How to ground to the rotating plate on a welding rotator?
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    Default How to ground to the rotating plate on a welding rotator?

    I am building a welding rotator that will feature a 4 3/4 through hole. It will turn on roller bearings and I obviously don't want to ground through the bearings. I plan on making a conductive graphite ring, 5" id 7" od 5/16 thick. This will be fastened to the central rotating tube and will turn with it. I Plan on spring loading 2 non rotating brass rings to sandwich the graphite ring and connecting the ground to them. is this too complicated? I wonder how many amps this setup will handle?
    Last edited by moonlight machine; 09-04-2011 at 09:42 AM. Reason: add a question

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    Talking

    Just a suggestion,you may find it easier to rotate a brass ring and the power is conducted through a set of carbon brushes.These come in many different sizes,have appropriate leads and also the springs,holders , etc,etc, are easily available.A set of 4 for the ground lead is perfect.

    Another source of these components is an electrical assembly called a "festoon" which is essentially what you want,although the current rating is important you may find a suitable
    unit already to bolt on.

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    Quote Originally Posted by moonlight machine View Post
    I am building a welding rotator that will feature a 4 3/4 through hole. It will turn on roller bearings and I obviously don't want to ground through the bearings. I plan on making a conductive graphite ring, 5" id 7" od 5/16 thick. This will be fastened to the central rotating tube and will turn with it. I Plan on spring loading 2 non rotating brass rings to sandwich the graphite ring and connecting the ground to them. is this too complicated? I wonder how many amps this setup will handle?

    I'd dispense with the graphite altogether in case you end up with a carbon arc!. What about slip rings made from bearing bronze?. Most bronze water valves are leaded bronze, you could get a 6" pipe bronze valve from the scrapyard and cut off the flanges to get two thick rings of bronze to mate together.

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    Why not just do it the simple way and run a cable to the center of the table? How many complete rotations must you make to weld up what you are doing?
    If it is less than two a simple cable should work.

    Contacts and slip rings will be neat, but trying to make something robust would be a pain compared to running a cable to the center of the table.

    Peter

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    I made such a turntable from a pair of discarded 3/4-ton truck brake rotors and a semi trailer axle bearing. The outer race fit into the recess in the rotor and is held in place by four 1/2" bolts. I drilled and tapped through the openings between cooling fins to accept the bolts. The bearing sits on top of the hub face and is held in place with 2 pieces of bent flat stock that have to be tapped on. I made a contactor from a piece of aluminum round stock and a used MIG gun tip. I bored the aluminum to accept the copper tip and fitted it with a spring, using an internal clip to hold it in place. I bored a hole in the bottom rotor to accept the assembly with a tight fit and faced the bottom side of the top rotor for good contact. I suppose that I could have used two of the contactors, but one has done the job so far.

    So far, I have used it for welding 1/2" flanges to pipe sleeves with good success and no problems with arcing or poor grounding. Furthermore, it has seved as defense for my habit of saving every used or scrap piece of anything metallic when my sons ask why I keep all of that 'junk'. The same could also apply to the 'powers that be', but she came into this mess understanding that it's just the way it is.

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    We made a rotator for the shop where I worked long ago. We wrapped a 1"-1 1/2" or so wide flat woven copper ground strap bolted to a tab on the base then wrapped it around the central tube and attached a spring to the other end which then went to another tab on the base. The spring kept tension on the strap and it never arced in use. As I recall the ground strap never wore very much either.

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    A general comment about slip ring materials. Because the loose outer orbit electrons that give copper its good conductivity are limited in number and are often locked up by bonds to alloying elements, the resistance increase is often much more than might be expected from the small amount of metal added. Brass would not be a good choice. Pure copper or the minimum alloy that will be hard enough is probably the best choice since you don't want to pay for silver. Because you would be rotating at low speed and few rotations compared to something like a motor, I would use pure a pure copper brush, or several, on a copper slip ring. Wiping a light coat of Vasoline would probably be all the lubrication needed.

    Bill

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    Quote Originally Posted by moonlight machine View Post
    I am building a welding rotator that will feature a 4 3/4 through hole. It will turn on roller bearings and I obviously don't want to ground through the bearings. I plan on making a conductive graphite ring, 5" id 7" od 5/16 thick. This will be fastened to the central rotating tube and will turn with it. I Plan on spring loading 2 non rotating brass rings to sandwich the graphite ring and connecting the ground to them. is this too complicated? I wonder how many amps this setup will handle?


    A simple ground strap from the local NAPA store, working directly on the shaft of my positioner, has had 300 amps through it on occasion with no issues.

    Electrical current, like water, will follow the path of least resistance. Even though the ground strap is between the shaft bearings, the bearings are unaffected by the current.


    Rex


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    Wsnyder & Bjorn don't speak with forked tongue....they tell truth.

    9100 speak in platitudes...take with grain of salt!

    Stuart

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    The common term for this feature is "slip ring". It passes heavy current between starionary to rotating parts via a conductive ring and brush arrangement. You see them in many places besides weld positioner, AC generators where the field rotates, static collection, car alternators, hydroelecctric generators, compass gimbol sensor connections.

