Acme vs UNC threaded rod 4

[jclough]

I was hoping some of you mechanical guys could help.

I'm designing a fixture and need to quantify and compare the amount of force a 3/4-10 UNC mild steel and a 1/2 -10 mild steel acme thread can develop before before failure. Each of them will be used to apply a static clamping pressure. I will be using standard 1*D nuts.

I've had no luck finding this information in the Machinery's Handbook.


Any help will be appreciated

Jon

 

[Cockroach]

The Machinist Handbook is used more appropriately for threading geometry rather than performance calculations. As such, the latter is somewhat limited to Acme and Stub Acme stress computations only.

What you need to calculate is the "shear area per unit length of thread engagement". Once this is computed for the nut and pin respectively, you will always find the nut has a much greater resistance to shear than the pin. Therefore we can refine the computations to computing the shear area per unit length engagement of the pins (external screws) for the 3/4 - 10 UNC-2A and 1/2 - 10 Acme-2A profiles. Obviously the one of lesser magnitude is the poorer choice since weaker.

In the first case, 3/4 - 10 UNC-2A, the thread geometry can be computed or looked up in the Machinist Handbook. Applying ANSI B1.1 - 1882 Appendix B, the shear area per unit length of thread engagement for the pin is 1.217 in^2/in. In the second case, 1/2 - 10 Acme-2A the threading geometry is calculated since the Machinist Handbook does not list the geometry. In this case the profiles for box-by-pin is:

c/w 45 Degree End Chmf(s)
EXTERNAL (2A) INTERNAL (2B)
Major Diameter: 0.500/0.495 Minor Diameter: 0.400/0.405
Pitch Diameter: 0.444/0.431 Pitch Diameter: 0.450/0.464
Minor Diameter: 0.390/0.369 Major Diameter: 0.510/0.520

to which the pin shear area per unit length thread engagement is 0.720 in^2/in.

Therefore the 3/4 - 10 UNC-2G thread offers superior performance. Since the length of engagement is the height of the nut and equals the thread diameter or 0.75 inches, the shear area is 1.217 in^2/in X 0.75 in = 0.91 in^2. Material is Mild Steel (43 ksi yield) so that the maximum expected force for thread shear impending is 39.1 ksi.

You will find the performance computation for Acme and Stub Acme threads in the Machinist Handbook, but no other thread profile. In Edition 23, it begins on page 1277; I expect that page number in other later editions, depending. As mentioned, the ANSI Specification was used for the Unified National Thread Form but thread geometry is listed in the Machinist Handbook.

I would go with the 3/4 - 10 UNC-2G box-by-pin. Hope this helps you out somewhat.

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

 

[FredGarvin]

Nice post Cockroach. Just to be a bit of a pain (and to clairify for jclough), the estimated max force is in kips, not ksi.

A star for your well presented post.

 

[Cockroach]

Yeah, not a problem.

Since we are talking 39.1 ksi for a thread machined from Mild Steel, the corresponding force is 12.2 kips in normal and 58.1 kips in shear.

Both calculations correspond to the minor diameter of the 3/4 - 10 UNC-2A thread.

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

 

[jclough]

Thanks cockroach...just what I was looking for!

Gave you a star.


Jon

 

[Cockroach]

.....thanks Jon, I'd buy you a beer tonight over the hockey game! Canucks over Slovacs by two (2) goals.

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