A gear may be thought of as a toothed wheel that, when meshed with another smaller-in-diameter toothed wheel (the pinion), will transmit rotational motion from one shaft to another. The primary function of a gear is to transfer power from one shaft to another while maintaining a definite ratio between the shaft angular velocities. The teeth of a driving gear push on the driven gear teeth, exerting a force with a component tangent to the gear periphery. Thus, a torque is transmitted, and because the gear is rotating, power is transferred. Gears are the most rugged and durable torque transmission devices. Their power transmission efficiency can be as high as 98%, but gears are usually more costly than other power transmitting machine elements, such as chain or belt drives.