Reducing Thread Length Engagement

[LanceM]

Background:

We have an internally threaded shaft connected to a threaded adaptor (see photo)

Due to the excess length of the shaft, we were thinking of decreasing the effective thread engagement of shaft and adapter by cutting 120 mm on the extruder shaft ends. Since the shaft end of the extruder shaft has a threaded portion for the adapter, cutting the extruder shaft end by 120mm will decrease the threaded area from 195mm to 75mm. This will ultimately decrease the effective thread engagement of the shaft and adapter from 60mm to 35mm. (photo)

Based on ASME B1.13M formula for tensile stress area and LE, the calculated length of thread engagement (LE) is 68 mm (vs 60 mm original length engagement).

Due to cutting of extruder shaft end, thread engagement is reduced to 35 mm. Also based on ASME B1.13M Table 7, at 100mm Major diameter and 6 mm Pitch, Minimum Normal thread engagement is 36 mm.

I want to know if:

1) Based on the formula, 68mm is recommended LE but why does manufacturer used 60 mm? I know it is within range of the Normal Thread LE based on Table 7. What should be used between formula or table 7?

2. Table 7 gives values based on Short, Normal, and Long LE. If in case my original LE is 60 and will be reduced to 36 mm, which are both within range of the Normal LE, how can I assure that it wont fail given that formula says 68mm calculated LE? what is the basis for minimum LE?

 

[desertfox]

Hi

In order to check for failure you need to know the loads that the joint experiences during operation.
If the female thread has an inferior strength to the male thread then the most likely mode of failure is stripping of the female thread, the joint really needs to be designed so that the bolt fails before the female thread strips.
That said if these parts are made from carbon steel then a thread engagement of over one bolt diameter won't improve the female stress area very much so I would base my design on a thread engagement of one bolt diameter.
What size thread and materials are you using?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[LanceM]

@desertfox during operation, the shaft experiences 1200 bars. the material is DIN 1.2344. Pitch is 6 mm at 100 major diameter and 94 minor diameter.

Original thread engagement is 60mm while adaptor major diameter is 100 so the one bolt diameter is not an acceptable rule of thumb in this case that is why i want to know the rationale of Minimum Length of Thread Engagement based from ASME.

 

[desertfox]

Hi LanceM

I mentioned the one diameter rule of thumb because in your original post I had no idea what the diameters were, so the engaged lengths could have been double the bolt diameter.

I can see now if the thread I 100 major diameter that the engagements are a fraction of this, I haven't got access to ASME but I can wok out if the thread will fail based on the thrust from 1200 bar, can you provide the yield and ultimate tensile strength stress for the material.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[LanceM]

Hello desertfox,

sorry about. The thread is M100 x 16. Material is DIN 1.2344. Based from

http://www.steel-grades.com/Steel-Grades/Tool-Steel/DIN-1-2344.html

Yield = 414 MPa and Tensile = 367 MPa

 

[desertfox]

hi LanceM

My scanners bust but according to my calculations on length engagement you need at least 74mm if you want to ensure the bolt fails before thread strips assuming both threads are of the same material.
Further if you reduce the thread engagement to 35mm the shear stress in the female thread reaches 112Mpa based on a thrust force of the 1200 bar over the 100mm diameter.
I think the site listing the materials is incorrect as I've never seen a yield stress higher then the ultimate tensile, so assuming 367Mpa is the yield the 112Mpa is about 30% and usually shear stresses maximum are usually between 50 and 70% of tensile but I usually use the yield figure.

What I would suggest if this adaptor is something you have purchased I would contact them because if you make any modification you or your company become liable in the event of any accidents, in addition you can ask them about the 68 versus the 60mm elongation.

the site I used for the thread calcs is:-

http://www.roymech.co.uk/Useful_Tables/Screws/Thread_Calcs.html

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[chicopee]

Looking at the drawing, the reduced thread of engagement shows only two active threads which my gut instinct tells me it is not enough. Many years ago, when I started as a mechanical design engineer, I learned that a minimum of three full threads of engagement for nuts and bolts is recommended and I verified that information by examining different size threaded nuts bearing that recommendation. Your case does not involve nuts and bolts so I suspect that informations posted above are appropriate.

