Slip ring questions

Alternator slip-rings (more like graphite brushes) are usually in the

5A-10A range. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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     Thinking outside the box... producing elegant solutions.

I think you'll be good to go. Industrial/military slip rings are expensive because they go into big expensive machines, which means that (A) they can be expensive and still be a small part of the whole cost, (B) if they break then a big expensive machine is broken because of some little "insignificant" part, and (C) because production volumes are low. Your alternator slip rings, if you can adapt them, should work fine.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!

If you can, look at an older car's pullies. Look and compare the diameter of the damper pulley with the alternator pulley. You will find it to be quite alot. If the crank is doing 5,000 RPMs, Just look at it.

The problem is finding them. Newer alternators do not do that, they are not "geared" as high so this someting to look into.

You better to get them refit for the time and materials to do the refit.

Slip rings for a car alternator would be fine. Car engines turn in the thousands of RPM and the alternator is usually run from a pulley that increases the speed a 2-4X over engine speed.

With motorcycles they turn at engine speed, but then red-line is often ~10,000 RPM or so, and they're fairly easy to take off of the rotor (just the rings without all the pole pieces coils etc.) They are also often radial whereas cars have axial slip rings.

The "one inch through" (hole?) might be hard to find on a car alternator, but would be easy with a bike alternator, or easy to ream out. The rings are a flat disk with coaxial copper circles imbedded in some 1/4" thick phenolic and with machine screws into tapped holes on the rotor pole pieces.

Slip rings may be something you could cobble together if you like to tinker. Copper tubing or fittings and some epoxy if the temperature is below a few hundred F. All you have to do is shim it in place until some epoxy sets. I'd find a way...

A set of radial rings may be possible to fabricate from some printed circuit board material, if you have access to that sort of gear.

Thinking outside the box. Put a coil of wire on the spinning shaft connected to the solenoid. Put another coil on the stationary part. Supply power to the coil and let it transfer like a transformer. You may even be able to use DC on the stationary coil and let it act like a generator.

I forget which vehicle had it but there was a brushless alternator where the coil was stationary and the magnetic field coupled to the pole pieces to supply a rotating field.

Maybe magnetize the shaft to move something attached to the shaft. Steel shafting would probably make a semi-permanent magnet, but then field reversal may work. Assuming centrifugal forces are balanced and reasonable.

Be nice to know what gizmo he's dealing with. Stepper motors make decent alternators so it may be possible to generate voltage with the shaft, but then presumably he still needs a trigger at a specific time to actuate the solenoid. Maybe do that with some sort of optical device.

It may also be possible to use something like an electric clutch - a ring or something that changes position around the axle pushing a rod and mechanical linkage to move whatever needs moving.

That leads me to think of balancing problems. But even a solenoid moving will likely result in balance problems at 5,000 RPM.

Thanks everyone for your answers. I now have two options. Get the rings from my old alternator or use some sort of transformer with a coil wrapped around the spinning tube coming out of the lathe. Just so everyone knows what I'm doing I'll describe it now. One of my repeat jobs is machining an O-ring groove in the underside of a flathead screw. There is not now a flathead screw available on the market that has an O-ring groove located in and normal to the angled surface of the screw head and this is why I am machining these screw heads. A customer asked me if this could be done and as a consequence I am doing them. In lots of 2000. Currently the machining time is 7 seconds. The CNC lathe I am running these on has a 15 horsepower spindle so you can imagine the inertia and energy it takes to start and stop the spindle. The hydraulic chuck actuator in the lathe will not work unless the spindle is stopped. I want to keep the spindle running. I could mount a through spindle air closer that is not connected to the machine control but it is not and cannot easily be modified with a dead length feature. Furthermore the top speed is only

3000 rpm and I want 4000 rpm. So to avoid stopping the spindle I am mounting a dead length collet closer meant for stationary mounting to the spindle nose of the lathe. I have figured out all the mecahnical stuff, balancing and all that, and how to get air to the setup. I just need a good way to power the tiny solenoid operated valve. Thanks, Eric

I would be tempted to price out the cost of getting the whole screw made on a CNC or screw machine, in lots of 10000. If you can break even by getting 10000 and selling 2000, and if the customer orders often enough, you can be in cycle where you break even once, make bux four times, break even once, etc.

