Gears are typically wheels with teeth or cogs. They are used to transmit
effort, change torque (turning force), change direction, or change speed.
There are several basic types of gears including:
If the driven gear has more teeth than the driving gear, the driven gear rotates at a slower rotational speed.
If the driven gear has less teeth than the driving gear, the driven gear rotates at a faster rotational speed.
The mechanical advantage of a pair of gears is the number of driven gear teeth divided by the number of driving gear teeth.
As an example, if the driving gear has 30 teeth and the driven gear 15 teeth, the mechanical advantage is 15/30 = 0.5. Conversely, if the driven gear has 30 teeth and the driving gear 15 teeth the mechanical advantage is 30/15 = 2.
Bikes use gears to increase the efficiency of the rider, depending on the surface and slope being tackled! Bikes are designed to have a mechanical advantage of less than one. That is, the input force of the rider acts over a short distance on the crank wheel gear, pushing the larger rear wheel over a longer distance. As a rider drops down through the gears on riding up a hill, the crank wheel gear is made larger and the rear gear wheel smaller until a MA of 1 is approached on very steep hills. Various mechanical advantages can be achieved by changing the rear and crank gear sizes, commonly know as "changing gears".
Rack and pinion
In a different application, railway locomotives pulling loads up steep inclines sometimes have a pinion wheel under the locomotive, engaging with a rack in the centre of the rail tracks. The Apt railway in Tasmania is a fine example of this type of gearing.