Is velcro use acceptable in explosive environments? 3

[zappedagain]

A colleague wants to use Velcro to attach cables to a robot arm in a paint booth that is classified as Class I Division 1. The only threads I see about this are from NASA in the 1970s. I see some people make anti-static Velcro straps for ESD controlled environments. I'm sure they work, but are they needed?
Thanks,

Z

 

[IRstuff]

Seems to me that's more of a question for your own people, i.e., why Velcro as opposed to a cable tie?

TTFN
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[Skogsgurra]

Velcro will probably saturate with paint and become very difficult to undo. Not that it has anything with your question to do - just mention what I experienced when I tried to change a sensor on such an arm.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

Methinks that the term "anti-static Velcro straps" is referring to the fact that they are anti-static straps, that use Velcro, as opposed to elastic bands that can pinch you or unwieldy buckles like on old watch bands, with the Velcro just making it easier to take on and off. I don't think it has anything to do with the static properties of Velcro.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[telecomguy]

Some anti-static Velcro straps I've seen have a tracer wire or some other conductive additive to make them conductive to avoid static buildup.

But I'm like Gunnar, I think you'll get paint buildup either way. Maybe not due to electrostatic attraction, but the powder will have plenty of places to accumulate in the fabric of the Velcro.

-tg

http://xternal.me

 

[zappedagain]

Good point on the paint clogging. I'll suggest the cable tie idea.

Z

 

[IRstuff]

Bear in mind that antistatic properties are needed if there are rubbing motions that might generate ESD, so if the Velcro were being released and applied, ESD could be generated, but that would only be in the context of a person doing that, and if they were working on the robot, they should be wearing ESD protection anyway, so the possibility of a serious ESD event would be unlikely. Moreover, given that this is simply to tie down a cable run, the insulation and shielding (there is shielding, hopefully?) would protect the wires from getting the ESD zap.

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[MikeHalloran]

I'd worry more about a triboelectic zap triggering a fire or explosion than affecting a machine control.



Mike Halloran
Pembroke Pines, FL, USA

 

[zappedagain]

Mike, you are correct. I'm worried about a spark event not a data corruption event; I am using ESD as a generic term.

Z

 

[DRWeig]

Ask the folks who make the Velcro you want to use.

Best to you,

Goober Dave

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[MikeHalloran]

Releasing Velcro involves rubbing dielectric on dielectric, many times. I wouldn't trust it in an explosive atmosphere no matter how much carbon black was compounded in.


Mike Halloran
Pembroke Pines, FL, USA

 

[moltenmetal]

Ask yourself: is someone going to be opening this Velcro WHILE the painting robot is spraying paint? Will there not be a hot work permit or similar maintenance procedure which will require the robot to be locked out before anyone touches it, much less starts ripping off cable tie straps?

There are all sorts of wiring methods, devices etc. which are approved for Div 1 or Div 2 that can be made unsafe by maintenance happening at the wrong time or done incorrectly. From a health and safety perspective alone, it seems to me that you wouldn't want people (probably in supplied air respirators) to be doing maintenance on this thing while it continues to spray paint.

Is the concern that the maintenance will be happening while other, adjacent robots are still operating? Or is there something else in the area making this location Div 1 other than just the paint spray?

Unless you're concerned about the Velcro coming loose due to motion of the robot and that motion causing a static-electric ignition spark, it would seem to me that you're making a mountain out of a molehill.

The point about the Velcro gluing together with overspray is a good one too of course.

 

[zappedagain]

My concern is the Velcro getting stressed during robot motion while painting (i.e. explosive environment) and loosening. While I'm waiting on a response from the safety engineer for the booth, I'm leaning toward Mike H's viewpoint.

Z

 

[MikeHalloran]

The exposive environment doesn't go away when the robots stop. The place remains loaded with dust.

Mike Halloran
Pembroke Pines, FL, USA

 

[Skogsgurra]

Is it powder (enough electrostatic charge already) or wet paint?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[moltenmetal]

If the place is so loaded with a dust which is explosive enough that you're worried about minor static electric discharge from parts rubbing together, you're suggesting that it's OK to send PEOPLE into that environment to do maintenance?! I have a hard time imagining Velco igniting dust, personally.

 

[IRstuff]

ESD events generally require a number of things to occur:

> Some sort of rubbing/contact motion event -- but this requires two dissimilar materials, one that strips electrons and one that accepts electrons. It would seem to me that Velcro pretty much has similar materials on either side of the rubbing event, so both sides are either trying to strip electrons or both trying to accept electrons, resulting in a net nothing

> One side has to be reasonably conductive -- a balloon can generate static charge, but it doesn't really generate discharges because it's not conductive enough to get all the generated charges into a single location where they can then jump to the other side of an arc. You never see carpets as part of a discharge, nor your shoes rubbing the carpet, nor the body of the door itself, only your finger and the door knob do anything, because they're both conductive


In general, if you're concerned about the Velcro loosening, then it's not the right solution, regardless of the ESD possibilities. And if it's on a robot, why does it need Velcro? Are the cables that bad that you need to replace them so often that you want Velcro and not cable ties? My impression is that cables on a robot arm are pretty much permanent installations, unless someone breaks the cable, or the entire arm.

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[Sparweb]

zappedagain,

Sounds like this is about the dust jackets that wrap around the whole robot body, and close around the connectors, too, not just follow a single harness of cables.
Strips of magnetic tape are useful for closing seams and being easy to re-open. You can get it with a self-adhesive backing to stick to the jackets, or sew-on tabs if you prefer. The material is likely to have a high content of ferrite (that is what is magnetized) so I don't think it can retain much ES charge.

Many things release static-electric discharges when they are separated. An example that I noticed last week is the wrapper on band-aids. Peel one in a dark room sometime if you don't believe me.


STF

 

[zappedagain]

The paint fumes are ventilated from the booth before service personnel enter, so it is a safe enough environment when people enter. Someone else makes that call, not me. Again, I'm concerned about what happens when the robot is moving through the explosive environment. Reproducibility (i.e. installer to installer variation) on the cable mounting is what has me thinking about this.

The user wants to remove the sensor when they clean the robot, so removing the "cable ties" needs to be part of the SOP.

I expect they can use the same thing that they use to hold the dust jackets in place. Unfortunately I'm at least four levels away in the contact chain from the user, so I'm looking for the most complete info that I can give when I hear the middle links talking about Velcro.

Thanks for all the insight.

Z

 

[IRstuff]

Well, many tribo-electric effects require a certain amount of speed, so unless the arm is running amuck, it's unlikely that any serious ESD event could occur during actual operations, since it would only be a fairly slow disengagement of the Velcro caused by the cable moving well beyond where it's supposed to go.

TTFN
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