Preload adjustments

[FC2008]

Hey,
I'm looking at some equations for torque and tension in fasteners and have a few questions.

The preload adjusment is to include the effects of the preload torsion to the axial tension requirements. (von Mises stress)

Does this mean that the adjusted preload should be used to calculate the torque required, or will this lead to too large stresses in the bolt?

I hope the question is somewhat clear, thanks.

 

[desertfox]

Hi FC2008

The normal procedure is that once you know the preload you require within the fastener you calculate the torque based on the required preload with friction factors to account for
lubrication of the fastener and any plating or surface finish of the fasteners in question. During tightening the maximum stress in the bolt shank is given as a resultant of the combined torsion and tension within the bolt or screw however once the spanner is removed the stress in the bolt is purely due to the axial tension ie the preload.
Have a look at this site it may help:-

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

regards

desertfox

 

[CoryPad]

What do you mean by adjusted preload?

When determining the required torque, you input the required tension to meet joint requirements. This required tension is a function the external loading, the settling losses, and thermal effects (if any).

Regards,

Cory

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

 

[plasgears]

One factor that needs attention is the relative coef of expansion of the clamped system. The bolt may have a greater expansion coef, therefore you may require more preload.

This is best analyzed on a constructed stress strain diag for the bolt. A bolt providing more expansion under temp will provide more relative expansion, and on the SS diag you will track the relaxation of room temp stress to a revised stress at temp. If this relaxation can be lived with, then accept. If not, plan to preload more to acct for bolt expansion under temp.

 

[FC2008]

CoryPad: The preload adjusment is to include the effects of the preload torsion to the axial tension requirements. That's what the machinery handbook says..

desertfox: Why is there then a section on preload adjustment in the machinery handbook, is it used for something clever?
Thanks

plasgears: nice, did not think of that! thanks

 

[desertfox]

Hi FC2008

Haven't got a copy of the machinery handbook but the preload adjustment chapter appears to demonstrate how to include friction etc to achieve your desired preload.
At least thats how I read it from your response to Corypad.
There are some formula that are more approximate than others
for calculating preload torque, so you might consider adjusting your preload from one calculation method to more accurate one. Alternatively you may make an initial tightening of a joint and later tighten up to the final preload thereby you could say your adjusting the preload.

Regards

desertfox

 

[CoryPad]

I just looked at the Machinery's handbook. The preload adjustment is the combined tension and torsion during assembly.

You use the tension value you require in the joint for the torque-tension equation. Then you use the von Mises equation as a check to insure that the combined stress during tightening won't overload the screw. VDI 2230 shows how to do this, as does Bolted Joint Engineering. You can find out about these references here: faq725-600

Regards,

Cory

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

 

[FC2008]

Thank you CoryPad, very nice FAQ as well.