crank, mechanical linkage consisting of a bar attached to a pivot at one of its ends in such a way that it is capable of rotating through a complete circle about the pivot. One of the principal uses of a crank is to turn reciprocating, or back and forth, motion into rotary motion or vice versa. A bell crank is one designed to change the direction of a linear motion.

In mechanics, an arm secured at right angles to a shaft with which it can rotate or oscillate. Next to the wheel, the crank is the most important motion-transmitting device, because, with the connecting rod, it provides means for converting linear to rotary motion, and vice versa. The first recognizable crank is said to have appeared in China in the 1st century AD. The carpenter's brace was invented circa 1400 by a Flemish carpenter. The first mechanical connecting rods were reportedly used on a treadle-operated machine in 1430. About this time, flywheels were added to the rotating members to carry the members over the “dead” positions when the rod and the crank arm are lined up with each other.

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