bolt/axle pretension and bending moment / prying 2

[daven123]

Hi. I am reviewing the stresses on a cantilevered axle with threaded shaft as shown in the first attachment, including comparing to formulas used for bolts. Not including pretension there is a bending moment in the axle due to horizontal forces keeping the axle in position. Is the bending moment nullified by the pretension? Reviewing how pretension works in bolts it appears that there is no added load on the axle due to the F.x force unless/until the F.x force would exceed the pretension force (causing separation). If that is true there would be no moment on the axle since the pretension forces are symmetric about the axle shoulder (bolt head) and about the axle nut. Specification for Structural Joints has a few sentences about limiting prying on bolts, but does not define prying. Introduction to the Design and Behavior of Bolted Joints shows prying as eccentric separation of the pieces being jointed by the bolt, but for an axle there is only a single member between the shoulder (bolt head) and nut. It appears to me that prying is different than the moment caused by these horizontal forces. Attached is also the information I found regarding pretension. Thanks for the help.

 

[CoryPad]

For practical purposes, the bending moment is nullified by pretension. This assumes relatively high pretension forces (kilonewtons, even for small thread sizes), and relatively low applied forces (kilonewtons only for large items like vehicles, bridges, etc.).

If you need to analyze this better, use VDI 2230 Systematic calculation of high duty bolted joints. There is software available for this as well.

I question the location of your F.x and L. How did you determine those? How about identifying the moment created by W and the distance from W to the joint center, where you would sum to zero. Then you can create a substitute eccentric force F from that moment. That is the prying force.

 

[daven123]

Hi CoryPad. Thanks for the reply and for the information. Attached are more detailed calculations, assuming a pretension equal to twice F.x to replace/balance the moment. Right now I'm using distances of between 2/3 and 3/4 of the actual lengths for the effective distances of the axle shoulder and the hex nut. Unfortunately we don't have VDI 2230 here and it looks like it's many hundreds of dollars. It will be a while to get approval to order it (if it's approved). Do you happen to know of any other printed showing pretension in relation to moments? Thanks!

 

[dhengr]

Daven123:
Your sketches look like a cam roller with a support plate which will not be strong enough to support the roller and the induced moment. What are you really trying to do and trying to prove?

 

[daven123]

It's a wheel axle attached to a frame. The sketches for clarity do not show the full width of the frame, etc.. If the pretension replaces the moment with just axial stresses (in addition to the shear stress from the weight), it allows the wheel to handle more weight than if the moment is still included in the calculations.