Forces on threaded rod that is in compression

[467ng]

How do you calculate the compressive forces on a rod that is inserted into a hollow shaft? The rod is held in place by nuts on both ends.

 

[israelkk]

calculate the ultimate shearing force on the mean diameter of the rod thread having the same number as in the nut.

 

[desertfox]

Hi 467ng

The threaded rod which passes through the sleeve will be in tension when the nuts are tightened and the sleeve will be in compression. It would help if you gave more details of your sleeve and rod ie:- materials and sizes and what you are trying to achieve, however the load that you place on the threaded rod will be the same as you place on the sleeve
so for a very rough estimate the axial load on the threaded rod can be determined from:-

F= T/(0,2*d)

where T = torque on threaded rod

d = threaded rod dia ie;- M12

0.2 = friction factor (dry threads)

F = axial force in rod

Now going a stage further if you require the stress carried
by the rod and sleeve then just divide F obtained from the above formula by the respective area's of the sleeve and threaded rod.
Note also the friction factor can vary greatly which obviously increases or decreases the axial force generated in the rod if you require very accurate forces, then practical tests must be carried out.

regards desertfox

 

[467ng]

desertfox,

Background – The purpose of the rod is to prevent a hollow steel shaft from breaking. The rod is 2.5 inches in diameter. There is 4 inches of threads (2.5”-4) on each end of the rod. The material of the rod is steel 4142.

The entire rod is heated with 1 nut already secured flush on one end so that the rod’s main body expands ever so slightly (~.098”- body = overall length minus the length of the nuts) prior to being installed into the hollow shaft. The remaining nut (again, the nut is just about flush with the end of the shaft) is secured onto to the other end after the rod after the rod is inside the shaft. As the rod cools, it will eventually put the hollow shaft in compression.

This is where I need assistance in calculating the force that is being applied as a result of the shaft being in compression.

 

[CoryPad]

This is linear thermal expansion:

F = [α] [Δ]T E A

where

F = force
[α] = linear thermal expansion coefficient
[Δ]T = change in temperature
E = Young's modulus
A = cross-sectional area


Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[desertfox]

Hi 467ng

The previous post by Corypad provides the correct formula to find the force in the rod, which will also be the force on the hollow shaft.The stress in the rod and sleeve can be found as before ie:- take the force F and divide it by the respective area's of rod and shaft.

regards

desertfox

 

[jetmaker]

This problem is one that is often covered in rudimentary staitics of mechanics indeterminant structures.

What you have is that the compressive displacement is equal, as is the force, in both parts. You must solve the 2 equations together to obtain an answer. The big trick is that the threaded rod has an initial displacement due to the heating.

So, the final displacement of the rod is

dL_rod = alpha*dT - delta_compression_cylinder

Hope this has helped,

jetmaker