Fasteners subject to Bending
Fasteners subject to Bending
(OP)
Hello smart people. I need your help. I've heard that screws should not be subject to bending loads, but I don't know where to reference that idea. And could you please explain why?
Thank you and best regards,
Irene Adler
Thank you and best regards,
Irene Adler





RE: Fasteners subject to Bending
- In an ideal joint, the screw is tightened very close to and even slightly over its proof stress such that an external load on the joint will affect the force between the clamped faces, only. The bolt will see constant force and stress. This eliminates any possibility that the bolt will fatigue.
- On a structural joint loaded in tension, the screws provide the least cross sectional area. All else being equal, the screws are the weak link in your design. If strength is critical, you need to maximize strength by, among other things, not bending the screws.
- In a highly stressed cantilever, you avoid stress raisers such as screw threads, especially at the fixed end where the high stresses are.
I am sure there are some other good reasons.For a given application, any structure can be made big enough that it will not break. A cantilevered screw is not an efficient structure, but you can do the analysis and make it work.
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JHG
RE: Fasteners subject to Bending
can you elaborate on your situation ?
another day in paradise, or is paradise one day closer ?
RE: Fasteners subject to Bending
RE: Fasteners subject to Bending
RE: Fasteners subject to Bending
I'd say if the clamping provided by the fastener is sufficient to keep the plates and spacer from sliding, and the two plates don't bend significantly then the fastener will see no bending.
The tendency of the fastened plate and principal material may be to align, as in the lower example here -
http://www.twi-global.com/EasysiteWeb/getresource....
As mentioned by others that will tend to increase tension on the fastener due to prying, and make the loading around the spacer very unbalanced and likely create further difficulties
RE: Fasteners subject to Bending
Does the spacer have to be PVC? The ideal way for this joint to work would be for you to clamp the screw down hard enough that everything would be held by friction force. In this scenario, the screw sees only tension. I would expect the PVC to creep and loosen the joint.
I appreciate that for safety analysis, it is assumed that the screw is loose.
--
JHG
RE: Fasteners subject to Bending
another day in paradise, or is paradise one day closer ?
RE: Fasteners subject to Bending
Joints very often are subjected to offset loads and in the diagram you've posted its looks to me that the bending would be negilable compared to the shear but we would need more information and dimensions within the joint in question.
Now if you preload any joint sufficiently to withstand an offset external tensile load then the joint behaves pretty much as described by others, in that the compressed members relieve some of there compression which exists between the compressed faces but in doing so the bolt also has an increase in tensile stress although this increase is usually small.
The reason the bolt stress increase is small, is due to the fact that the clamped faces usually exhibit a stiffness of 4 or 5 times that of the bolt and therefore dissipate the majority of the external force.
I've posted a link with information about bolted joints
http://viewmold.com/Products/Technical%20Reference...
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Fasteners subject to Bending
Joint prying can impart bending into the bolt head. This can be a consideration for bolt fatigue. Bolt tensile load does not change much due to external joint loads, however prying can impart bending into the bolt which cycles with each joint loading/unloading, even for a non-separated joint.
John Bickford's book "An Introduction to the Design and Behavior of Bolted Joints" has a discussion of prying and a methodology for estimating its impact.