Shear Fatigue in bolted Connection

[yojo]

I am wondring if there is an endurance limit to adjust the ultimate tensile strength of bolts in fatigue shear loading. I am using Se=.5*Sut for Sut<200ksi developed from the Rotaing-Beam test for tensile fatigue loading.
I am designing a frame that will hold a disc brake system to stop a 1000hp GE Mill motor. I have already reduced the Sp(74000kpsi proof strength) by half Sp=.5*Sps to get a Shear proof strength. I do not know if I have to further reduce my proof strength do to the nature of the Repeated loading conditions (20400lb).

Any help would be great
Thanks

 

[yojo]

Sorry I stated the formula incorrectly
Sps (shear proof strength) = .5*Sp (Proof strength)

 

[CoryPad]

Do the joint members actually bear against the fastener, or is there a clearance hole and the preload provides restraint? There are few joints that have cyclic shear stresses that lead to failure.

Regards,

Cory

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

 

[yojo]

Yes it has a clearance hole and I think you are referring to alignment pins and dowels that are usually in pure shear but I know our maintenance department and I fear they will not tighten the bolts with the proper pre load to prevent the bolts from seeing a cyclical shear load. Do you recommend a different analysis approach?

Thanks

 

[desertfox]

Hi yojo

Is the joint in pure shear or is the force offset to the joint which would cause the joint to rotate.

regards
desertfox

 

[CoryPad]

yojo,

My experience is with joints that have high preload to withstand the applied forces.

A recent SAE paper deals with fatigue of joints with low preload. It should provide some insight for you:

http://www.sae.org/servlets/productDetail?PROD_TYP=PAPER&PROD_CD=2005-01-1320

Regards,

Cory

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

 

[Tmoose]

Is the load reversing? Without enough preload it will wiggle to death. Sounds like a good application for nylock type lock nuts ( to prevent loss total bolt ), LONG (L/D > 6) bolts to better maintain some preload after some joint wiggling and fretting, and field fit welded stops to handle the shear if the bolts do loosen. I'm starting to think many 10 mm bolts get tightened "better" than a few bigger ones.