Working of a Gearbox

How does a gearbox work?

In the picture below we can see the internals of a normal gearbox. It consists of helical gears meshing with each other. There are two shafts in the gearbox.

The lower shaft in this image is called the layshaft - it's the one connected to the clutch - the one driven directly by the engine. The output shaft is the upper shaft in this image. By looking at the image we can determine that it is a 5 speed gearbox.

 

This is understood by looking at the output shaft .It has 5 helical gears and 3 sets of selector forks, that tells this is a 5-speed box. When the clutch is engaged, the layshaft is always turning. All the helical gears on the layshaft are permanently attached to it so they all turn at the same rate. They mesh with a series of gears on the output shaft that are mounted on slip rings so they actually spin around the output shaft without turning it. This means that they can spin independently without turning either the output shaft or the other gears on the output shaft.

As long as any gear is not engaged the gears on the output shaft it keeps spinning without turning the output shaft. When any gear is selected the following actions are done, the selector forks which are visible between the gears are slipped around a series of collars with teeth on the inside. Those are the dog gears and the teeth are the dog teeth. The dog gears are mounted to the output shaft on a splined section which allows them to slide back and forth. When a gear is engaged, a series of mechanical pushrod connections move the various selector forks, sliding the dog gears back and forth, thereby engaging the gears. Once the dog gears are engaged then the gear on the output shaft will start to spin and as the dog gear is locking the output shaft with the gear the output shaft also spins with it. When the next gear is selected the dog gear moves to the next gear thereby causing the output shaft to spin at a different speed.

 

 

 

 

 

 

 

What about reverse?

So far we have seen how the gearbox works. Since all the forward gears are directly meshed with the output shaft they will all turn in the same direction. However in the case of the reverse gear there is a requirement to reverse the direction of rotation.

This requirement is met by introducing an idler gear in between the output shaft and the layshaft which ensures that the output shaft and the layshaft spin in the same direction. Typically, there will be three gears that mesh together at one point in the gearbox instead of the customary two. There will be a gear each on the layshaft and output shaft as usual, but there will be a small gear in between them called the idler gear. The inclusion of this extra mini gear causes the last helical gear on the output shaft to spin in the opposite direction to all the others. The principle of engaging reverse is the same as for any other gear - a dog gear is slid into place with a selector fork. Because the reverse gear is spinning in the opposite direction, when the clutch is releases, the gearbox output shaft spins the other way - in reverse.