Bolt Group Analysis

[dengmech]

I'm looking for a theoretical treatement of bolt group analysis. I have seen allot of examples / theory on off axis shearing forces on a bolt group, although I am looking for an analysis where you would have a bolt group and you pull on it in tension eccentrically, and then calculate the resultant bolt loads. I believe the same method is used whether the applied load is off axis shear or off axis tension, although I have never seen a design example with off axis tension. I believe the two methods are the elastic method and center of rotation method.

 

[canwesteng]

You are correct, instantaneous center of rotation and elastic analysis are the two methods of analysis if there is a torsion on the bolt group. Elastic analysis is simpler but more conservative. Instantaneous center of rotation does not apply if there is an eccentric tension though.

 

[desertfox]

hi

This might help

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

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Jamesg83]

The application of the elastic method is similar for bolt groups in torsion and bending, both require the neutral axis to be found initially. However, the moment of inertia calculations are slightly different.

This method, whilst easy to calculate, is conservative. The principal of superpositioning is assumed for the translational and rotational actions of bolt groups in torsion when in reality these actions are dependent on each other. A method known as the Instantaneous Center of Rotation was developed to take this into account. A good description of the two methods can be found here. Link

A web based software tool can be used to find these forces for random bolt groups. This software can analyse both pre-loaded and non pre-loaded connections. It uses the more conservative elastic method for calculations. See link below.

Link

 

[Jamesg83]

One more thing to mention. If you're wanting to calculate off centre tensile forces you'll need to calculate the moment induced by said force to use with the software above. To do this you'll need to find the neutral axis in both x and y directions. For bolt groups this can be considered the arithmetic mean position of all bolt coordinates for each axis. The moment is then found by multiplying the force by the distance to the neutral axis.