US Patent 6988432 Multi-tiered-recess screws

A screw and driver system includes a range of screws and a driver. Each screw has a head having a driving recess in its surface for engagement by the driver. The recess of larger screws in the range have a plurality of superimposed recess tiers of decreasing size with increasing depth from said surface. Each tier has substantially parallel driving surfaces and torque applied to the driver is transmitted to the screw through the driving surfaces. At least one tier of at least smaller screws in the range has a rib. Engagement of the driver with the recess causes deformation of rib, and hence creation of an interference fit of the driver in the recess.