Strength of ACME threads 2

[razmech2012]

Hi,

I have often wondered what people mean when they say that ACME threads are stronger than 60° triangular thread forms (ISO and unified).

My own calculations show that ISO threads are stronger than ACME threads when you look at thread stripping strength and bolt tensile strength for threads with the same nominal diameter (Thread geometries according to ASME B1.5 and B1.8 for ACME/Stub ACME and ISO 965 for ISO threads) . For ISO threads I normally use the "Alexander method" as outlined in SAE 770420 but since this cannot be applied to ACME threads I am using calculation where an equivalent stress is calculated (Von Mise) by adding the bending (thread modeled as cantilever beam) and the shear stress. The calculations may be incorrect but I have seen them beeing used by the one of the biggest oilfield service companies.

The difference in flank angles migh of course mean that ACME threads are not as pronce to galling but in my head this is not the same as stronger.

I have searched exensively on the internet for a comparison between the thread types but all I find is that "ACME threads are stronger than than triangular threads" without any real explanation of what stronger means. There are som many calculations beeing made on threads but I have not found any that can explain why ACME threads should be stronger, I am looking for numbers rather than suggestions here.

I appreciate any input.

Best Regards

 

[TVP]

There is some discussion of this topic in the following document:

http://hexagon.de/dose/dose-1e.pdf

 

[Cockroach]

You're kidding me, right? I mean seriously!

Regards,
Cockroach

 

[razmech2012]

Cockroach, I assume by your reply that the answer is very simple. This is a good thing, I am just looking to understand rather than accept.
I do understand that since ACME threads are being used as high strength threads they must be stronger I have just not found any calculations that can show this comparison.

Let me explain why I posted this question. I have tried to compare stress calculations to get an idea of the difference between ACME and v-shaped threads. I am fully aware of the fact that stress in threads is a very complicated thing where handcalculations normally utilize many assumptions (load equally divided on all threads etc) but since we base alot of our design by handcalculations I thought it was important to show that ACME threads are stronger with handcalculations.

Many of the sources("an introduction to the design and behaviour of bolted joints" by Bickford,

http://www.roymech.co.uk,

SAE 770420) I have looked at seem to start off with the standard formulas for calculating stress (excluding stress concentration factors etc for now):
Stress = F / A[sub]tensile[/sub] For tensile stress
Stress = F / A[sub]thread stripping[/sub] for thread stripping strength (most often Von Mise is utilizied for comparing shear stress with tensile stress data)

I had a look at the different stress areas and compared an ISO thread (ISO 965) with an ACME (ASME B1.5) thread with the same nominal diameter ( 30mm) and pitch (3,5mm). The stress area was calculated by using the "minor diameter". I normally use the minimum value as stated in the standards but for this example I have used the nominal values.
d[sub]ISO 30x3,5 ,minor[/sub]= 26,211mm
d[sub]ACME 30mm - 7,26 TPI minor[/sub]= 26,5mm
A[sub]tensile[/sub]= π d[sub]minor[/sub][sup]2[/sup] / 4
Which gives:
A[sub]Tensile, ISO[/sub]= 539,6mm[sup]2[/sup]
A[sub]Tensile, ACME[/sub]= 551,5mm[sup]2[/sup]
The thread stripping area was calculated at the thread root, again I realise that there are many different theories (such as defined in the link by TVP or SAE 770420 on where the thread stripping occurs and how to calculate it). I have calculated it at the root of the screw according to the following formula:
π L[sub]engagement[/sub] d[sub]minor[/sub] (p - F[sub]rs[/sub]) / p
where p =pitch (3,5mm) L[sub]engagement[/sub]=length of engagement (Set equal to diameter which is 30mm) and F[sub]rs[/sub]=flat width rooot screw (1,209mm for ACME according to ASME B1.5) and F[sub]rs[/sub]=0mm for ISO. This gives the following thread strip areas:
A[sub]thread stripping, ACME[/sub]=1635mm[sup]2[/sup]
A[sub]thread stripping, ISO[/sub]=2470[sup]2[/sup]
Looking at the geometries of the ACME vs ISO I guess it is evident that the thread strip area is larger for ISO when looking at the root.

So for the tensile stress area at the root of the screw the ACME is marginally higher, for the thread stripping area the ACME is considerably lower. After this step there are two other things that I have considered:
1. Bending of the threads.
Some people have suggested that the ACME threads have a higher bending coefficient then the ISO threads. I have done the calculations (which I could post if necessary) but again the ISO outperforms the ACME.
2. Stress concentration factors
SAE 770420 outlines some different coefficients but they are based on numerical data rather than mathematical formulas it is hard to use this while comapring ACME and ISO.

Now, could you please advise me on how you calculate ACME vs ISO threads or why my calculations are wrong?

Thanks in Advance.

Best Regards,

 

[Cockroach]

Actually, threads subjected to shear does not rely on the assumption that load is equally spread on each tooth. In reality in order for one tooth to be in contact, then so to must the others. Two reasonss, geometry is such that if one tooth on the pin is in contact, then so to are the others noting invariant pitch. Second, in the thread shear equation, the second term in the equation changes the half angle profile. In other words, the Acme thread being 29 degrees, must chane to 60 degrees for Vee threads. Acme threads are therefore more robust than Vee threads of equal size. Period.

Your model is giving you issues because of the mathematics. I may take the time to do this, which is involved if you do it correctly from forst principles. I note your method does not discuss principle loads, you must necessarily used combined loads of axial load with bending, and account for shear at the same time. I particularly like Mohr's Circle for that, something you have omitted from the discussion.

I have posted many of these computations in legacy threads to this forum. It should be obvious, I would of thought. You're post caught me by surprise, hence my remark, not meant to be sarcastic.

Regards,
Cockroach

 

[GogiBOB]

I am very interested in this discussion as I was wondering something.

 

[Cockroach]

Check the legacy discussions concerning threads in this very forum.

Regards,
Cockroach