    A big brass washer or pipe makes a good start for a rotating contact ring. The stationary contact can be wire braid as in Bjorn's post, leaf contacts or merely a big spring loaded bolt head witrh a copper slug brazed to the head are good places to start. B=rass against copper with a little light grease.

    A carbon brush arrangement has to be sized for the current and you need large brushes with braided copper wire shunts to carry welding current. There's a rule of so many Amps per square inch of brush but I can't recall the value. If you have a source for free big DC electric motor brushes then that's probably the way to go. If you have to buy them, figure on major money for enough to pass the current. Maybe a couple sets of starter motor brushes or scrounge at a golf cart shop.

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    before the advent of brushless motors for model airplanes some of the competition types were called "flying welders" as they were passing something like 100 amps through the small brushed motors. so a big copper ring and a few brushes should work just fine if you want the neato factor and technical achievement award....if you just want to get it done then use one of the various strap methods shown above.

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    Quote Originally Posted by bjorn toulouse View Post
    A simple ground strap from the local NAPA store, working directly on the shaft of my positioner, has had 300 amps through it on occasion with no issues.

    Electrical current, like water, will follow the path of least resistance. Even though the ground strap is between the shaft bearings, the bearings are unaffected by the current.


    Rex

    Not doubting that your setup has worked just fine, but disagree with your last statement, current will follow all available paths. Your bearings may never be hurt, but they are conducting some current.
    James

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    Zagnut one thing you have to remember about RC stuff ratings: there's more outrageous ratings lies perpetrated on a faithful following of RC enthusiasts than political parties, televangelists, and penile enhancement hucksters combined. They probably make stuff from "billet" too.

    100 Amps on battery RC stuff my a$$. You can't even draw 50 amps from the li-po batteries they use with a dead short. Apparently internal resistance just doesn't happen to RC batteries. RC batteries are capable of impressive current pulses but 100 Amps? Even for an instant? It's just an unprovable number to awe the gullible.

    Anyway, one can overload carbon brushes. They just heat up a little or a lot depending.

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    Forest you might not be able to draw 50 amps from a Li po battery but good old Nicad or latest generation nickle metal hydroxide are good for pulses of several hundreds of amps. I have had stuff on my testing gear and those numbers are sound. Im talking about sub 13 turn 540 motors that more or less are a dead short when stationary. Cooling for both the motors and brushes are a serious issue. Brushes often have very high silver content to get those currents and work.

    The person i use to do that work for thought nothing of a few new motors over a weekend. New brushes every few races, commutators on new motors turned down to the bare minimum to increase rpm - reduce drag. Battery packs swapped after less than 50 charge cycles. Thats the sort of approach and facts, not hype it took to win races.

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    Quote Originally Posted by J. Randall View Post
    Not doubting that your setup has worked just fine, but disagree with your last statement, current will follow all available paths. Your bearings may never be hurt, but they are conducting some current.
    James

    Boy howdy, James, if my Nanosemantic Folicular Spliterizer (Pat.Pend.) wasn't down for annual maintenance you'd have an argument on your hands!


    Rex

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    Nanosemantic Folicular Spliterizer

    I love that!!!!

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    Quote Originally Posted by J. Randall View Post
    ...Current will follow all available paths. Your bearings may never be hurt, but they are conducting some current.
    James
    If you were worried about it, isolating the bearings electrically would do the trick. A pad under the bearing base and a couple shouldered bushings to isolate the fasteners would do the trick.

    Very funky, resourceful and elegant solution there, Bjorn. It does look way too clean in your shop, though.

    Neil

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    A bearing will conduct an arc welding current just fine. Arcing only occurs when there is an air gap(not present in a bearing).

    As an example virtually all rig welders here use welding reals that make connection through wheel bearings from the end of trailer axles. These are used pushing a 3/16 5p.

    My weld rotator has a brush rubbing on the cast iron table. Several times I have caught my help welding with the ground attached to the framework and the current passing through bearings gearbox etc. to no apparent ill effect other than my anger.
    Last edited by Heavey Metal; 09-04-2011 at 11:51 PM. Reason: spelling

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    Quote Originally Posted by Heavey Metal View Post
    A bearing will conduct an arc welding current just fine. Arcing only occurs when there is an air gap(not present in a bearing).

    As an example virtually all rig welders here use welding reals that make connection through wheel bearings from the end of trailer axles. These are used pushing a 3/16 5p.

    My weld rotator has a brush rubbing on the cast iron table. Several times I have caught my help welding with the ground attached to the framework and the current passing through bearings gearbox etc. to no apparent ill effect other than my anger.
    I am using tapered roller bearings, 84 rollers between the 2 bearings. You are saying it won't hurt them to ground through them?

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    It might not hurt them enough to make them not work for a rotating welding table..............

    But why bother finding out when a good solution like Bjorn's exists?

    or just clamp an overhead hanging ground cord to the work.... if it is in the way, take a half-second and move it.... Run the stinger cable with it and clear up the floor area of tripping hazards as well as keeping them together.


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