Captive screws When the protective cover or inspection cover of a machine/device is removed, captive screws will not drop out (conforms to CE marking, etc.). Also suitable for covers that are frequently removed as well as for machines related to food and packaging. Many types are available, including those with thinned thread roots and those on which rings (washers) and pins can be attached. The head is also available in hexagon socket head, hexagon, knurled, button, truss, low head, wing bolt, etc.