Bearing area under a washer 3

[jball1]

When calculating the bearing area under a washer, I was taught to assume that the washer extends the diameter of the bearing surface past the diameter of the fastener head by twice the thickness of a washer (i.e. a 45 degree cone extending from the OD of the fastener head through the thickness of the washer). I was taught that this approach applies if the washer is 3/16 or less in thickness, but if it exceeds 3/16, you can assume that the OD of the washer is the OD of the bearing surface. Does anyone else use this method? Do you have a reference for this method (I can't find one)? Do you use a different method?

Thanks!

Andrew

 

[CoryPad]

The 45 degree cone is a traditional approach, which can be found in Handbook of Bolts and Bolted Joints. More recent analysis shows the angle to be smaller and varies with thickness and diameter. A detailed account is included in VDI 2230 Systematic calculation of high duty bolted joints.

 

[jball1]

Thanks CoryPad. Exactly what I needed.

 

[Compositepro]

The little purple (pink?) star at the bottom of the post is for showing appreciation. I gave one because I learned something useful and because CoryPad's answer exemplifies what eng-tips is all about.

 

[racookpe1978]

Technically, a very thick "washer" is going to start behaving like a punch: It will not deform under the compression load from the bolt (which is always applied by the bolt well within the ID of the washer, then allows the washer to deflect "up" away from the base metal as the washer deflects under extreme load.)

Thus, a thin washer will bend (a little bit) under the center load from the bolt, but that outside ring of the washer will deform the base metal very little. A thick washer will try to "cut" the ring that it touches of the base metal, and so there is no justification for the 45 degree cone of extra area.

 

[drawoh]

Search McMaster Carr for belleville springs. A while ago, I analyzed one of those things, and the compression force approached the yield strength of the screw. We now understand that spring washers do not lock screws down, but this could be a good way to distribute the force of your screw.

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JHG