Acme Nuts - Back Nut 4

[MDGroup]

With traditional threads (UNF/UNC), you sometimes install a second nut behind the first nut to lock everything in place so the first nut doesn't loosen or back off at all.

I am looking to use acme threads on a project because they should ride much smoother. I want the first nut to be tightened into position, and need to ensure that it doesn't back off at all as the device is being used.

Can I put a second acme nut behind the first to lock it in place? Does it work about the same as standard threads? The acme thread seems to ride easier, so I am worried that it would back off pretty easily and not hold its position.

I was considering pinning it in place, or using detents or a set screw, but that gets pretty complicated and would require additional pieces. If I can just back-nut it to firmly lock it in position that would be great.

Anyone have any experience with anything like this??

Thanks.

 

[MikeHalloran]

The phrase "ride easily" suggests you are using it as a leadscrew, in which case you might be better off to machine a shoulder on each end and build the assembly more like a machine tool, with controlled dimensions and/or shims.

Using another regular Acme nut as a jacknut may not work well, because of the helix angle. I'd definitely drill and pin something if I went that way. Maybe a pin diametrally through a nut and shaft, maybe axially through both nuts after torquing. I definitely would not trust a setscrew.



Mike Halloran
Pembroke Pines, FL, USA

 

[JohnRBaker]

I agree with Mike, I don't think it's practical to use a 'second' acme 'nut' to jam another one. Acme threads, by definition, are not designed to 'retain' parts but rather to 'position/drive' them.

John R. Baker, P.E.
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[KENAT]

There have been many threads about jam/jamb nut & double nutting/second nut.

thread725-229669
thread725-55085
thread725-188550

Some links given in some of the threads include:

http://www.boltscience.com/pages/faq.htm#15

http://www.du.edu/~jcalvert/tech/jamnut.htm

I'm not sure about your second sentence John, I've seen ACME used on a number of locking ring applications where very high loads would be expected - arguably in lieu of true buttress threads.

However, I don't explicitly recall seeing ACME jam nuts - but I'm not convinced they wouldn't work.

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[JohnRBaker]

My comment was made because there is, as you mentioned, a 'Buttress' thread form which I think would be more appropriate for this sort of application (perhaps I should have mentioned it then).

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

 

[MDGroup]

Thanks for all the good info.

My application is very similar to the way you set the depth gauge on a drill press.
The nut needs to be adjusted frequently, so we cannot use any thread locker or nylon patch to hold it in place.
There is not a lot of pre-load on the first nut when it is tightened.
The first nut essentially just establishes a position and I was hoping the second would hold it in place.

I think the double-nut would work, but don't know if the acme thread would behave the same as standard threads.

 

[KENAT]

Is it a simple 'nut' or is it a locking ring or similar?

I've seen locking rings which then had set screws (or similar) set through the ring parallel to the thread axis to lock them off.

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[ornerynorsk]

A simple method, similar to the one Kenat just described, and one I've seen employed on older machine tools, is simply to drill and tap a setscrew hole perpendicular to the main thread axis in the nut, and simply use a very short segment of plastic, brass, or wood dowel rod and a thumbscrew to effectively lock the position. If inadequate clearance exists for a thumbscrew, a standard setcrew will suffice.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[MDGroup]

It is a machined locking ring I am working with, not just a simple nut. It can be designed with whatever features I want to use.

I think I run with all of the ideas and try them out to see which works best.

Design the locking ring with holes in the sides to accept thumbscrews. There are plastic thumbscrews that shouldn't damage the acme thread (and I am not dealing with much load at all anyway).
If the thread is enough by itself, remove the thumbscrews and be done with it. If not, use the thumbscrews as set screws to hold it in place.
If that doesn't work, run a second nut on the back of the locking ring and see if it holds.

The buttress thread looks very interesting. It looks like it might be more of what I want; easy to go in one direction, but resists motion in the other. I will probably try that instead of the acme.

Thanks to all for the comments.

 

[MikeHalloran]

Ah.
Features I'd like to see in a locking nut:

A jam nut built in, to take out the normal lash, and compensate for wear. Maybe manually adjustable and secured with a setscrew, maybe automatic and preloaded by a spring.

A very light detent, at a fine pitch, so it goes click-click-click as it's rotated. This might comprise an axial knurl on the male thread crests, and a ball detent in the nut.




Mike Halloran
Pembroke Pines, FL, USA