Including preload in shear/tension interaction 1

[jetmaker]

Looking for some comments regarding the following.

A bolted access hatch exists on a bulkhead which is subjected to both shear and internal pressure loads. The hatch is considered to be structural such that shear loads are transfered across it.

The internal pressure is such that it produces a tension load in the bolts. The bolts are preloaded to 50% of their proof strength to prevent hatch separation and pressure leakage. There is no gasket between the hatch and bulkhead per design.

My question is this:

When checking the bolt capabilities, is it too conservative to use a bolt interaction equation in which the shear load and preload are combined together? Comments?

My feeling is that if the preload exists, some of the shear load will transfer to the hatch by friction rather than through shear of the bolt shank. What I think is more appropriate is the use of the actual tension load due to the pressure rather than the bolt preload.

Thanks,
jetmaker

 

[UcfSE]

Check out this link. It helped me understand the effects of pretensioning on bolt behavior. It's not exhaustive by any means but is a nice little explanation.

http://www.unified-eng.com/scitech/bolt/clamping.html

 

[CoryPad]

You need to determine if the joint is shear-friction or shear-bearing. If shear-friction, the bolt doesn't see any shear force until the joint slips. Regardless of configuration, you need to account for bolt pretension and external forces that increase the bolt tension.

Regards,

Cory

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

 

[BATman2]

Jetmaker,

I agree with CoryPad. If it is not clear what the situation is,as is often the case, I would check both scenarios to be sure.

The preload you noted sounds low to me. Is there a particular reason for 50%. Typically 75% of proof is used.

Also check out:

http://www.boltscience.com/pages/basics3.htm

It's a little more detailed than UcFse's link, but not by much.

Batman2

 

[UcfSE]

Holy pretensioned bolts BATman, nice link

 

[desertfox]

Hi jetmaker

I would calculate the combined stress due to the pressure loading and shear loading (ie the principle stress).
I would then use this stress figure to size my bolts and select the bolt material. The pre-load that the bolts recieve would be based on the tensile loading due to the pressure altough I would make sure the cover couldn't slip
due to self weight. I agree with Corypad the bolts won't see shear untill cover comes into direct contact with the bolts.


regards desertfox

 

[jetmaker]

Thanks all for the replies.

BATman, what is the justification for applying a preload of 75% proof? My understanding is that one should preload the bolts only to the necessary load to prevent separation, or to keep it above the alternating stress. Anything above that is just gravy.

Thanks,

jetmaker

 

[BATman2]

Jetmaker,

First, know that I come from a "fit-and-forget" environment, where frequent inspection can not be depended on.

Yes, gravy against loosening unless you know the actual loads accurately and can control torque application very accurately. Basically, I would say you want the highest preload achieveable within the limits of the joint, bolt and working loads to ensure nothing loosens.

In terms of design, if a smaller bolt gives the required preload (with a factor of safety, working loads, etc.) at 75% proof, there could be a cost and weight savings.
Again, it must be within the limits of the working loads and checked carefully. Check out J. Bickford: "An Introduction to the Design and Behavior of Bolted Joints. ISBN: 0-8247-1508

Regards,
Batman2

 

[Screwman]

Good reply Batman. The 75% of Proof generally gives an adequate saftey margin if the service load is such that it utilizes the full capacity of the fasteners. Any additional strength that is available is excess (if you are dealing with a known service load environment) and gives you a good chance to down size the fastener.
One of the interesting things to check is just how much load carrying capacity even a single fastener has. The vast majority of 'commercial' joints are grossly over designed for the service loads. Of course one of the problems in the 'real world' is knowing precisly what that service load is going to be; so we all over design to compensate for the depth of the pot holes that may be in the road.

Dick

 

[BATman2]

Screwman,

Thanks. In our business (rail) we tend to over-design for sure out of necessity to ensure no failures and because the real loads are not always truly known.

Interesting discussion from all.

Usfse - nice BAT pun.

Batman2