Rubber pad mount bolt torque value

[1952mike]

I seem to be having a fine time calculating bolts tightening torque value for the following situation:
A 1000 lbs rectangular box sits on two rubber pads of 1" thick and 4" wide x 18" long at the ends of box.
There are (4) 1/2" bolts (2 each end) that need to be tightened, but the max allowed rubber stress is 50 PSI and fasteners are lubed (approx. K=.12).
Pads are continuously supported by base foundation and the box (sandwiched in between).
How can I determine what torque value to tighten these bolts so the max stress is not exceeded?

Thanks in advance for any advice.
Mike

 

[rb1957]

what area of the rubber pad is reacting the applied bolt load ?

50psi sounds mightly low ... what happens to the rubber if you subject if to 51 psi ? does it crumble to dust ?? or does it compress ??

what load will the 1000 lbs box put on the rubber pad ?

 

[1952mike]

The pad is sandwiched between a foundation and the rigid box and is "completely" supported (sandwiched) by the foundation and the box (so the bolt load will be distributed almost equally).
Rubber is 40 Durometer. If more than 50 PSi is applied it just compresses and its isolation eff. is affected negatively with compression.
For a pad of 4 x 18 = 72 sq. in area & 500 lbs half the box weight will be about 7 PSI.
I hope I answered all your questions.

Mike

 

[rb1957]

yeah, i expected it would just compress (so 50psi isn't a hard failure). Assume a fastener compresses 1in2 of pad (a dia of about 1"), so you want about 50 lbs preload. T = Pd/5 = 5 in.lbs (boy that looks "silly" on a 1/2" bolt, probably not much more than finger tight !)

what's the vibration environment like ?

 

[1952mike]

Yes, I also imagined torque value will be low, that is why I thought to post my question to make sure I am not making a mistake.

Currently the box is mounted rigidly on the foundation and is cracking at the bolted area due to high vib.

If we use rubber pads, we will have to use lock nuts & loctite:-D

Thanks anyway!

 

[MintJulep]

It really doesn't matter. The rubber will take a set and all preload will be gone in a short time.

 

[Tmoose]

Rubber on one side of the flange preloaded by a bolt compressing from the other isn't really an "isolator."
If isolation from vibration is required something more like this is needed.

http://www.antivibrationmethodsrubber.co.uk/assets/images/autogen/a_Bolted_Through_Isolators_2.jpg

Rubber on both sides, rubber surrounding the body of the thru bolt and keeping it away from the flange, controlled compression (metal sleeve to limit bolt compression) and a specific stiffness to tune the system's resonant frequency relative to the troublesome vibration frequency.

 

[dhengr]

1952mike:
I think you should rethink your problem. The rubber pad will change the frequency and amplitude of the box vibration, but it probably won’t solve your box failure (cracking) problem. The bolt loads won’t be distributed very evenly to the rubber pads or the box, they are fairly concentrated, and that is exactly what is causing your low cycle failure (cracking of the box). But, now you sill have hard box, hard ½" bolts, to hard found., with a spring btwn. the box and found. You say nothing about the design of the box, pl. thick., joinery, location of bolts wrt side pls., etc. and the box is what’s cracking and failing. I think you should look at the design of the box, and more specifically how you put the bolt loads into it. While I know nothing of the box design and its C.G. v. vib., etc.; if you could distribute the bolt loads, in shear, up into the sides; rather than in bending into bottom pl., you would probably solve your problem. Move the bolts further apart will lessen their loading due to vib., then tighten down so box amplitude never completely unloads any bolt. You always want some prestress left, and rubber which might take a permanent set may not be your best choice. For example; put the bolts through a piece of 3" long 2" sq. tube, and weld that to the sides in the corners of the box, with a 4x4" block of rubber under each corner.

 

[1952mike]

Thanks for your reply.
That is right, we have added load distributing large area steel pads to distribute bolt head concen. load to a larger area, and some other changes.
My only intention for posting this was to obtain info on bolt torque so not to exceed rubber compressive load limit.

Mike