Loosening preventive screw

摘要

A cross-recessed head screw has in its head a tool engagement recess including first releasing surfaces which are adapted to be engaged with generally flat front edges of an end of a cross-headed tip screwdriver and have the function of moving the screwdriver backward when the screwdriver is turned in the reverse direction for loosening the screw. The tool engagement recess further has second releasing surfaces each contiguous to a corresponding one of the first releasing surfaces and serve to guide the backward movement of the screwdriver as the latter is turned in the screw-loosening reverse direction. By virtue of engagement between the first releasing surfaces and the front edges of the screwdriver, the reverse rotation of the screwdriver produces only a small friction torque which is quite insufficient to cause rotation of the screw in the screw-loosening direction. The first releasing surfaces project from the second releasing surfaces toward the axis of the tool engagement recess, so that when a screwdriver one size smaller than the specified screwdriver is inserted into the tool engagement recess, a tip end of the one-size-smaller screwdriver comes into abutment with only a small area extending in the vicinity of inner edges of the respective first releasing surface. With this small contact area provided between the tool tip and the first releasing surfaces, the screwdriver is likely to slip or turn freely when it is rotated in the reverse direction.