Almost everyone in this world must be aware of this word "gear". At the least in any of our basic amenities of our life we must have been using gears in one form or the other. Whichever component or device which we use in our day to day life has a spinning part then it must use a gear. Starting from pendulum clocks, fans, watches, VCR, power meter to car engines we can see the extent of usage of this simple but powerful component. Gear is just nothing but a circle with toothed edge that can mesh with another gear or any other mechanical device of similar type. This is used to provide rotational force to another meshing gear. Gears are used in innumerous mechanical devices. In particular they are used to provide gear reduction ie providing high torque by reducing the output speed. This is useful for motorized equipments like screwdriver. Also it helps in adjusting the direction of rotation. It can increase or decrease the speed of rotation. It helps to move the rotational motion to a different axis and also to keep the rotation of axes to be synchronized.
Gears generally work on the concept called gear ratio which is defined given as the number of turns the smaller gear makes to complete one full rotation of the larger gear. In general considering two gears A and B where A is smaller than B, the number of teeth on A should be non-divisible by the number of teeth on B. This is deliberate because this will otherwise result in too much wear. Assuming A has 10 teeth and B has 30 teeth then for every 3 turns of A same teeth would mesh together thus resulting in excessive wear. Thus making it non-divisible ratio then would probably increase the number of turns at which same teeth would mesh and thus reducing the wear of the gears.