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rsg disk material

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LAN_403

Coronafix

Wed Apr 09 2008, 10:28PM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Dr. Shock wrote ...

A propellar is a lot more dangerous. A RSG uses short electrodes, and moves at slower speeds, usually because it has either 4, 6, 8 (or even more) electrodes.

I don't see that making sense. Where the RSG has the many electrodes on the disk spinning at high speed, the propeller has the many electrodes stationary, but the speed is the same. The RSG has a larger inertial mass and would take longer to stop than the relative light propeller gap.
Both would be equally dangerous though.

HV Enthusiast

Wed Apr 09 2008, 11:15PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Goldsphere wrote ...

Dr. Shock wrote ...

A propellar is a lot more dangerous. A RSG uses short electrodes, and moves at slower speeds, usually because it has either 4, 6, 8 (or even more) electrodes.

well, your right. i all depends on the configuration.
but the propellar makes me shiver - very nasty.
i always ran my inside 1" thick lexan box.

Herr Zapp

Thu Apr 10 2008, 12:42AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Having seen many different implementations of home-made RSG's on the web, I'll have to say that many (if not most) are quite frightening from a mechanical-engineering viewpoint. Dynamic balancing, attachment of the rotating member to the motor shaft, and the material used for the disk or hub are the usual weak points. I'd have to agree that most of the "propeller gaps" I've seen would be more dangerous than the average disk-type RSG.

Composite materials will have the greatest tensile strength and (more importantly) greater resistance to shattering under high centripetal or shock loads. Materials like G-series fiberglass-reinforced epoxy, linen-reinforced phenolic, etc are about the only materials that have the combination of electrical insulation, resistance to carbon tracking, mechanical strength, etc that are needed in an RSG rotor.

Lexan or acrylic ("Plexaglass") are absolutely unsuitable for an RSG disk, as both of these materials shatter explosively when their tensile strength is exceeded, especially when they have stress-risers (like holes for the electrodes) in them. Nylon or polypropylene (polypropylene is the common cutting board material) are soft and have very low heat-deflection temperatures and so are also unsuitable.

A relatively safe propeller-type RSG could be designed, but I've never seen one. All I've seen are relatively crude, basically a chunk of soft plastic with a hole drilled in it, hammered onto a motor shaft, with a piece of tungsten rod poked transversly through the plastic. The reason why a propeller gap is intrinsically more dangerous than a disk-type RSG is because of the sectional density of the rotating electrode. In all "propeller gaps" I have seen, the rotating electrode is a single piece of tungsten rod, usually 1/8" diameter X 7" long. This is the readily-available TIG welding electrode. The problem is that when the tungsten rod breaks loose from the propeller gap hub and tries to fly across the room (and through the wall), the great sectional density of the tungsten rod gives it great penetrating power, and a substantial blast shield is required to contain it. This assumes a worst-case failure of the RSG, where the tungsten rod leaves the RSG assembly intact.

Shan

Coronafix

Thu Apr 10 2008, 04:36AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Mmmm...good point!
Like a tungsten arrow.

Dr. Slack

Thu Apr 10 2008, 07:19AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Just to be completely different (as I usually am), my RSG disk is 5mm thick aluminium. I figure practically no reduction in strength with temperature, and the critical parts around the electrodes are well heatsunk by the rest of the disc, so it stays cooler than composites anyway. Any additional heatsinking of the rotary electrodes is probably irrelevant compared to the blast cooling they get anyway.

The more alert will have noticed a possible problem with electrical conductivity. The alli disk is araldited into a GRP hub (araldite to eliminate any air-path disc to shaft) giving >12mm of glass and epoxy between the disk and the motor shaft, which should leave my breakdown dominated by the air-gaps, as will any RSG be. If the charging circuit has enough inductance to prevent power-arcing (which is also a consideration with a standard insulating disc RSG), then there is no reason a spark should strike from the stationary electrodes to the metal disc, for any practical length of rotating electrode.

Thoughts on my iconoclasm?

Steve Conner

Thu Apr 10 2008, 10:16AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, it sounds like my old propeller gap from 2003

FWIW, those electrodes turned out to be brass-plated steel! The ends eventually burnt off them completely.

Herr Zapp

Thu Apr 10 2008, 07:36PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Neil -

Your aluminum RSG disk sounds interesting, with the insulated hub, but I'd be a little concerned about the flashover distance. How much greater than 12mm is the actual flashover distance? If the peak voltage in your tank circuit is any more than 10 KV (I'll bet it is), you probably have some corona discharge occurring between your disk and the motor shaft. Over a long period of time, this can damage the ball bearings in the motor via micro-pitting of the balls and races. (Obviously not problem if it's a sleeve-bearing motor). You can add a spring-brass "wiper" riding on part of the motor shaft to shunt any "leakage" voltage on the motor shaft directly to ground.

Shan

bob golding

Fri Apr 11 2008, 10:50PM

Registered Member #1084 Joined: Mon Oct 29 2007, 08:58PM
Location:
Posts: 28

i made my 12inch rsg from 1/2inch G10 and mounted it vertically. that is the motor was vertical. i used an old centrifuge motor. big top bearing and a proper hub. it still scared the brown stuff out of me when it was doing 4000 rpm. and it was balanced both statically and dynamically. you could use a direct drive upright spin drier motor if you can find one. i ran mine inside a 16 inch tube.

bob golding

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