Granted, you'd be a salesman for someone else -- but money is money.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!

I didn't look close but a lot of hits for flat head screw with o-ring seal.

George H.

I cannot get these made cheaper at 10,000 pieces. Not in the USA that is. Already been there. But I am thinking about larger quantities. The screw is headed stamping after threading and the groove has to be added after heading. The groove cannot be stamped because it is normal to the head angle. I have been thinking about some way to feed and load the screws automatically, which is what a screw machine shop would do. When my customer asked if I could do the job he had only the desire, no drawing or dimensions. Since I had to do the actual design and engineering he has no problem with me making these for myself and selling them to whomever I please. And since the design works so well because it doesn't require a precise hole diameter and because it retains the O-ring so securely I think there might be a market big enough for these to be made in very large quantities. Eric

they only carry a few hundered milliamps, the high current is on the stator.

somewhere 1000 to 4000 RPM I think, same speed as the enine RPM multiplied by the pulley-size ratio.

an automobile altenator shaft is unlikely to be that big, perhaps use copper pipe and epoxy?

for an automobile 300000 km is about the same as 4 to 8 months run-time you're using a larger diameter and so that means more wear but you're only running it 10 hours a day,

maybe it'll last 6 months.

Another option is to put a wound rotor on the shaft and a stator around it and vary the stator current to control the power generated in the rotor... this is how brushless altenators feed their main rotor coil, you want a laminated rotor, maybe from a power tool, or vacuum-cleaner motor, but bored out to fit the shaft. almost certainly cost more than slip rings. but frictionless, so should last forever.

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It's unlikely that the rings from an auto part will be mechanically adaptable, but all you need is cylinder (or annulus) elements that spin, and a way to hold brushes against 'em.

Carbon brushes are available as overpriced repair parts, and as inexpensive imports, and as parts of every broken power drill.

These should make good reliable rubbing contact on bronze, copper, or even brass rotating parts. It's not uncommon to see assemblies built of linen-based micarta that can hold the low-voltage elements away from the nearby steel and such; swarf and coolant might be issues, though.

Greetings George, Thanks for the link but both my customer and I have seen these screws and they won't work. None of those are like what I make and all require a special hole. The one flat head screw they offer is a 100 degree flat head. Even though my customer could have a custom 82 degree head made by these folks it would not seal in the marine industry standard holes. Aircraft standard holes have less clearance and tighter tolerances. But thanks again for looking. Cheers, Eric

Reading your other replies:

You have a lathe, and that means you have the know how to make slip rings.

There is no magic to it. Get a short piece of rigget brass pipe of the size you can deal with and dice off a couple of rings. Get piece of micarta to make the through hole mounts and pressed form mounts for the rings. use set screws to mount on shaft, drill hole in the side of the micarta for the lead wires and pull tyes to hold them on the shaft or slot the shaft and lay the wires into that if it needs to go through a bearing race.

I can usually find short pieces of stock in lock hardware outlets.

if you're cheap you can use construction copper pipe which has a thick wall./

schedule 40 is ok, 80 is better of course ;)

No problems Eric, it took almost no time, so worth what I put into it.

I didn't mean to bother you, (It seems likely there is some screw place, that could make what you need... just jammin on Tim's idea.)

George H.

I wasn't bothered George. Thanks for taking the time. Lots of the time when I look for something or a solution for something and come up empty somebody else points out something that I just plain missed. The way I machine the screws, the location of the O-ring groove, allows these screws to seal holes with really oversize and corroded holes. Much of the marine stuff is made with oversized holes to allow easy assembly of parts with large hole location tolerances as well as allowing easy disassembly of parts with corrosion products in the holes. Cheers, Eric