Engage large threads without cross threading 1

[Philrock]

I'm designing an axial adjuster for positioning the outer ring of a tapered roller bearing, to control end play. One method for doing this is to use shims under the retaining flange. My client and his customers have been doing this for decades and have found it very awkward, and have asked me to come up with something better.

The bearing outer ring o.d. is 7.125". I am considering making a threaded adjusting ring to adjust the position of the bearing outer ring. The adjusting ring would have 7.375-8 male threads which would engage with female threads in the housing. However, I am always leery of large threads that will be assembled by hand, because it is so easy to cross thread them. I thought it might help to have a cylindrical male pilot diameter that registers with the minor diameter of the female threads before the male and female threads actually start to engage, but I'm not sure this would be enough to assure reliable thread engagement.

Any suggestions regarding ways to help assure reliable thread engagement would be appreciated.

 

[geesamand]

I've considered that concept but never moved forward for a different reason: how fine the adjustments are for a typical tapered roller bearing. Take the total tolerance for bearing endplay, divide that 8tpi and calculate what fraction of a turn encompasses the entire range of bearing setting. Then decide how you're going to keep it there. I haven't tried actually building this kind of setup, but that adjustment issue had me lose interest.

 

[GregLocock]

Zero run in taper on the threads, maybe even broach the end of the thread away so it just stops dead. Then when you run it backwards there'll be a distinct click as the thread engages.



Cheers

Greg Locock


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[MintJulep]

Geesamand is right on. Sometimes there is actually a reason why something has been done the same way for decades.

 

[MikeHalloran]

Differential threads would allow construction without fine threads.

Kinematically, think of a steamboat jack, with two differences from classical form:
Instead of right and left hand threads on opposing ends of the adjusting spindle, both threads would be of the same hand.
The threads would be of slightly differing lead.

I.e., the adjusting ring would be double ended, with 7.375" threads as described, except one end would have a lead of 0.1250", and the other end would have a lead of 0.1240" or maybe 0.1230", depending on how much range of adjustment you want.

Distributing the tolerances will be fun....



Mike Halloran
Pembroke Pines, FL, USA

 

[Philrock]

Guys,

Thanks very much for your input.

Geesamand - speed and running accuracy running requirements are low, so end play allowance is fairly generous - it amounts to about 29 degrees of rotation of the adjuster.

Greglocock - I like your idea - I'm going to give it a try. I'm also going to mark the parts where the threads end, so the assembler can line up the parts to immediately start a clean engagement.

 

[tbuelna]

It would likely be more expensive to produce a set of mating precision ground threaded parts for adjusting the tapered roller bearing preload than it would be to provide a set of shims and a retainer flange.

Also consider that a threaded retainer would need to incorporate some sort of device to lock it after adjustment. One benefit of using shims and a flanged retainer is that no adjustment should be required if the bearing was simply removed and reinstalled, while a threaded retainer would require readjustment.