ASME Clamp Bolting Material - Allowed to lower the allowable stress?

[sema79]

Hi everyone.

I am a structural engineer but have been working at a mechanical firm for years. When things get slow I sometimes work on the Mechanical side under the direction of the MEs.

I have been working on a clamp design governed by ASME Section VIII Div 1 Appendix 24. By code I am told to use the bolt material allowable stress at room temperature. This may sometimes result in a very high assembly condition bolt load when high strength bolt materials are used. I have been directed to lower the allowable value which will increase the required bolt size. This at first seems ok but there are other calculations (Hub longitudinal stress) that rely on the assembly condition bolt load. If I lower the allowable stress then the hub bending forces will lower as well. I have read through the code and it doesn't make sense to lower the value. On the other hand I am not exactly sure about the intent of the code as it applies here. Maybe someone knows if/why this approach is allowed? Thanks in advance!

 

[prex]

You can always use an allowable stress lower than the maximum allowed for any material. You must however be consistent with everything else: so the bolt tightening load should be adapted to the lower stress, otherwise you risk to shear the hub or overstress the clamp.
Note that only the shear stress in the hub, not the bending due to hydrostatic end force, is controlled by the assembly bolt load.
If controlled tightening is not usually employed for your clamps, this doesn't seem to be a good idea, though.

prex

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