Pipe Coupling Strength 1

[Wolfman7665]

A question came to mind today. Say a pipe is threaded on its outer diameter in order to couple to another pipe to achieve a desired length. Material is taken from the pipe to make the threads which weakens the pipe, and as I understand it pipe threading is tapered, so that would take even more material. However, it is then reinforced with a coupling. How does this coupled pipe compare to one single piece that runs the desired length?

Say this pipe is used for a guard railing, and no fluid ever passes through the pipe. Let's say it is schedule 40 pipe and I am only concerned with the outside forces that may act upon said pipe. Is there a rule of thumb for the strength at the point of coupling or perhaps someone is aware of a simple calculation that can satisfy my curiosity.

 

[TBP]

The calculations in the various ASME pressure piping codes determine the wall thickness for a given pressure/temperature application. Reductions in wall thickness from threading, grooving, etc have to be taken into condsideration, as do the effects of corrosion and/or erosion.

 

[TpaRAF]

Hi, Wolfman:

Where is the coupler at? The rail/piost junctions or the middle of the (horizontal) railing?

RAF

 

[Tmoose]

If the joint is placed where the moment is low, the reduced bending strength does not matter

http://www.mdsolids.com/screenshots/beamdiagram.gif

http://www-robot.mes.titech.ac.jp/robot/snake/sg/fig1.gif

 

[Wolfman7665]

Perhaps there is another way to look at it. Imagine a pipe coupling welded to a steel floor and a pipe is threaded into the coupling. The pipe has a high moment due to its length and until now I have simply welded it to the floor. More of a simple post than a railing now. I am curious as to how much different the two methods are, I would think that welding would be stronger, but how much abuse will it withstand threaded into a coupling?

 

[MikeHalloran]

Nobody actually builds railings that way. Not twice, anyway. Too easy for teenagers to snap off. Too susceptible to crevice corrosion.

Pipe joints are intended to carry fluid under pressure, not moments.

Misapply them and you're on your own.





Mike Halloran
Pembroke Pines, FL, USA

 

[moltenmetal]

If your interest is in joining long sections of horizontal railing, perhaps the pipe coupling is OK- but I can't help you on calculating the ultimate strength of the assembly. The key weakness of threading in this application is the fact that the threading concentrates stresses at a notch, right at the point where the pipe threads into the coupling- it's a natural point for breakage, especially if helped along by corrosion or low temperatures.

If you're interested in using couplings to provide removability for vertical posts, consider using something more standard like Kee Klamp

http://commercecenter.keeklamp.com/Main.asp

. No threading or welding required, meets OSHA-required design loadings, and permits the use of galvanized pipe or round HSS (i.e. galvanized on both the OD and ID to minimize the risk of hidden corrosion. Worth their weight in gold in my opinion.