Threaded Retaining Ring Design

[Gumpmaster]

I have an annular retaining ring that's holding in a spherical bearing. The spherical bearing has some side load that will get transferred into the retaining ring, so I'll need to design the ring to take that load. The specifics are a 7" OD, 6" ID retaining ring holding in a 6.5" OD spherical bearing. the side load is ~15 kips.

Is there a design guide or example that would help me in the design of the retaining ring? The failure modes I can think of are 1)striping the threads; 2) torquing the annular ring and disengaging the threads; 3) torquing the annular ring and tearing it. Any other failure modes?

Thanks.

 

[MikeHalloran]

Losing axial preload, followed by fretting failure of the threads, or the nut loosening until it falls out.


Mike Halloran
Pembroke Pines, FL, USA

 

[DiamondDave]

Hi Gumpmaster,

i'm not familiar with your 15kips do you mean kilo pounds per square inch?

i've seen similar things on a much smaller scale in lathes for example. Could you manufacture something with a course thread that would screw in against the bearing and then perhaps lock it with another ring? Either manufacture from Hex material or make a ring with notches cut into it to allow a C-spanner to tighten it?

 

[Gumpmaster]

Kips is our structural engineer speak for kilo pounds. It's our half-hearted nod to the metric system.

I had a recommendation to provide a star pattern lock washer (obviously custom fabricated), that should work to prevent backing out of the threaded ring.

 

[Gumpmaster]

Thanks for the additional failure modes Mike.

 

[MintJulep]

I had a recommendation to provide a star pattern lock washer (obviously custom fabricated), that should work to prevent backing out of the threaded ring.

No, it won't.

 

[tbuelna]

Gumpmaster,

Can you provide a sketch and more detailed loading conditions? For example, with combined radial and axial loads on the spherical bearing, if the radial force is substantially greater than the axial force it may produce enough friction at the bearing OD/housing contact to negate the axial force.

From your description it sounds like the retaining ring has a 6.5" thread(?). If so, with such a large thread diameter even a minimum engagement of only 2 or 3 pitches would provide more than adequate root shear area for a 15,000# axial force, even with low strength materials.

As for locking the threaded retaining ring, large diameter (ie. 6.5") threads naturally have exaggerated geometry errors like run out, etc. Thus they tend to have large running frictions and have an inherent "self-locking" effect. Another suitable method for locking the threaded retaining ring would be staking.

Hope that helps.
Terry

 

[chicopee]

About using tapered bearings?

 

[mfgenggear]

like terry said staking then thread locker,IE locktite 232

Mfgenggear

 

[Tmoose]

If the 15 kips is an occassional or static axial load that is one thing. But if it dynamic, especially if it is caused by rotating machinery i would want to secure the bearing OD with a slight interference, and as Mike H said, reliable preload >> than the axial load. Methods that merely prevent sleeve un-rotation will not be sufficient long term.

http://www.boltscience.com/images/boltf6.jpg

 

[Gumpmaster]

The rotating part of this is quite low speed, 1 rpm or so. the thrust load is a seismic load (conservatively caclulated).

The bearing itself has an ANSI LT2 fit.

tbuelna, the radial force is ~180 kips, so it is substantially greater than the thrust load. Unfortunately, during the seismic event, there may be some vertical acceleration which could negate some of the radial load. Sorry, no sketch.

Thanks everyone for your advice.

 

[Cockroach]

Drill and tap a hole above the thread of the nut to retain a brass set screw. This will lock the nut down and keep it from backing off. Typical in oilfield assemblies where backing out is a concern from vibration or left hand torque. I occasionally put a small solid teflon plug into the hole and then come down with a set screw on top of that if concern for thread damage, maintenance applications are of concern.

But very easily solved without the use of Locktite, which must be heated in order to disengage the nut. Doesn't make sense if you need to service the piece in the future. Anything but Locktite....

Regards,
Cockroach