ACME thread vs. UNC threads

[steris]

I am designing a system to translate rotary motion from a motor to linear motion. For my application I can't use a rack/pinion or pulley/cable, the only thing that works is a screw gear arrangement.

I am looking at the trade-offs between using a ball-screw, ACME threaded rod with ACME nut or UNC threaded rod with UNC nut. I don't need precision or repeatability. I have a small load (only about 200 lbs). The same 6" of thread will be traversed up to 10x/day.

The ACME threads and ball screws are much too expensive especially since I don't need precision or large load capabilities. I am leaning towards using a UNC threaded rod with nut 2"-4 because of cost but I am worried about wear. Are there other options that offer decent wear resistance without high cost? Also is there anything I can do to minimize the binding between the nut & screw?

Any advise would be greatly appreciated! Thanks in advance!

Best,
-Steris

 

[CorBlimeyLimey]

Materials?
Lubrication?

 

[steris]

If I was using UNC threaded material, it would likely be Grade 5 steel and a thick oil-based grease.

 

[mpcgage]

Can't help you there.

 

[CorBlimeyLimey]

Use a bronze or even brass nut to minimse binding?

 

[steris]

The brass or bronze nut would work fine. Thanks for the suggestion CBL!

About the wear though, I know that ACME threads have better wear resistance than UNC. Does UNC perform very poorly? Does anyone have experience with wear on UNC threads when used for linear motion?

Thanks again!

-Steris

 

[BobM3]

Can you get rolled or ground threads that size? The finer surface finish might keep the friction (and wear) lower. An anti-seize lubricant might also help keep the friction lower

 

[KENAT]

Do you really mean UNC or just some kind of unified 'saw tooth' thread form.

We use small unified thread form, high pitch screws with brass boss's and stainless screws for adjustmens, a few of them motor driven I believe.

However, ours are sizes like 1/4-80 UNS-3 or similar for precision, doesn't seem like this is your requirement.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[dinjin]

I would use undersize threads on the rods or oversize
nuts to help reduce any binding.

 

[IceStationZebra]

the ACME threads will work much better in the presence of debris or dust, won't gall like normal thread.

ISZ

 

[lumenharold]

I have had good luck with Kerk anti-backlash nuts and lead screw assemblies from these people:

http://www.haydonkerk.com/Products/LeadscrewsAndNuts/tabid/68/Default.aspx

It has been a few years but the last time I used them they were less expensive than the ball screw equivalent.

Harold
SW2009 SP2.0 OPW2009 SP0 Win XP Pro 2002 SP3
Dell 690, Xeon 5160 @3.00GHz, 3.25GB RAM
nVidia Quadro FX4600

http://www.lumenflow.com

 

[adrag]

If I remember correctly, some of the Chinese mini lathes use cut metric threads on their lead screws. It was strongly suggested to deburr it before running the lathe and then it supposedly lasts forever....or until something else breaks. Seems like an similar application to your but with potentially greater load. They use brass or bronze split nuts.

Al

 

[Wrenchbender]

Steris,
Either thread form would suffice but I would vote for Acme (ref ANSI B1.5), which is traditionally used for your specific purpose. Manufacturing costs for any thread form in that size range should not be much of an issue. Only difference would be in the profile of the cutting tool insert. External thread can be chased; internal is best milled on a CNC.

Although I cannot readily back this up with references, I believe Acme is a slightly stronger thread form than UN, more wear resistant (because of the steeper flanks), and mechanically efficient. I also like Acme threads because the flat crests on the external thd are more resistant to accidental impact abuse.

I would suggest an alloy steel rod, quenched & tempered, with a softer nut (e.g. steel, beryllium-copper, bronze) because the nut portion would probably be easier and less expensive to replace than the rod. Hope that helps. Also, lube du jour and a phosphate surface treatment on the steel part(s) wouldn’t hurt. (BTW, 4-pitch seems rather coarse for that diameter; try something like 2.000-8ACME-3G.)

 

[steris]

Hi everyone. Thanks for all the suggestions!! The surface finish will definitely play an important roll in the friction and wear. While precision ground or custom machined threads would be better. I was hoping to use an off the shelf component.

Kenat - yes, I am using UNC as a generic term for saw-tooth thread profile.

Adrag - that is really interesting to learn. I will have to research that. Do you know any of the manufacturers or lathe sizes?

Bestwrench - I agree that ACME is of higher quality however it seems that purchasing ACME threads vs UNC (saw tooth) is significantly more expensive. If we were manufacturing in house it would be a different story. Thanks for the other suggestions.

Once again - I really appreciate all the help! Thank you so much.

 

[drawoh]

steris,

Are ACME threads really that expensive?

You can buy ACME threaded rod, and you can buy ACME taps. Look in the McMaster Carr catalogue.

JHG

 

[Cockroach]

Typically the Acme threads are used for high load applications, something you stated that you don't need. I would choose the UNC threading geometry.

I always case harden threads by liquid nitration, 0.005 inch to 0.008 inch depth. This hardens the threads to about HRc 45/55 which is advantageous to mitigate metal-to-metal transfer mechanisms like wear, galling, thread pickup. The process also mitigates against corrosion since the free iron is chemically activated to bond with the nitrates, thus removing much of the oxidization component to this corrosion mechanism.

I do this a lot, never had an issue. Be sure to machine the threads using edge breaking inserts. This will remove the rag edge from the flanks of the threading. Also, run a small triangular shaped file over the threads and then emery them right in the lathe. You need to have good cut threads BEFORE liquid nitration.

Hope this helps.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Frederick]

Cryogenically treat the nut and the threads if wear is a problem. It should double or triple the life of the system.

 

[MachinaMan]

Hi Steris,

What wasn't discussed is that power screws like an ACME would most likely be self-locking while a UNC likely would not be.

This may be a very important feature in your application.

 

[steris]

MachinaMan -

Do you mean that UNC threads could be back driven? If so, I was not aware that this was possible. Can you please explain what you mean by self-locking?

Thanks!

 

[MachinaMan]

Hi Steris,

"Self-locking refers to a condition in which the screw cannot be turned by the application of an magnitude of force applied axially (not as torque) to the nut."
Machine Design by Norton

I can't confirm about the UNC being self locking or not, analysis is required to confirm or there must be a source of information on this issue.

In my opinion, it's something that you need to consider.

Regards,