by Larry Borowski, President/Engineer; Greenslade & Company, Inc.
Mechanical fasteners (screws, bolts, nuts, rivet, etc.) must be both dimensionally and physically compliant to their applicable specifications to perform properly for end-users. Unfortunately, many fastener suppliers only evaluate the dimensional size of the parts they supply and ignore the required physical tests that evaluate the strength and fastening performance of the parts.
The torsional strength test is one of the most widely specified physical tests in screw specifications. The torsional strength test determines if a screw has adequate strength to perform its intended function when put into final use. In most screw specifications in which the torsional strength test is required, the parts are not required to be tested for tensile strength. The torsional strength of a screw determines its ability to resist being twisted into two pieces where as the tensile test determines a screw or bolt’s ability to resist being pulled from end-to-end into two pieces.