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# Help with CBUSH Fastener Flexibility Value Calculation

Valued Contributor

Hello,

I am in need of some clarification from experienced stress engineers in the aero industry about right procedure to employ for calculation of fastener flexibility values for Nastran CBUSH entry in case of double shear joints. I also posting the question here in this forum as well!

I am posting an image which hopefully will illustrate my question.

So essentially my questions are as follows:

1. Is it an accepted practice to use two CBUSH elements to represent a fastener under double shear (like the above) in industry?

If yes, then should I use:
(a). n=1 (i.e. single shear condition) for each CBUSH element and consider thickness of one of the plates as 't/2' during calculation of fastener shear flexibility using Huth equation?

(b). n=2 in Huth equation for each CBUSH element?

I understand the above question in specifically not Femap related and the answer may be proprietary but I am hoping for some guidance.

- B

2 REPLIES

# Re: Help with CBUSH Fastener Flexibility Value Calculation

Legend
1. Yes, it is acceptable to use two CBUSH elements to represent a fastener.

b) Huth formulation, n=2

There has been a few developments in fastener representation which you could baseline off, using the Rutman modelling technique: https://web.mscsoftware.com/events/vpd2007/na/presentations/tech_papers/8.pdf

Where the actual bearing and rotational bearing stiffness are calculated per layer and the bending and shear is handled by a CBEAM element. We have found that locking the axial DOF in the outermost CBUSHES simulate the head/nut well enough.

# Re: Help with CBUSH Fastener Flexibility Value Calculation

Valued Contributor

^^Sir. Thanks a lot for your response.

Definitely provided the needed clarification.