A gear transmission trades speed for torque. A motor drives a small gear that meshes with larger gears, so the output shaft turns slowly but with far more twisting force. Watch how the drive passes through the train.
A gear transmission is a set of toothed wheels that hands motion from one shaft to another while changing its speed and twisting force. Motors spin fast but push weakly; gears trade some of that speed away so a machine can turn a heavy load slowly and steadily.
On the outside a gearbox is just a quiet metal case. The whole point of it — the way the teeth mesh and the sizes step up — is completely hidden. Opening the housing shows why a small fast gear next to a big slow gear is such a useful trick.
The motor spins fast but with little twisting force. It turns the input shaft, which carries the smallest gear in the train — the driving gear.
The driving gear's teeth push into the idler's teeth. Because the idler is larger, it must make fewer turns for every turn of the small gear, and it is driven in the opposite direction.
The idler meshes with the largest gear on the output shaft. Again the bigger gear turns more slowly. An idler alone does not change the overall ratio, but it sets the direction and the shaft spacing.
Each mesh that turns a larger gear slows the output down but multiplies the twisting force. By the time the motion reaches the output shaft it is slow and powerful — exactly what a heavy load needs.
Bearings hold each shaft in place and let it spin freely, while the housing keeps the gear centres at exactly the right spacing. If that spacing drifted, the teeth would jam or wear out.
It uses gear size ratio. A small gear turning a larger gear must turn it more slowly, and in exchange the larger gear twists harder. The trade is fixed by the ratio of the gear sizes, so you gain torque by giving up speed.
An idler passes the drive along without changing the ratio (when it is alone between the two). Its main job is to make both outer gears spin the same way, and to let the shafts sit where the machine needs them.
Because their teeth push against each other. One gear pushes the next, so the neighbour is driven the other way — which is why a third gear is often added when the output must spin the same way as the input.
It holds the shafts in precise positions, keeps oil in to lubricate the gears, and keeps dirt out. If the gear centres drifted apart the teeth would wear or jam, so the housing's accuracy is as important as the gears.
No. A reduction gearbox slows the output and boosts torque, but the same gears reversed become an overdrive that speeds the output up with less torque — for example in a car's top gear.