Internal threads - Required wall thickness 2

[jackboot]

Question:

What required wall thickness is necessary to develop the full strength of internal threads for a given length?

Example: a cylinder with 3.75- 10 UNS-2B has a major dia of 3.75; now if the cylinder had an OD of 3.80 it would raise some questions as to how strong the connection would be. So what should the required wall thickness be for a given thread diameter to develop the full strength of the thread?

Is there a standard, a specification, a rule of thumb, or even a thought -

jackboot

 

[CoryPad]

You are referring to the strength reduction factor due to nut dilation, C₁. The nut will not dilate significantly if it has sufficient wall thickness. Below is an equation for this factor:

C₁ = [-(s/D)² + 3.8 &[ignore]middot[/ignore]; (s/D) - 2.61]

for 1.4 &[ignore]le[/ignore]; (s/D) < 1.9

where

s = nut outer diameter
D = nut nominal diameter

The C₁ factor is used to modify the nut stripping load F_strip,nut according to:

F_strip,nut = &[ignore]sigma[/ignore];_n &[ignore]middot[/ignore]; A_shear,nut &[ignore]middot[/ignore]; C₁ &[ignore]middot[/ignore]; C₃ &[ignore]middot[/ignore]; 0.6

where

&[ignore]sigma[/ignore];_n is the nut material ultimate tensile strength
A_shear,nut is the nut shear area
C₃ is the nut thread bending factor

To insure that screw threads fracture or strip before nut threads strip, set C₃ = 0.897

 

[Hush]

If sizing isn't critical the my rule of thumb is to use the dimensions of a Hvy Hex Nut as a minimum. 1.5D + .125&quot; in diameter x 1D long. If I need to make it smaller then I do the calcs.

 

[jackboot]

Thank-you both.

Looking the Machinist Handbook - the 1.5D rule is the standard.

jackboot

 

[Cockroach]

Actually &quot;Jackboot&quot; has a major problem here. Threads as specified, 3 3/4 - 10 UNS-2B machined internal to a pipe of 3.80 OD leaves only 0.022/0.021 inches of wall. As such, there is probably little room for a thread relief at run out. I'd hate to be the machinist cutting this one!

Noting the internal minor, pitch and major diameters as 3.642/3.663, 3.685/3.697 and 3.750/3.755 inches, a mild steel tube (43 ksi yield) would only withstand around 11480 lbf loading. Depending on your seal diameter, say 3.328 inches, the wall would not take an ANSI Class 600 application and bearly an ANSI Class 300.

You need to worry about crack initiation at the root of the thread as a probably mode of failure. Depending on the design around the box threads, you have a good case for pipe swaging here. Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[roseda]

CoryPad,

please could you tell me where you found that formula.

 

[CoryPad]

SAE Technical Paper 770420 Analysis and Design of Threaded Assemblies by E. M. Alexander. This work is referenced in many subsequent sources such as Handbook of Bolts and Bolted Joints edited by Bickford and Nassar and VDI 2230 Systematic Calculation of High Duty Bolted Joints.