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Mechanical device that transmits intermittent motion or permits a shaft to rotate in one direction but not in the opposite one. Reversible ratchets are used on socket wrench handles and are convenient for tightening or loosening bolts in positions where a complete revolution of a wrench handle is impossible. They are used in mechanical jacks to lock the jack rod after each successive lift.
This entry comes from Encyclopædia Britannica Concise. For the full entry on ratchet, visit Britannica.com.