American Fastener Journal - September/October 2020

Hex Tap Bolt vs. Hex Tap Screw

Kevin Connolly 2020-09-16 02:11:04

Can a Hex Tap Bolt be used interchangeably with a Partially Threaded Hex Cap Screw?

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 cap screws (or hex bolts) is 60% of their minimum ultimate tensile strength. Designers prefer to use a partially threaded hex screws 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; therefore, the body provides a higher shear strength than the threads. On average, the load required to shear a cap screw is about 1.2 to 1.5 times higher across the body than it is across the threads, for coarse threaded hex cap screws 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, the use of a partially threaded hex cap screw provides the same shear strength that a fully threaded tap bolt provides.

In answer to the original question, a partially threaded hex cap screw should be used in applications where shear forces are exerted across the shoulder, and either a hex cap screw or a hex tap bolt can be used in applications that encounter tensile loads.

KEVIN CONNOLLY

Kevin Connolly received his bachelor’s degree in Mechanical Engineering Technology from Cleveland State University. He has 38 years of experience in the fastener industry and is currently Earnest Machine’s resident Applications Engineer. Email Kevin at kfc@earnestmachine.com, or call 1-800-EARNEST.

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Hex Tap Bolt vs. Hex Tap Screw
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