Comments on: Bolt Shear Strength Considerations

By: matt

matt — Wed, 08 Dec 2010 00:13:52 +0000

if you are using a bolt in a high movement/lateral stress area and it keeps breaking

you might try using either lots of washers or thick spacers. so you end up with a longer bolt.

examples of this can be found on exhaust manifolds on cars.

the longer bolt means when the items flexs the angle is less than that of a shorter bolt. sometimes a super tight bolt hole can cause you more problems than benefits. but obviously this is job specific.

By: Gilbert

Gilbert — Fri, 17 Sep 2010 06:43:52 +0000

It is true that grade 8 bolts are more brittle than grade 5. In general (although not always the case), steels that have a higher strength are less ductile. Grade 8 bolts have a higher strength and, therefore, would require fewer bolts in a joint than grade 5. This may be of benefit in joints that are tight for space. However, depending on the application, it may be preferable to use the weaker and more ductile grade grade 5 bolts. The statement that grade 8 bolts are better than grade 5 cannot be generalized. Consult with an engineer (structural or mechanical) about your particular application.

The shear strength of bolts is taken as approximately 60% of the tensile strength. This is not a rule of thumb. A large number of tests on high strength bolts tested in shear has indicated that the shear strength is about 62% of the tensile strength. A purely theoretical relationship between shear strength and tensile strength indicates that the shear strength is 58% of the tensile strength. In practice we use 60% as a good approximation of test results and theory.

By: ben

ben — Fri, 23 Oct 2009 18:18:52 +0000

Herb, you will be better off with a grade 8.

By: David Benyon

David Benyon — Tue, 19 Aug 2008 22:48:58 +0000

There is a UK company called Bolt Science who have computer programs on bolt tensile strengths (there used to be a free time limited demo as well!) There is also a rule of thumb that puts shear strength as 60% of tensile strength. Put the two together and voila! Note however that antique solid mounted diesel engines can wreak havoc on their mounting frames and won’t be the least impressed with ones clever calculations! Salt can also give unexpected results as tests done a great many years ago on chains showed that they were weaker when immersed in seawater. Ideally the bolts should only be loaded to about 10% of their failure load, possibly 20% on a rough job.

By: Adam Oakley

Adam Oakley — Fri, 11 Jul 2008 17:55:59 +0000

Thanks for adding the helpful link, Eric. Even though this is from an automotive standpoint, it looks like there is some good information for industrial and construction applications.

By: Eric

Eric — Fri, 11 Jul 2008 15:30:17 +0000

I found this, which might help.

http://www.rockcrawler.com/techreports/fasteners/index.asp

By: Greg Lindsay

Greg Lindsay — Thu, 24 Apr 2008 15:50:48 +0000

@Herb: Due to liability issues and given the fact that Portland Bolt does not have any engineers on staff, we are not in a position to answer questions regarding the use of specific quantities, sizes and grades of fasteners for a specific application. Please consider consulting a structural engineer for assistance with this question. Sorry we couldn’t be of more assistance.

By: Herb

Herb — Thu, 24 Apr 2008 12:58:26 +0000

When a bolt is used in a sheer load is it better to use grade 5 or grade 8? I’ve heard that grade 8 bolts are harder and therfore more brittle. Assuming a good fit with no slop in the assembly or a slightly loose fit – say 25-30 thousandths.