Thermal Bolt Pretension

[jfymsam86]

Hi
According thread799-338002
Bolts are A490 and 1.25 in. So A=7.91 cm^2
Pt-bolt= 138.7 kips= 62900 kg
E=2.05 e6 kg/cm^2
I’m going to define 70% Pt = 44000 kg
I defined orthotropic expansin as:
0, 0, 2.1e-6
So AEalpha=34.08 and then T= -1290 degrees
So in predefined load I assigned -1290 for surface of shanks in the first step.
(Direct specification> constant through region> Ramp> and propagate for 2nd step)
But they provide the Force only 8000 kg instead of 44000 kg at the end of the step (according to free body force)‼
What’s my fault?
Should I define a bolt load and a Predefined load together for a bolt?
thanks

 

[jfymsam86]

when we use thermal pretension for bolt, the narrowing will be removed, so it's good.
but why it won't provide the desired pretension?

thanks in advance

 

[TGS4]

It won't provide exactly what you want, because of the compliance of the entire joint. In other words, you are imposing a prescribed displacement on the bolt, not a prescribed load. Owing to the stiffness of the entire joint system, you are going to get less than what you expect for load.

 

[jfymsam86]

Hi
Thanks for your reply.
First I defined pretension in BOLT LOAD. But bolts got narrowed during the analys. then I imposed thermal pretensin as i told above and the result of above.
I need to impose 70% of capcity of bolts to them as without any large narrowing. also I expect when I get output as free body diagram, the minimum load after imposing pretension, shows the 70% that I imposed?
what is the best way for it?

 

[TGS4]

I prefer the thermal pre-tension method. However, you need to iterate to find the over-tightening that you need to achieve the "actual" bolt stress.