Bolt preload vs failure in shear 1

[Chiwahua]

We have a case where we have a bolt that fails in shear.

Am I wrong to think that:

the higher the tension in the bolt, the less it takes to be able to shear the bolt?!

For example:

Bolt 1 is torqued at 600 lbs.ft
Bolt 2 is torqued at 200 lbs.ft

Let's pretend the friction due to the clamping force doesn't exist, it would takes less force to shear the bolt 1, right?

Thanks a lot for your help!

 

[rb1957]

that feels odd to me ... I see the bolt under tension, initially from preload then increasing with applied tension until the joint gaps (consistent with relaxed preload) and all the applied tension is now carried by the bolt.

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

 

[hydtools]

rb1957, the bolt failed in shear.
I don't understand how you see additional tension being applied to the bolt. Unless you see the pivot pin seriously bending and prying the collar into an ob-round shape through which the bolt runs.

Ted

 

[rb1957]

I was replying to the general situation, as the previous post discussed.

In this case, of course, there is no additional tension from the load. In this case I see no mechanism to relieve the preload (as suggested by the previous post).

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

 

[3DDave]

My first guess is that it is fatigue failure. Since the holes and the pivot are not reamed for a close fit one end will see the majority of the load and partially fracture, allowing enough motion for the other end to take the load and then fracture. Eventually they even up, taking turns and advancing the crack until one end completely fails, and then the other is too weak and it also fails.

It looks like side loading on the arm could provide a significant prying load, overcoming any lubrication. The outer bores should probably be the bearing surfaces, allowing the pin to be fixed to the arm and simplifying lubrication.

Having seen similar pin applications, I can say that the grease passages are very likely inadequate to clear the thickener when attempting to introduce new grease, that the volume of grease involved is inadequate to store enough oil, and that what grease does manage to get in will not be well distributed. The grease to the center will end up as solidified thickener and the grease jockey will see grease take the short path right out the ends, making a mess and not lubricating the joint.

 

[seaaggie]

My apologies rb1957, I did not include enough context for the Kulak quote.
The explanation is in regards to pre-loaded bolts under pure shear testing. Tension pre-loading to levels near yield did not reduce the pure shear failure loading.

Thanks,
Mark

 

[rb1957]

'k; I wonder why the preload relaxes under shear loading ?

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

 

[hydtools]

It could be that the shear deformation increases the effective length of the bolt enough to reduce the preload stretch.

Ted

 

[AWDMIKE]

While I glossed over some of the middle of this thread, I caught the end of it and wanted to throw something out there.

Some time ago I had a discussion with a structural engineer on anchor bolts / anchor rods who mentioned that under a shear load the effect of Poisson's ratio was to elongate the bolt, thereby relieving a bit of the preload / pretension in the bolt. I hadn't thought about that until our discussion.

Somewhat related, I found a paper that address lap joints and the effect of longitudinal loads on the preloads. I didn't read the entire article, but it seemed like the Poisson's effect of pulling at the lap joints reduced the thickness slightly enough to reduce the preload.

Link