Allowable bearing stress due to preload

[jball1]

I have always considered that a bearing stress due to preload (sigma_brg = P/Ab, where P = preload and Ab = area under head of fastener or washer) below yield was acceptable. In other words, a safety factor of 1.0 (sigma_yield/sigma_brg). I have always treated bearing due to preload differently than bearing due to applied loads. In the case of bearing due to applied loads I use 1.5*sigma_yield as the allowable stress.

However, I do not have (and cannot find) a reference for allowable bearing stress due to preload. Is 1.0*sigma_yield the allowable stress that other people use? Or do you use a different allowable stress? Maybe 1.5*sigma_yield for both preload and applied loads? Does anyone have a reference for what they use (specifically in the case of preload)?

Thanks!

 

[desertfox]

Hi Jball1

I also treat bearing stress under the bolt head in the same manner as you but I've never found a reference either.

 

[Tmoose]

"TIGHTENING TORQUES. At the tightening torques listed standard Holo-Krome alloy steel 1960 Series screws, used under the conditions
described, will be preloaded to approximately 75% of the tension induced at yield. The bearing stress under the head at these preloads
will be approximately 80,000 psi, so indentation should not occur when the parts clamped are of steel or cast iron with a hardness equal to or in excess of Rockwell B 85. With softer materials, washers may be required under the heads of the screws to avoid indentation."

from page 32 here -

http://www.holo-krome.com/pdf/consolidated_tech_manual.pdf

for those thinking metric page see page 83

 

[CoryPad]

The best reference is VDI 2230. That standard does not compare contact pressure vs. yield strength, it shows actual measured values for allowable contact pressure, and provides estimates based on strength and hardness for a variety of materials. In general, allowable contact pressure is approximately equal to the tensile strength of the material, due to constraint of the surrounding material. You can get some additional insight from analysis of plane strain indentation.

 

[desertfox]

Hi

I found this which is related to VDI 2230 mentioned previously:-

5.3 Surface Pressure at the Bolt Head/Nut Bearing Areas
Surface pressures capable of causing creep processes (time-dependent plastic flow) giving a loss in
preload (relaxation) should not be allowed in the bearing areas between the bolt head or nut and the
clamped parts, either as a result of the preload or as a result of the maximum service load. Thus, the
surface pressure that is calculated from the maximum load should not exceed the compressive yield point
of the clamped material.
If creep cannot be avoided, estimation must be made of the remaining preload after the end of the
relaxation process.
The initial preload must then be set high enough so that the remaining clamp load guarantees safe
operation of the joint.
Experimentally determined limiting surface pressures are given.
If washers are used to reduce the surface pressure, care must be taken to ensure that they have
sufficient strength and thickness.
Information on the computation of limiting surface pressures is given in Section 4.7 (calculation step R10).

 

[tbuelna]

The actual bearing stress at the bolt/nut face contact can be a bit tricky to determine accurately. You must consider the worst case max/min tolerance conditions of the bolt/nut face and the mating hole. And even a slight amount of angular misalignment at the contact surfaces can significantly increase local bearing stress levels. If the intent of your analysis is to ensure there will be no bearing yield at the bolt/nut/flange contact faces, then an analysis FoS of 1.5*fastener preload is probably a bit low, due to the large uncertainty in the actual bolt/nut/flange bearing area the preload is applied over.