Member slippage/ bolts in bearing? 10

[DJL Eng]

I am trying to determine bolt stresses from a resulting shock load. A component is bolted to a mounting plate and the mounting plate is bolted to a fixed structure (see attached sketch). The system sees some acceleration due to shock. We cannot bolt the component directly to the structure since it would require significant modifications and a design change (negative impacts to cost, schedule, yada yada...). Assume symmetrical loading, bolt patterns, and components. The qty. 12 bolts sit in counterbored holes to prevent contact with the component.

I am mainly concerned with the stresses in the qty. 12 mounting plate bolts. I determined that the bolts are sized such that a preload could be applied to provide sufficient clamping force (thus friction) to prevent slippage of the members. I then treated this like a cantilevered beam and the fixed end is the mounting plate bolted connection. So, due to symmetry of loading and geometry, I assumed half (qty. 6) of the bolts would be in tension. I only solved for stresses due to tension in these bolts after finding bolt/member stiffness.

A coworker of mine says that I should account for shear as well. He says that due to possible variations in workmanship, the bolts could be in bearing when installed and the clamping force would not help in this case. I disagree, but I have been wrong before; what do you guys think?

 

[DJL Eng]

Again, many thanks for this discussion, y'all. I feel like I have more than enough information to go forward.
 
@Tmoose

Currently the design calls for 12 bolts/ nuts and through holes. We have not yet machined the bolt pattern in the structure.

@SWComposites

Lol, I'm pretty surprised at the amount of feedback I'm getting on this thread. Leave it to a bunch of engineers to pummel a problem down to submission.
Thank you for the reply. The breakdown helps me understand the logic behind your previous post.

@dvd

I could add shear dowels if I can't get a bolting pattern to work for my loading scenario.

@desertfox

Thanks for that link! Yup, I think we beat the dead horse here.

 

[desertfox]

Hi DLJ Eng

You’re welcome, one more thing I would add if you haven’t considered already is ensure the clamped components can take the bolt preload without yielding, from my own experience I have checked lots of designs where the bolt preload as caused the clamp material to yield and when that happens you never get the preload specified.

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

 

[dik]

thanks, fox... good article.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[desertfox]

Hi dik

You’re very welcome

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