Stub Acme or UN Threads

[Davros2013]

Hi folks,

Was just wondering if anyone could tell me the specific differences over Stub Acme Vs UN threads? As you know, you can design any thread size you like with these two types but why would you choose one over the other? I have heard in the past that stub acme threads have a tendency to back off easier - hence the reason that we usually use locking grub screws in conjunction with the SA threads.

Any help would be appreciated. My business sector is Oil/ Gas design

Thanks

 

[CoryPad]

Ace threads are for traversing motion/power transfer (dynamic), while UN threads are for clamping (static). If the mating parts move during use, select the Acme. If they don't move, use UN.

 

[Screwman1]

And please don't come up with your own thread size. As part of that process the manufacturers will have to create gaging for your special thread too. If at all feasible, use an existing standard thread size, it will result in a lot less headache in the long run.

 

[Davros2013]

not sure if you are familiar with oilfield tools but more times than not, non standard sized threads need to be used. We've been doing it for years - tools can range up to 15" diameter.

 

[Cockroach]

Typically the shallower Stub Acme thread is used in places where wall thickness is an issue. Acme threads are much deeper and are therefore more robust in comparison to the Stub Acme alternative. The UN thread is also of shallow geometry but achieve their strength over length of engagement and good for thin wall applications. Probably the strongest thread form is the Acme, I've done my own tapered threads in oil field applications using that thread geometry and know they take quite the load, static and cyclical.

I come up with my own thread sizes all the time. I gauge the pin using a three wire measure and then gage the box accordingly. This gets around the necessity of either designing and cutting gauges. Rarely do I have an interchangeability problem since I specify major, pitch and minor diameters on the print and measurement over wires for pins. You can get online calculators to do this for you, there are a number of good ones. But I prefer to write my own programs using the equations in the ASME specifications and developing make-up/break torques.

Regards,
Cockroach

 

[Davros2013]

thanks Cockroach. We use a programme called Thread Disk by Gagemaker - this allows design of a few types of threads incl stub acme, UN etc.