What the difference is between Stub Acme & Acme thread?

#1
Hello Everyone,

Can someone tell me briefly what the difference is between Stub Acme & Acme thread?
I'm asking because I just molded a internal thread that has a flat root but a rounded crest. (The print is calling out a Acme thread)
Now I haven't been dealing with Acme threads long at all, but can this be a Acme thread?

Thanks in advanced for your help.
 

Marc

Retired Old Goat
Staff member
Admin
#2
As a quickie from a search - Stub Acme, what is it?

Stub Acme, what is it?

From the link:
I cut stub ACME quite a bit usaully for motor operated valve stems.

Forrest Addy got it right, the stub are basically 2/3 the thread death of the standard.

Having dealt with various factory reps over the years that build them, I've been told that the thread strength is basically the same as a standard ACME but quicker and easier to produce.

So you have the same thread format with the same strength, but easier to manufacture and it takes less time. If using a stub ACME tap, you'll use less horsepower to run it than you would for a standard.

25 years ago when I started as a Machinist at the Nuclear plant where I still work,everything was standard ACME. Its been in the last 15 or so years that the stub ACME had become popular.

Even so, it is not uncommon for a stub acme threaded valves stem to run in a standard acme bronze driver nut. I can think of a few cases where it is specified solely for the ability to be able to withstand higher clearances due to extreme length of a stem, or one that may be less than perfectly straight.

About a month ago a 12 foot long drive stem that was 2 inches in dia. that operated a 4'dia. butterfly valve was brought into the shop. That sucker was bent all over the place with about 3 different bends that deviated about 6 inches off plane. Believe it or not, it still operated the valve perfectly. The mechanics brought it to us to straighten, thinking that it was so ugly that they might as well get us to fix it while they were changing the rubber seating surface on the butterfly. I told them to bring me the drive nut as I was sure it had to be shot. I couldn't imagine that it couldn't be. As it was, there was nothing wrong with the drive nut and it actually looked pretty good.

We straightened out the shaft and put it back in operation. After looking at the tech manual AFTER the mechs put it in
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I noticed that is specified a Stub acme thread with a standard drive bushing. I have never heard that before but the manufacutuer confirmed that it was standard practice on some valves as regular threads on a regular drive nut would bind up and lock up the thread, rendering the valve inoperable.

Anyhow, in that case it worked. I have noticed that some stub acme taps come in a 2 tap set verses the standard acme which is usually a 3 tap set.
DISCLAIMER: I know little about threads.
 
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