Application of fastener preload in tension/shear interaction 1

[jetmaker]

Morning folks,

Looking for some opinions and thoughts here.

I have a situation where I am using bolts/screws c/w a nut securing 2 parts together. The joint experiences a combination of shear and tension loads. Now the bolt tension and shear allowable data that I have is for the bolt itself, and does not specify mating nut.

A lead in another group does not believe that I should be including the preload when I do a fastener tension/shear interaction equation. I tend to disagree.

Any opinions?

jetmaker

 

[desertfox]

Hi jetmaker

Look at this site:-

http://www.roymech.co.uk/Useful_Tables/Screws/Bolted_Joint.html

regards desertfox

 

[jetmaker]

desertfox,

I've looked at the site, and not sure how it is of use in answering my question. I know that it is conservative to apply the preload directly to the applied load during ultimate loading. Often way too conservative. Therefore, when this is the case, I use a method derived by Wileman to calculate the incremental load on the bolt when subjected to preload.

However, the question still lies in whether it is appropriate to include preload in the calculation of total bolt load when comparing to published tensile allowables for a bolt. With Hi-loks/Lockbolts, this is resolved in that the tensile capability is based on a specified collar/fastener combination which includes preload.

Thanks.

jetmaker

 

[CoryPad]

How is the shear force applied to the fastener? Does a mating component bear against the fastener and create a shear stress?

It is appropriate to consider fastener pretension and applied shear stresses in combination. There is information on this in NASA RP1228 Fastener Design Manual, Handbook of Bolts and Bolted Joints, and ASTM STP 1236 Structural Integrity of Fasteners.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[jetmaker]

CoryPad,

Thanks for the reference. I think I understand it now. For a pure tension load, one does not include the preload in the ultimate analysis as the preload is always below Ptu, and once the bolt load exceeds the preload, then the bolt sees just the external applied load.

Now for tension and shear acting together, it would be prudent to include preload as one would expect to get different joint strengths if the preload were varied and a constant shear maintained assuming a fictionless faying surface. I guess the only reason not to include preload would be to assume that the greater the preload, the less shear load the fastener would see due to friction.

jetmaker

 

[CoryPad]

Yes