Set screws are designed to provide torsional holding power in a compressive force application. The tighter they are applied, the more holding and less prone to vibration they become. If excessive torque is used during these applications, reaming or cracking of the socket can occur. Hence there is a need to perform torque testing to standard specifications to achieve the recommended torque per ASTM F880 which is used for stainless and ASTM F912 is used for alloy.
High hardness values are required for alloy set screws during heat treatment. It is imperative that a balanced heat treating is achieved that will result in strength of the socket. Too hard and the socket could crack, too soft and the socket may ream under the stress of the hex or spline bit used during testing.
Set screws should never be used under Tensile applications. Using as a stud or with a nut will place tensile stress on the set screw which opens the door to cracking, fracture, and failure due to the high hardness range to which it is heat treated.
A typical torque test fixture for testing socket set screws would involve a torque wrench with the proper socket sized bit, applying force in a vertical manner to a test piece installed in a threaded testing block. A backing screw is inserted in the opposite end of the block to provide a hard stopping point for the set screw being tested. Once the test reaches the minimum torque recommended by the specification, the set screw is considered acceptable.
Since this is a destructive form of testing, approval is based on acceptance sampling of the entire lot and not performed at 100 percent.