission gears in one place, in a multi-ring structure. This is not unlike a small solar system, hence the name Planetary Gear Sets.
The transmission is an impressive puzzle for most drivers, consisting of many complex connections between the many parts of the transmission. The complexity of these parts and how they fit together is intimidating, especially if you're faced with transmission repairs and don't understand what's going on or why.
While we have to trust our transmission mechanics to take care of the specifics, knowing the parts of the transmission that can go wrong can help give you peace of mind during the transmission repair process.
The first step to a basic understanding of transmission parts is to understand the purpose of the transmission as a whole. Your automatic transmission is responsible for transferring the power of your engine to the drive shaft and wheels so that your car can move in the optimal range of revolutions per minute (RPM). The transmission maintains this optimal range by changing gears when driving faster or slower.
The main parts of the transmission that must work together are:
Planetary gear sets
Hydraulic system
Torque converter
Computer
Planetary gear sets
Manual transmissions use a clutch to connect the engine and transmission. They require the driver to shift gears, which in effect means moving the gears in a somewhat linear, sliding gear, to engage the coordinating gears needed to maintain the correct rpm ratio. Automatic transmissions keep the transmission gears in one place, in a multi-ring structure. This is not unlike a small solar system, hence the name Planetary Gear Sets.
Using a combination of an outer ring gear, a central "sun" gear, and two or more smaller "planet" gears, all of which are constantly engaged, the transmission takes over shifting from the driver. Like the solar system, the central wheel is in the center and remains stationary, and the smaller planet wheels mesh with it and the ring gear to keep the car moving smoothly.
The ring gear is connected to the input shaft, which delivers power from the engine. Planetary gears are located in a housing or carrier that is connected to an output shaft that transmits power to the drive train and wheels. The planetary gears are also connected to the clutch kit. The center wheel is connected to the drum, which is connected to the other half of the clutch set.
A transmission clutch set is a series of discs, half with grooves on the outer edge and half with grooves on the inner edge. These alternating discs are made to fit together to lock and rotate together. They do this using a hydraulic function.
Hydraulic system
All transmission parts are constantly immersed in transmission fluid. This fluid is adjusted to create a pressure that presses the transmission clutch pack together at the correct time. A complex system of pipes moves fluid around the transmission and torque converter to create this pressure. A transmission's hydraulic system has three main purposes: to help control gear shifting, to lubricate transmission parts to prevent frictional damage, and to keep the transmission cool. Transmission fluid pressure must be maintained at all times to prevent damage.
The tubes that carry the transmission fluid have two main outer seals at the front and rear. The front seal protects the connection to the torque converter, and the rear seal contains fluid where the gearbox meets the output shaft. The seals are made of neoprene. There is another type of seal in the gearbox, called a seal, which connects and protects the two stationary parts of the gearbox. Gaskets can be made from any of a variety of materials such as rubber or silicone. Seals and gaskets can harden over time, causing leaks and a drop in transmission fluid pressure, both of which can lead to transmission damage.
Torque converter
When driving with a manual transmission, the driver must engage the clutch or shift into neutral when the vehicle stops, such as at a red light, or the engine will die. A torque converter in an automatic transmission allows the engine to continue running when the vehicle is stopped and still in gear. Torque is defined as the force that causes rotation. A torque converter uses transmission fluid pressure to control the rotation of its parts. When you stop at this red light, one half of the torque converter rotates while the other is stationary. When you accelerate, fluid pressure forces the second half to spin in conjunction with the first half, so the vehicle moves forward.
Computer
In most cars today, a computer controls the function of the transmission, allowing all the vehicle's systems to work together for optimal fuel economy and performance. Up to 30 sensors read all the different factors such as vehicle speed, engine temperature, engine speed, etc., which control gear shifting to ensure the optimum shift points are used.
Many transmission parts in your vehicle can remain a mystery, but understanding some of the basics can help you have a more informed conversation with transmission mechanics before you put it in their capable hands.
