Tension Fitting Anlaysis - Gapping and Bolt Strength

[Stezza]

Hi All,

I’ve been recently analyzing a tension fitting with pre torqued bolts.

I haven’t seen anything on the effect of shim’s when checking Gapping. Do shim’s have a detrimental effect on the gapping load? Any references to look at?

Also, when I got up to checking the bolt itself, the Stress Memo I had been following, stated “When the tension fitting faces are in contact, shear loads shall be combined with only the External tension loads in the analysis of the bolt.”

Now, the pre-torque puts the bolts into a significant amount of tension, up to about 75% of ult tension capability when you allow for variation in frictional resistance and other safety factors.

My first concern was that the Stress Memo direction seemed non-conservative in the case where the External tension load was less than the tension pre-load.

However, if the Test Setup that the shear-tension allowables are generated from included pre-tensioning of the bolts before testing began, this would effectively “include” the pre-tension effects and leave the External tension as a variable in the test.

What do you guys think? Is this correct, or is there another explanation?

Regards
Stezza

 

[rb1957]

IMHO shims have a minor affect on preload, i'd include the shim thickness with the thickness being bolted together (in determining the bolt/joint stiffness).

with the faces in contact I'd have thought that most of the shear force would be carried by friction between the faces (rather than the bolt). The other extreme is when the joint is gapped, and so all the shear (and all the tension) is carried by the bolt. This is the typical design point, since the external tension is higher than the preload.

obviously the shear capacity of the bolt is significantly reduced by the preload, but the shear capacity of the joint is increased by the preload (increasing the friction between the shear surfaces). i think your stress memo is indicating that the the design condition is a gapped bolt. you should verify this is the case, with your design tension load, but if your faces are still in contact, then a portion of the shear can be carried by the friction between the faces.

clear as mud ?

 

[MintJulep]

Embeddment and relaxation are both functions of the number of mating surface pairs within the joint.

So, depending upon things like the surface finish and material of the shims, shims will cause an increase in preload loss due to embeddment and relaxation, which will in-turn change the amount of load carried by the bolt.