Tie Rod Torque Question (Hydraulic Cylinder)

These don't seem to be the proper type of tie rods for this application....

"Tie-Rods are usually high tensile steel with either cut or rolled threads, prestressed during assembly. Prestressing with proper torque prevents separation of parts when subjected to pressure and reduces the need for locknuts, although locknuts are sometimes used."

http://www.hydraulicguide.com/hydraulic-cylinders-construction/

..."maximum allowable stress is 0.4 x the yield strength"...

http://www.physicsforums.com/showthread.php?t=447419

Thanks for the response SolarEagle, the tierod material are either one of the following 1144, 4140 HT or 1045 and heat treated to give sufficient strength. the pre-stressing with proper torque value calculation is what I am after for assembly.

Samantha,

I think you are wanting to look at the elongation of the tie rods but also ensure that they are under the maximum stresses when the cylinder is fully loaded.

Your choice of 6 tie rods is arbitrary until you set the maximum allowable elongation, whereupon you can proceed to look at stresses in the tie rods and zero in on what you require.

The tie rods are there pretty well to keep the cylinder from leaking when pressurized, although there are situations where you will see a bit more load through the rods from how the cylinder is mounted and which end of the cylinder is pressurized.

It might help to visualized them as springs that you will allow to move only say, 0.025" between zero and full pressure. Calculate the pull and therefore stress required on the rod to create this pull and convert it to a bolt torque.

I've noticed that cylinder manufacturers typically limit tie rod cylinders to about 2500 psi, probably because of this issue. I'm guessing that above this pressure it takes too many tie rods to meet the requirements economically and therefore the threaded or welded head design takes over.

Hope this helps,

Jon.

The size of the bolts handle the push and pull forces not their torqued down valve. How tight they are torque down only has to insure a good seal on what ever sealing medium is used. Tight enough that there is no movement of the ends. Over torquing can damage this depending on how the seal is installed. Some seals are installed to make contact with inner or outer surface of the tube. Some are installed directly under the edge of the tube. These can be damage if over torqued. Also if torque to tight can distort the tube.

The rods need to be fully tensioned to yield to account for the pressure and the rest to keep the seal. The tension should be set with proper equipment, not a torque wrench.

Tie rod hydraulic cylinders depend upon the quality of raw material and strokes.

What did the manufacturer recommend?

I have found a torque spec from an internet search, below is the link, the tie rod torque is located on the last page (page 6):

http://www.parker.com/literature/Industrial%20Cylinder/cylinder/Serv/1110-M3.pdf

I am very interested in knowing how they came up with those values or what was the procedure used, can anyone offer some suggestions.

Call the original equipment manufacturer, and ask.