Importance

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Abstract

A screw-type lift having a ratchet lifting nut assembly providing for the safe lowering of a load while preventing additional lifting of the load upon failure of a lifting nut is disclosed. The ratchet lifting nut assembly includes a lifting nut and an auxiliary nut threadably engaging a lifting screw and a wear gap separating the lifting nut from the auxiliary nut. A ratchet portion is supported on the auxiliary nut and is aligned for engagement with a pawl supported on the lifting nut. During a normal mode of operation, the lifting nut supports the entire weight of a load and the ratchet portion and pawl cooperate to restrain rotational movement of the auxiliary nut relative to the lifting nut. A failure mode of operation is defined when the lifting nut is excessively worn in that it can no longer safely support the load. In this mode, the wear gap is eliminated wherein the auxiliary nut is supported on the lifting nut and the load is supported solely by the auxiliary nut. The ratchet portion and the pawl cooperate to restrain rotational movement of the auxiliary nut relative to the lifting nut in a first direction permitting the lowering of the load, while the auxiliary nut is permitted to rotate relative to the lifting nut in a second direction preventing the raising of the load. Therefore, when the lifting nut fails, the load may be safely lowered but the ratchet lifting nut assembly will prevent any further lifting of the load.

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First Claim