Threaded Pipe Tensile Strength

[dodson]

I have a Timber space truss that is using a combination of steel pipes and rods as the compression and tension members. The contractor wants to use turnbuckles on the rods for erection purposes to be able to rack and plumb the truss, but the architect doesn't like the look of turnbuckles. One idea has been to take a pipe section and thread the inside to act as a turnbuckle. The rods are 1 3/4" diameter. I was thinking about using a 2"-XX pipe, which has plenty of tensile strength (max tension is 35 kips) and an inside diameter of 1 1/2". My question is how to calculate the strength of the threaded section on a pipe?

 

[DBronson]

dodson:

Pipe (ASTM A53) has a weld bead on the inside that will make threading difficult. There are seamless versions of ASTM 500 tube listed, but I don't know their availability. I'd call a local fabricator or steel center to see what you can easily get. Alternatively, Cold Drawn Seamless and Drawn Over Mandrel Tubing - DOM (ASTM A 519 & A 513) should be readily available. These tubes have smooth internal bores, are offered in a wide variety of outside diameters and wall thicknesses, and it would be a straight forward task for a machine shop to thread them.

Regards

DB

 

[JStephen]

You should be able to find thread dimensions online or in reference books; just figure the remaining material between the bottoms of threads and the OD as "root" area and work from that.

How about these:

http://www.portlandbolt.com/products/nuts/sleeve/

(Edit: Scroll down and the see the "round with wrench flats" option.)

One drawback with the pipe idea is that it would take a pipe wrench to turn the pipe, so it'd get all chewed up in the tightening process.
I would for sure inquire with a machine shop/bolt supplier about threading those things before assuming it was quick and easy.

(Edit- to the above, note that A106-B pipe should be readily available, and it is seamless. Some A53 pipe may be, for that matter.

 

[oldestguy]

Unless you can find actual usable information on the tensile strength of threaded pipes, I'd use a very healthy safety factor. Alternative is have a testing lab run a test.

 

[moltenmetal]

The tensile and yield strengths of pipe materials are well known and well controlled per the specification.

Threaded vs seamless shouldn't matter to this application, but in my experience you will find XX easier to find in seamless than in welded seam at 2" NPS size. In internal pressure applications a modest de-rating (a weld quality factor) is used for welded seam pipe vs seamless, which will likely be within your safety factors for design anyway..

Threading changes the section thickness, but being a cold machining operation it should not affect properties at all.

IF you are doing to use a thread you could accomplish with a tap, you're going to need to pre-bore the pipe to the correct diameter. There are large wall and hence ID tolerances on seamless pipe. Obviously the boring operation is easier done in a lathe than trying to do this in a pipe threading machine with a drill bit- pipe threaders are meant for external threading and tooling is as such. The pre-boring will take care of the ovality and bore tolerances which are common in seamless pipe, or any bore irregularity from welding. The welded product we routinely buy has no bore irregularity noticeable from the seam weld and has better controlled wall thickness and hence ID, unlike round HSS which is often welded by other means and which frequently has a pronounced ID irregularity at the weld.