1
: the main driving wheel or gear of a machine, usually the largest and strongest wheel in the train of mechanism, in some machines (such as a shaper) transmitting the power received by the machine to the various mechanisms, in others (such as a tractor) delivering to the driving wheels the power transmitted to it from the motor
2
: a drum on which a rope is wound for hauling or lifting something (such as logs or well-boring tools)
3
: a wheel attached to the boom of a derrick at the base and used to swing the boom about a vertical axis
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Merriam-Webster unabridged
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