Retaining screws (securing covers on cases)

The retaining screw is a simple screw with a reduced shank. It is used to secure a cover on a case. Many times during disassembly screws are lost or mixed between them with different lengths causing headaches to maintenance workers. Retaining screw main advantage compare to other common screw used for this purpose is that it remains on the cover. The screw hole on the cover is half drilled to a diameter larger than screw nominal diameter. Drilling and tapping the other half (with the same screw thread) but with a larger than standard minor diameter of the internal thread allows to complete remove the screw only if we unscrew it from the cover also (see figures below).

Retaining screws can be purchased from standard stock (DIN 7964 - Hexagon bolts with reduced shank) or they can be made accordingly to design intends. When securing parts with the DIN 7964 retaining screw always consider a minimum distance A between screw’s round slot abutment on cover and case’s face (see figure above). The A distance must be larger than retaining screw’s thread length in order to disassembly.  If we don’t like the idea of make threads and round slots on the cover (movable part) we can use another solution. Instead of making a thread we can use a threaded part. The threaded part (usually a cylinder) is fitted in a hole drilled vertical to the retaining screw’s hole (see figure below). In order to assembly the retaining screw we must screw it first through the threaded part and after with the corresponding threaded hole of the case. In this case it is much easier to make a side round hole than a half threaded hole.