Lead screw overthrust protection

Claim 1

In apparatus having a movable member, a drive including a rotatable shaft threaded to said member and operative to move said member when rotated, power means for rotating said shaft, mechanism for protecting the apparatus from damage due to excessive overloads when movement of said member is obstructed, said mechanism comprising means supporting said shaft for axial movement, resilient means locating said shaft axially in operative position and preventing axial movement of said shaft in both directions from said operative position under ordinary operating conditions when movement of said member is not obstructed and said shaft is subjected to axial loading below that which is excessive, said resilient means including a first spring means acting axially in one direction against said shaft and in the same direction against a first fixed abutment, said resilient means also including a second spring means acting in the opposite direction against said shaft and in said opposite direction against a second fixed abutment, said shaft during rotation being caused to move axially from its operative position against the force of one of said spring means when movement of said member is obstructed to create an excessive overload but without assist from the other of said spring means the force of which is blocked by the fixed abutment against which it acts, braking elements spaced axially in relation to said shaft on opposite sides of a part on said shaft, one braking element or the other being engaged by said part on said shaft when said shaft moves axially from its operative position to stop said shaft from rotating, and yieldable brake support means for said braking elements resisting with a yielding pressure movement of said braking elements when engaged by said part on said shaft.