Fasteners subject to Bending 1

[IreneAdler]

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

 

[drawoh]

IreneAdler,

[ol]
[li]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.[/li]
[li]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.[/li]
[li]In a highly stressed cantilever, you avoid stress raisers such as screw threads, especially at the fixed end where the high stresses are.[/li]
[/ol]

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.

--
JHG

 

[rb1957]

screws can work in bending, they're not very good at it, and most times the moment is carried by the preload into the surrounding structure, rather than bending the screw itself (as explained above).

can you elaborate on your situation ?

another day in paradise, or is paradise one day closer ?

 

[IreneAdler]

So here's a picture of the scenario. The space between the principal material and the fastened plate is one inch. Won't the fastener be subject to bending? Also, most codes I look at doesn't seem to show any criteria or consideration for bending. It's always shear or tension loads. This has been a long time question to me. What are your points of view ?

 

[Jboggs]

Irene, If the two surfaces pulling against each other are also touching each other then the loading on the screw is close to perfect shear. The opposing forces are in line with each other, or close to it. There is little force pulling the screw thread out of the tapped hole. By separating the two loads you create a larger moment effect causing a much greater twisting action on the screw, thus pulling the thread out of the tapped hole. Bottom line - keep your opposing forces either in line with each other or balanced by some other means.

 

[Tmoose]

Hi IreneAdler,

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.axd?AssetID=12219&type=full&servicetype=Inline

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

 

[drawoh]

IreneAdler,

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

 

[rb1957]

I suspect you're showing us a small detail of a larger installation, and a larger view would show us more. How large are the loads ? how stiff the PVC block ? could it react the bending and then the fastener only feels the shear ? I hope there's a nut somewhere (hopefully not on my side of the screen) and you're not pushing shear over a threaded shaft ?

another day in paradise, or is paradise one day closer ?

 

[desertfox]

Hi IreneAdler

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/Techni...ner Theory and Application/Joint-Diagrams.pdf

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[flash3780]

While bolt bending is generally small, for thin flanges it can be a significant consideration.

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.