Torque Values for Retaining Rings?

[rhmeng]

Does anyone have a good reference for acceptable torque values for retaining rings?

For fasteners it is pretty straight forward to calculate the stress on the screw for a given torque, and there is a lot of information online about it also. An example is on this site:
Link Some of our screws are vented, and we just calculate the new cross sectional area (because of the vent hole in the screw) of the screw and then this changes the stress (F/Area).

One of my colleagues mentioned (jokingly) that a retaining ring is just like a screw with a large vent hole.. I think that the limiting factor for retaining rings will be the stress in the threads and not the axial stress in the cross sectional area of the retaining ring. I am not sure how to go about calculating what we should use for torque values.. We were thinking of getting a force gauge and just testing it ourselves. Any body have ideas?

 

[SwinnyGG]

Threaded retaining rings are a very interesting case in fastener design.

When you use a retaining ring to secure a component in a pocket, you are basically using whatever housing design is present as the tension member in the system- the retaining ring functions as a nut would, and is governed by the same rules- failure is controlled by shear stress in the thread, not axial stress in the main body (as you have already stated).

If the goal is to apply accurate preload with a retaining ring, you have a tricky problem on your hands- because to do that, you really need to know how stiff your enclosure or housing is along the axis through which the ring applies tension. You can get half way there by calculating simple applied forces due to torque and the thread pitch, but it's also important to remember that a retaining ring usually has very large diameter threads, so frictional losses are much, much larger than they would be for a nut-and-bolt assembly, meaning that the simple calculation method is not going to be accurate.

The simplest way is *probably* to measure directly, using either a force gauge, or measuring the change in length of a part based on torque applied to the retaining ring- with the caveat that this 'test part' must be simple enough that its strain rate can be very easily and accurately calculated.

 

[rhmeng]

thanks for the input. Yea I think we are going to have to go with the force gauge measurement. We have severalll different optical systems/trains/lens assy's that use retaining rings. The retaining rings all hold different masses and different diameter optical components, on top of this each optical assy has its own shock/vibe requirements. So to get this right we would have to calculate required holding forces so the optics dont rattle, and then use a force gauge to measure what the required torque is for that optical train. I was just hoping that there was an easier way..

 

[desertfox]

Hi rhmeng

I've done some calculations on retaining rings and yes I've based it on the threads being sheared.
Formula's for thread shearing can be found at

http://www.roymech.co.uk/Useful_Tables/Screws/Thread_Calcs.html.

Now regarding the axial load I've used the general formula F= T/(0.2*d) and ignored the hole in the centre of the ring.

So basically in my application the retaining ring is holding a plug in place which is subject to pressure on one side, so step 1 I calculated the axial force on the plug from the pressure and then used this force to stress the threads using the formula on the roymech site above.

Finally I used F= T/(0.2*d) to determine the minimum required torque for the ring.

hope this helps

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