bolt/joint stiffness ratio & preload 1

[horstr1]

I found a good discussion at thread725-79154 that's close to answering my question, but not quite a conclusive answer.

I am working on a bolted joint where the joint is only about 4 times stiffer than the bolt. (The 'active' length of the bolt is about 4 diameters, but about half of that is compressing a sleeve whose sectional area isn't much more than the bolt's.) It has a pretty high working load that's almost purely axial to the bolt. I'm trying to figure out how strong my bolt needs to be, to apply enough preload so that the working load doesn't overload the bolt. It seems like kind of a chicken-egg thing.

I've been trying to work out an algebraic relation that'll help me converge on the answer, and had the sudden thought that I may be inventing a wheel. Someone must have done this before, so I came here.

In the referenced thread, israelkk used an example with a 10:1 stiffness ratio, and 1/10 of the applied load went to the bolt. At 2:1 it was 1/2. Is it that simple? Assuming it's below the joint separation point, is my increase in bolt load 1/4 of the applied load?

Also, if the bolt is going into a tapped hole, how much of that bolt length is part of the 'active' length of the bolt?

I know I can get this worked out, but I know there is a lot of bolting experience represented in this community, so I would appreciate any guidance towards a quicker, more confident solution.

 

[CoryPad]

Preloaded bolted joints act like springs in parallel. The equation for the amount of external force added to the screw is:

and can be found here:

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

So, if the joint is 10 times stiffer than the bolt, the extra force added to the bolt preload is 1/(1+10). If your ratio is 1:4, then 20 % of the external force is added to the bolt preload.

You can back calculate this by determining the required preload after external forces have been applied. Use the corresponding equation for remaining joint compression:

VDI 2230 states that the active length for engaged threads is 0.5d, where d is the thread major diameter.