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Definition of CRANK
: a bent part of an axle or shaft or an arm keyed at right angles to the end of a shaft by which circular motion is imparted to or received from the shaft or by which reciprocating motion is changed into circular motion or vice versa
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 c. 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.