Can a Hex Set Screw be used instead of a partially threaded Hex Head Bolt?
April 5, 2022
In applications where the working loads are exerted in line with the length of the bolt (called tensile loads), both a fully threaded and a partially threaded fastener are going to be able to support the same load. If the working forces exceed the strength of the fastener, both of these fasteners will fail in the threads at the same load.
The cross-sectional area (the amount of material that supports the load) is smaller across the thread as compared to across the body (also called the shoulder or unthreaded length). In applications where the bolt is subjected to just tensile loads, having a shoulder on the bolt does not provide a significant benefit.
Applications where there are shearing forces (the working loads are exerted across the bolt) are the ones where the presence of a shoulder will provide increased strength–as long as the shear loading is across the shoulder.
The shear strength of grade 5 and grade 8 hex head bolts is 60% of their minimum ultimate tensile strength. Designers prefer to use a partially threaded hex head bolts in applications that encounter shear loading. The best designs make sure that any shear forces are exerted across the body and not across the threads. The body has a larger cross-sectional area than the threads and therefore the body provides a higher shear strength than the threads. On average, the load required to shear a hex head bolt is about 1.2 to 1.5 times higher across the body than it is across the threads, for coarse threaded hex head bolt in diameters from 1/4” to 1 1/4”.
Note that in applications where shear forces are encountered and the shear loads are exerted across the threads, then the use of a partially threaded hex head bolt provides the same shear strength that a fully threaded hex set screw provides.
In answer to the original question, a partially threaded hex head bolt should be used in applications where shear forces are exerted across the shoulder, and either a hex head bolt or a hex set screw can be used in applications that encounter tensile loads.
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