Understanding the Way Transmissions Work


When somebody asks “What’s inside my transmission?” they probably don’t want to read a lengthy mechanical dissertation. So, let’s talk about how transmissions work, manual and automatic.

Gears and Collars

Cars need more than one gear ratio, just like a bicycle rider. Maybe not 10, but at least three, and possibly as many as six speeds.


To grasp how that works, picture a strong metal shaft that holds several strong gears of different diameters. All the gears are locked to this shaft. This shaft will receive power from the engine, through the clutch assembly.

Now devise another strong metal shaft to hold the same number of gears, also with different diameters from small to large. But these gears spin freely on this shaft.

Bring the two shafts together and let each gear on the first shaft mesh with a gear on the second shaft. The second shaft will be connected to the drive wheels and transmit power to them.

To understand the different gear ratios, picture that the smallest gear on the first shaft drives the largest gear on the second shaft. That is first gear, just like on a bicycle. Make sense?

Likewise, the largest gear on the first shaft drives the smallest gear on the second shaft. That would be overdrive. Technically, high gear in 3-speed or 4-speed gearboxes bypasses the gears on the second shaft altogether, and merely transmits power straight through the transmission.

Since the gears on the second shaft spin freely they can’t transmit any power. Only one gear at a time on the first shaft can be allowed to transmit power to its matching gear (its driven gear) on the second shaft.


To put the car into a gear, we need a sliding device – a collar- that slides in grooves on the second shaft. Moving the collar close to a driven gear will engage a separate set of tiny outer teeth on that gear. This locks the collar to that gear. Then the driven gear can transmit power through the collar, then through the second shaft, and on to the wheels.

Since the engine runs at different speeds and the car rolls at different speeds through all of this, we need a way to equalize the speeds of the sliding collar and driven gears. Otherwise there would be a lot of grinding of gears going on as we tried to shift. Simply insert a soft brass ring to float between the collar and its matching driven gear. That brass ring is called a synchronizer, because it speeds up or slows down the driven gear as needed to allow smooth shifting.

Automatic Transmissions

To explain how the gears in automatic transmissions work, let’s use a simple manual pencil sharpener as an example. The device has three parts. Combined they are called a planetary transmission. Their positions somewhat resemble the planets orbiting the sun.

Turning the handle drives a housing, which contains two rollers that have the cutting ridges, or teeth, on them. The rollers spin, but in the opposite direction their housing is turning, because they are attached to a large ring gear surrounding them.


If we freed the large ring gear and allowed it to rotate freely, that would also let us lock the rollers to their housing, causing the pencil to rotate like a driveshaft. Or, we could apply the drive power from the handle to the rollers, causing the housing to rotate, but in the opposite direction, as in a reverse gear.

That was a simple example. More of the same components could be added for more speeds, like more drive housings, rollers and ring gears. We could then apply power to any component we choose, and lock any component we chose, which would give us the separate gear ratios we wanted.

Manually selecting the components to drive or anchor isn’t necessary. We could devise hydraulic circuits that would engage or release (simple on or off) clutches or bands, that can hold or release individual components. Then put a hydraulic valve body, or an electronic computer, in charge of controlling those hydraulic circuits. That’s what makes the transmission shift through the gears.

The Basics Are Easy to Grasp

To see videos of how planetary gear sets work, you could Google “Simpson planetary gear set” and “Ravigneaux planetary gear set.” Share this article with someone you know who works on their own car. They will thank you for it.