 

[LanceM]

Hello desertfox,

I am using the same formula too. I was wondering how you got the 74mm LE.

I'm getting 96.1 mm using the simplified formula

A t = (π/4)*(D - 0.64952*p )^2
Le (min) = 2*At / [0.5*π*(D - 0.64952*p )] where D = 100 mm , p = 6 mm


while I'm getting 68 mm using

Le (min) = 2*At/ Knmax * π[0.5 + 0.57735*(1/p)*(Esmin - Knmax)] where Esmin = 96 mm , Knmax = 94, p = 6 mm

Also, how did you use the tensile and yield strength because the formula didn't use them?

 

[LanceM]

Hello chicopee,

Sorry, The drawing is only for the general picture of the situation but doesn't represent ta actual threads in use. You were right about my case not involving nuts and bolts as these are threaded shafts. Do you know of any other references aside from the one I'm using (ASME 1.13M)?

 

[desertfox]

Hi LanceM

I used the formula for the shear area of female thread and my Dsmin was 99.32 and Enmax was 99.328mm from BS3643.

Once I worked out the shear area of the female thread I divided it into the force generated by the 1200 bar:-

120*3.142*100^2/4 = 942600N.



“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[SwinnyGG]

The one thing you haven't clarified is how this shaft is loaded.

Is the adapter loaded in tension or compression?

If compression, the flange is handling the load, the threads are just keeping the assembly together, and thread engagement length will matter very little.

 

[LanceM]

desertfox,

Ive check ASME and Dsmin is 99.4mm and Enmax is 96.5 mm. Also, the male thread is the adaptor with major diameter of 100mm and not the female shaft in your solution. Here's a copy of ASME1.13M for your reference :

https://www.scribd.com/doc/124894297/Asme-b1-13M-2005

Also, we've tested the material where Tensile Strength = 1022MPa; Yield Strength = 856 MPa

Hi jgKRI,
The adaptor is in tension. However, I am still wondering on why ASME doesn't use tensile stress and strength experienced by the material.


Guys, I came up with a different solution using the tensile stress area. Can you please confirm if I'm doing it right?:

At = (π/4)*(D - 0.9382*p )^2 , Tensile stress Area formula as per ASME (

https://www.scribd.com/doc/124894297/Asme-b1-13M-2005)

At = (π/4)*(100 - 0.9382*96 )^2
At = 6994.6 mm

Then equating the computed At to this to solve for number of threads:

At = (π/4)(Dsmax^2 - Ksmin^2)*Nt,

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where Dsmax = Maximum Major diameter of external thread,

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Ksmiin = minimum minor diameter of external thread

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Nt = number of threads
6994.6 = (π/4)(100^2 - 93.5^2)*Nt

Nt = 7.1 threads

LE = Nt*pitch
LE = 7.1*6
LE = 42.5 mm

 

[desertfox]

Hi

Cannot see the ASME spec through scribe, however Inmight have posted one of the values above incorrectly but I used the right figure when I did the calculation. The differences in the sizes could be the class of thread and also I now notice the threads you are talking about are an M profile, I used standard metric threads.
I will check later about the calculations to me now you have the correct material values if you divide the force from the pressure by the shear area of the engaged thread you will obtain the shear stress on the threads, if this is less than half the yield stress it should be okay.
What I don't know is if this adaptor is subject to cyclic stresses and if so would need to be checked.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Screwman1]

Depending on the relative material strengths, the failure mode can be breaking of the male shaft through the thread root, stripping of the male thread through the pitch (pressure) diameter or stripping of the female thread through the pitch diameter. You need to calculate the tensile strength of those three cases.