Using liquidtight conduit for flex joint?

[iXod]

I use liquidtight flex conduit for most HV (230v) runs in the industrial machines I wire up. One such machine has a large cast arm on bearings that oscillates about 20(?) degrees. I could easily bridge this joint with liquidtight--others have done it in the same application.

See the liquidtight at this joint (0:20):

https://youtu.be/3TQbhnDVqck?t=15

Is liquidtight designed to flex? I always thought it was flexible solely for ease of routing in environments such as this where rigid or EMT would be nearly impossible, but not for flexing. It's not so much the liquidtight as the conductors inside--are they at risk of work-hardening and increasing resistance and/or breaking?

I realize eevblog is electronics forum. If anyone knows a better forum to ask such questions, I'm all ears! (c:

Thanks.

[Ian.M]

Of course the conductors are at risk of work hardening and breaking unless you use high strand count wire and have a long enough loop to accommodate the motion without excessive flexing.   Contact the conduit manufacturer's technical department for advice on what type of conduit you should be using for an aplication involving cyclic movement.

[ajb]

There is "continuous flex" cable specifically designed for these sorts of applications. I guess it's a combination of insulation and conductor properties that have been selected to improve resistance to fatigue. 

Either way, you'll want to leave as much slack in the flexing area as you can to spread the stress out over more of the cable/conduit.

[T3sl4co1l]

Oh, hmm, the other genericized brand name I guess -- we always call it sealtite around here, but of course Seal-Tite(R) is the legitimate brand.

Anways, the conduit is a jointed metal spiral.  It will definitely slide under such flex, and I would be concerned about wear from all that siding.  Is it well enough greased, from the factory?  Would it pay to squirt more down it?  I'd definitely call the factory, or see if they have some appnotes or something.

I don't see that copper wires would have a problem, but just in case, you could specify flexible wire (usually neoprene jacket, fine stranded -- "welding cable" in the larger sizes) to be sure.

The next best thing would be a corrugated conduit, that is able to flex (elastically) without sliding or plastic deformation (which would fatigue and break in short order).  I'm not sure where you can get this.  It may be quite expensive (the same construction is often used for high vacuum flex tubing, but that wouldn't carry a NEMA rating).

After that, if a nonmetallic conduit, or even just strain relief, is acceptable, then one of those plastic duct tubing things that's got a spiral rib and thin webbing would be good.  The fatigue life should be reasonable.  If it doesn't even need to seal*, just some plastic spiral wrap or split loom or braid will do fine, too!

*If the environment is greasy, make sure the wire is "oil and gas resistant", THHN or what have you.

Tim

[floobydust]

Armored flex cable (teck cable), common in the oil and gas industry, tolerates equipment flex from tilt, temperature or mild vibration. Also used on SAGD wellheads which move a lot due to thermal expansion. It meets electrical codes and good for outdoor use, OK for stuff that moves, at mains power levels. For instrumentation, you can just pull out the conductors and use the jacket.

As long as your bend radius is not too small, and you don't make a mechanical resonance with the cable shaking, it works really well even in cold weather.

[tpowell1830]

I use liquidtight flex conduit for most HV (230v) runs in the industrial machines I wire up. One such machine has a large cast arm on bearings that oscillates about 20(?) degrees. I could easily bridge this joint with liquidtight--others have done it in the same application.

See the liquidtight at this joint (0:20):

https://youtu.be/3TQbhnDVqck?t=15

Is liquidtight designed to flex? I always thought it was flexible solely for ease of routing in environments such as this where rigid or EMT would be nearly impossible, but not for flexing. It's not so much the liquidtight as the conductors inside--are they at risk of work-hardening and increasing resistance and/or breaking?

I realize eevblog is electronics forum. If anyone knows a better forum to ask such questions, I'm all ears! (c:

Thanks.

I have not seen another forum which speaks of electrical wiring and conduit from a practical point of view, to answer your specific question. As far as the way that you have ran the flexible conduit, and yes Sealtite is a brand name, but there are others out there.
For moving parts on a machine, this is commonly used. I was directly involved in wiring industrial machinery as an electrician back in the 1980s and we used the same type of conduit for parts of the machine that moved. This was not only used for mains power, but more often there were control elements that needed wiring on moving parts as well, even though this may have normally been at 120 VAC potential, quite often we isolated these type of control signals with a small isolation transformer installed in the control cabinet.

For powering a unit or wiring up a control element such as a limit switch, we would use the flexible type conduit that you describe. However, we usually did not use individual stranded wire inside the flextite conduit, but a cable that had individual stranded wire would be used such as an "SO" (SO, SOOW) cable would be placed inside this moving flextite conduit. SO cable can be used in situations where it is not deemed necessary to armor the wiring if you did not require much protection, however when armor was required, the SO cable was ran inside the flextite where movement was involved. This reduces the chance of scuffing and abrading individual wire insulations with the added flexible sheath of the SO cable and it's underlying construction, which lends itself to flexing (i.e. multiple stranded twisted wires surrounded by twists of flexible material such as paper, cloth thread and other low stress materials). The twisted wires and materials used to construct SO type cable lends itself very nicely to control/power runs, even you do not use the flextite conduit for further armor protection. I would say that in many cases, the flextite is not necessary if you have used SO cable.

I have seen such units do many 10s of thousands cycles without any degradation of the copper or flextite conduit and/or SO cable. However, if you do this incorrectly, as some have said where you do not allow large radii of flex area, the mechanical stress will not only start damaging the flextite conduit, but, as also been said, work hardening the copper into a failure mode. The key is correct placement of fixed mounts, such as "P" clamps or "U" clamps.

I hope this helps.