Lobular screw with means for improved stress distribution

摘要

A screw, such as a self-tapping screw, comprises a threaded shank with a lobular cross-section and adapted for engagement with a mating thread of circular cross section. At the head end of the screw there is a bearing surface for pressure engagement against a member to be retained by the screw. This bearing surface has alternately arranged, circumferentially disposed recesses and ridges and with the recesses being in line with the lobes. Upon tightening of the screw at high torques the ridges engage the member to be retained and the recesses cause relief from stress concentrations at the lobes by permitting axial movement of the screw so as to increase the pressure flank areas of engagement of the screw and the mating thread. A more positive locking engagement of the thread is also provided. The bearing surface may be integral with the screw head or may be provided on a washer on the screw.