The nitty-gritty of your gearbox

In an automatic transmission, the gears are never physically moved and are always engaged to the same gears. Photo / Supplied

I have been asked on numerous occasions how exactly an automatic gearbox works - and I'll be the first to admit although I've got an idea in my head, I can't really explain it.

Therefore, I did some research and below is a basic outline on how they work.

A gearbox, either manual or automatic, is the piece of kit bolted to the engine that allows the power developed to be transmitted to the wheels at the front, rear or both ends of the car.

As we all know, an engine runs best at its optimum revs where it develops the best power and to achieve this it needs the gear ratios in a gearbox.

For example, the engine revs a lot higher in first gear than the wheels turn thus allowing for more pulling power and making the car easier to get going.

On the other hand, in fifth or sixth gear the car can be travelling at 100km/h but the engine barely turning over.

In reverse gear the cogs are meshed in such a way to allow the wheels to turn in the opposite direction, allowing the driver to back up.

Neutral disengages the engine from the drive shaft and thus the wheels.

Park acts as like a door bolt and inserts a pin into the drive shaft stopping it from turning.

On a rear-wheel drive car, the gearbox is located behind the engine and is that hump just below the gear lever and the drive shaft. It runs under the tunnel that runs the length of the car.

This is the most basic setup where power leaves the engine, goes through the gearbox into the differential - situated in the middle of the rear axle - and on to the rear wheels.

Front-wheel drive is a bit different. The power is transmitted through a transaxle. The engine is normally situated in and east/west (sideways) configuration and the transaxle is mounted underneath.

The front axles are connected directly to the transaxle and hence drive the front wheels.

With this setup the power goes through the torque converter to a chain that coverts the power 180 degrees to the transmission, which sends it to the final drive, which directs it to the wheels.

An automatic gearbox is a complex mechanism made up of mechanical, hydraulic and electrical components.

In an effort to keep it relatively simple, with the help of familycar.com, listed below are the main components in an automatic gearbox.

* Planetary gear sets are the mechanical systems that provide the various forward gear ratios as well as reverse.

* The hydraulic system uses a special transmission fluid sent under pressure by an oil pump through the valve body to control the clutches and the bands in order to control the planetary gear sets.

* Seals and gaskets are used to keep the oil where it is supposed to be and prevent it from leaking out.

* The torque converter which acts like a clutch to allow the vehicle to come to a stop in gear while the engine is still running.

* The governor and the modulator or throttle cable that monitor speed and throttle position in order to determine when to shift.

* On newer vehicles, shift points are controlled by computer, which directs electrical solenoids to shift oil flow to the appropriate component at the right instant.

In a manual transmission, gears slide along shafts as you move the shift lever from one position to another, engaging various-sized gears as required in order to provide the correct gear ratio.

In an automatic transmission, the gears are never physically moved and are always engaged to the same gears. This is accomplished through the use of planetary gear sets.

The basic planetary gear set consists of a sun gear, a ring gear and two or more planet gears, all remaining in constant mesh. The planet gears are connected to each other through a common carrier that allows the gears to spin on shafts called pinions that are attached to the carrier.

In its simplest form, the ring gear is connected to the input shaft coming from the engine, connecting the planet carrier to the output shaft, and locking the sun gear so it can't move.

For example, when the ring gear is turned the planets will walk along the sun gear (held stationary) causing the planet carrier to turn the output shaft in the same direction as the input shaft but at a slower speed, causing gear reduction - similar to first gear in a manual gearbox.

If the sun gear is unlocked and any other two elements are locked together, all three elements will rotate at the speed as the output shaft. This is similar to a manual gearbox operating in third or fourth gear.

To get reverse, the planet carrier is locked and the power is transmitted to the ring gear that will cause the sun gear to turn in the opposite direction and hence the drive shaft as well turning the wheels in the opposite direction.

A clutch pack is used to lock the planet carrier with the sun gear forcing both to turn at the same speed.

If both the clutch pack and the band were released, the system would be in neutral.

Turning the input shaft would turn the planet gears against the sun gear but since nothing is holding the sun gear it will just spin free and have no effect on the output shaft.

To place the unit in first gear, the band is applied to hold the sun gear from moving.

To shift from first to high gear, the band is released and the clutch is applied causing the output shaft to turn at the same speed as the input shaft.

Again, I'll let familycar.com explain how the clutch packs work. The clutch pack consists of alternating disks that fit inside a clutch drum.

Half of the disks are steel and have splines that fit into grooves on the inside of the drum.

The other half have a friction material bonded to their surface and have splines on the inside edge that fit grooves on the outer surface of the adjoining hub.

There is a piston inside the drum that is activated by oil pressure at the appropriate time to squeeze the clutch pack together so that the two components become locked and turn as one.

A band is a steel strap with friction material bonded to the inside surface. One end of the band is anchored against the transmission case, while the other end is connected to a servo.

At the appropriate time, hydraulic oil is sent to the servo under pressure to tighten the band around the drum to stop the drum from turning.

On automatic transmissions, the torque converter takes the place of the clutch found on a manual gearbox. It is there to allow the engine to continue running when the vehicle comes to a stop.

The principle behind a torque converter is like taking a fan that is plugged into the wall and blowing air into another fan that is unplugged.

If you grab the blade on the unplugged fan, you are able to hold it from turning but as soon as you let go, it will begin to speed up until it comes close to the speed of the powered fan.

The difference with a torque converter is that instead of using air, it uses oil, or transmission fluid. The hydraulic system is a complex maze of passages and tubes that sends transmission fluid under pressure to all parts of the transmission and torque converter.

Most of the components in an automatic gearbox are constantly submerged in fluid including the clutch packs and bands.

The friction surfaces on these parts are designed to operate properly only when they are submerged in oil, so make sure your automatic gearbox is always topped up.