Power tool

Claim 1

A power tool, particularly a power screw driver, wherein the transmission between a drive and a tool holder is automatically terminated when a predetermined torque is exceeded, comprising a housing accommodating a drive, said drive comprising an air motor and a valve connecting said air motor with a source of compressed air and being movable to open position against a biasing force; a tool holder rotatably mounted in said housing; a rotatable component permanently connected with said drive to be rotated by the same, said rotatable component being formed with a transverse bore accommodating a plurality of ball members, and with a longitudinal bore intersecting said transverse bore; and means including coupling means for connecting said tool holder with said rotatable component in torque transmitting relationship and for terminating the transmission when the torque exceeds a predetermined value, said coupling means including a plurality of spherical coupling elements mounted on said rotatable component at fixed circumferentially spaced locations for rotation with said component, a plurality of circumferentially spaced axial recesses formed in an axial end of said tool holder which faces said coupling elements, and said connecting means includng further biasing means urging said coupling elements into respective ones of said recesses, a rod which holds said valve in open position when said torque is below said predetermined value, said rod being axially movable in said longitudinal bore of said rotating component in response to said torque exceeding said predetermined value so that said biasing force can move said valve to said closed position, a member surrounding said rotatable component and being formed with a stepped bore having a portion of larger and a portion of smaller diameter, said member being slidable axially of said rotatable component against the action of said biasing means from a position in which said smaller diameter portion of said bore surrounds said transverse bore and urges said ball members inwardly of the latter to block sliding of said valve rod, to a position in which said large diameter portion of said bore surrounds said transverse bore and said valve rod displaces said ball members radially outwardly of said transverse bore so as to be slidable in said longitudinal bore, said spherical coupling elements constituting the only drive connection between said component and said tool holder.