Standard vs. Stub thread 3

[dennis45]

What is the advantage of using a stub Acme thread vs. a standard Acme thread? And what about a 60 degree stub thread? Right now I am driving/actuating a piston using a standard Acme and need to know if there would be a gain in mechanical advantage or a friction reduction using a different thread type. Thanks. DM

 

[Cockroach]

Stub Acme threads are utilized primarily where thin walls are incurred. Typically there are other mechanical and metallurgical issues, but a coarse pitch thread of shallow depth becomes the design criterion.

As such, the Stub Acme thread is about 57% the height of a Regular Acme profile. One major disadvantage is that a true Stub Acme thread is a Class 2G fit; Regular Acme threads can be 1G, 2G, 3G, 4G or 5G(s).

In surface, sub surface oilfield equipment applications, the Stub Acme profile is commonly found because of the limited space found in downhole applications. Most tubes are "thin walled", hence the advantage using Stub Acme(s).

Hope this answers your question. Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[zakk]

Cockroach:

I too use Stub Acme in place of Standard ACME due to space contraints and I often wondered if, from a stress and shear standpoint, if the two thread forms are equivalent. Any thoughts?

 

[plasgears]

Stub or modified acme is beneficial to the thread rolling process. It minimizes metal deformation and maximizes tool life. In multiple start acme's like two-start, three-start, etc., a pressure angle of 14.5 deg is common, which gives an included angle on the thread of 29 deg. On single starts we generally specify 20 deg pressure angle, which gives a tooth included angle of 40 deg. A 60 deg included angle would produce a higher friction thread for a power screw.

 

[Cockroach]

From a manufacturing view, "plasgears" is correct for thread rolling. However, the vast majority of Acme, Stub Acme threads are cut, profile inserts being extremely common from supply houses. Any other discussion regarding friction is completely off base since from a conceptual view, stresses associated with thread stripping are not addressed.

To answer your question "zakk", consider a 4.000-6 Acme-2G thread in comparison to 4.000-6 Stub Acme-2G. The shear area of the pin (screw) are less than that of the box (nut), therefore the pin ALWAYS fails before the box threads. In terms of pin shear area per unit length of thread the Acme is 8.512% greater than the Stub Acme. This is due to threading geometry for basic major, pitch and minor diameters: 4.000, 3.917, 3.833 for the Acme and 4.000, 3.950, 3.900 for Stub Acme.

I would concurr with your thoughts, the Acme and Stub Acme threads are virtually identical, stress performance wise. Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada