Fastener with multilobular internal recess and tool entry ramps.

摘要

A recessed drive socket (20) in a fastener (10) or the like, is driven by a mating, drive tool (14) received within the socket. The socket cavity (20) has a multilobular cross-section defined by an alternating, annular series of adjacent socket flutes (24) and socket lobes (26) extending parallel along a general central axis (28) from an entrance opening (21) into the cavity circumscribed by a peripheral support collar. Each socket lobe (26) is truncated by a guide surface (27) inclined from the opening (21) at a radially outer edge of an adjacent socket flute (24) into the cavity to form a tool clearance recess portion (29) of the guide surface (27). The guide surface (27) extends from the tool clearance recess portion (29) to a termination (31) of the guide surface within the cavity axially recessed from the cavity opening (21) in order to provide cammed guidance of drive lobes (23) on the drive tool (14) against the inclined guide surface (27) into the respective socket flutes (24) and to prevent driving engagement of the drive lobes (23) with the socket lobes (26) until the cammed guidance results in axially extended depth (D) of the drive lobes (23) into the socket flutes (24) for efficient and balanced torque transmission.