Attempting to rebuild / rewind an 8278 PTO clutch.

[BurtB 88]

The 8278 PTO clutch on my '69 GT-14 is shot. Can't get any continuity with a multimeter.

 

Finished rewiring my garage this week. Today I'm taking the day off and going to start working on this clutch.

 

Have 400' of 18 gauge copper magnet wire and Epoxy Seal 9000 electronic grade epoxy resin for potting.

 

Today I will dissect the old coil, work on turning a two piece bobbin, try and make a spool holder with some sort of friction tensioner. (Probably won't get all that done)

 

Manufacturer specs on old coil:

 

Self bonding aluminum wire, 18.25 gauge at .0419 diameter. (the 18.25 at .0419 is weird, my wire is .0413 in diameter, would expect the old wire to be smaller than that)

12 turns per layer, 20 layers, 237 turns.

2.88 ohms, 4.17 amps

12V tape wrapped

OD 4.915, ID 3.374, .655 thick

 

My general idea is to wind the coil at reasonable tightness, line the bobbin with wax paper, super glue occasionally, may also use cotton thread to hold it together. I'll be hand winding with the bobbin held in the lathe. Try and get as many windings as possible with the above given ID/OD/thickness subject to what I find when dissecting old coil. Once I've got a coil wound, I'll see what resistance I've got and do some calculating.

[ri702bill 8,318]

Sounds like a plan.... was the original wire copper?? If so, the resistance will be different than alumunum....

[BurtB 88]

The original is supposed to be aluminum. I'm hoping that what I wind with copper will be close enough. Going to wind it and check the resistance and see where I land.

 

The the old potting has a bunch of fiber in it that I don't trust not to be asbestos, so the P100 is going on.

[BurtB 88]

Old potting was sand and some sort of adhesive. Fiber was from the tape wrapped in three places. Wire is copper plated aluminum. Calipers say .043 wire diam, .500 thick, 3.375 ID, 4.7 OD.

 

Shell is 3.290 minor, 5.140 major, .625 deep.

[peter lena 8,623]

@Burt B  you going to RE PACK THOSE BEARINGS ?  combo of wide rubber side shields , and  lucas 550 deg , polyurea grease , would be a  simple improvement , or new bearings  , and a clean out  and repack , rolling ease is dramatic , pete  

[BurtB 88]

@peter lena Have already ordered a replacement bearing, the current one doesn't feel great.

[BurtB 88]

Here goes nothing:

 

 

[BurtB 88]

Now we wait on the super glue to cure. Getting 1.2 ohms.

[BurtB 88]

So, 1.2ohms would mean I need to have a 1.5ohm power resistor in series otherwise it will draw like 10amps. Coil uses enough juice that I need a 100watt resistor.

 

This would probably work, but I don't really like it.

 

I'm going to take my first attempt out of the bobbin today and see how it turned out structurally. Don't know if I like the super glue.

 

I'm doing what is called a scramble wind: i.e. I'm not trying to get the rows of wire neatly laid down. Since this is a DC electromagnet, scramble winding is totally fine for the application and much easier to produce; as long as I can fit it into the available space. Scramble wind is less compact than a tight wind.

 

I did wind to the max dims that I can fit in the housing 5.06 OD, 3.375 ID, .530 thick.

 

I've ordered 20 gauge magnet wire, that should let me get the resistance high enough that I don't need a resistor. Field strength will likely go up.

 

Wonder if I'll need to adjust the clutch engage distance or add shims or something?

[953 nut 55,217]

5 minutes ago, BurtB said:

I need to have a 1.5ohm power resistor in series otherwise it will draw like 10amps

As you know there would be a lot of heat generated with no heatsink and it would probably not last very long. 

 

7 minutes ago, BurtB said:

20 gauge magnet wire

At 0.01015 ohms per foot a piece of 20 gauge 273.7 feet long should get you in the 2,88 ohm range with about four amp current flow.    sounds like a plan.

[BurtB 88]

So, structurally, it turned out well. I thought I had purchased water thin super glue, but it was more of a gel. Will get thinner glue next time, but the super glue worked.

 

I will probably line the ID of the coil with insulating tape or plastic. Or maybe, I'll insulate the minor of the housing.

 

Technique is good, next is hitting a resistance that is good enough. 20ga wire is supposed to be here Friday.

 

 

[BurtB 88]

Magnet wire came in yesterday but I didn't have time to wind another coil. Will try and make the attempt today.

 

Been head scratching on the epoxy I'm going to use for potting. Is it worth pulling a vacuum on the epoxy when I pour? Maybe I should also let it cure under pressure?

 

I have a Venturi vacuum pump and an old pressure cooker. Would need a thick piece of polycarb or acrylic to make the lid.

 

Harbor freight has a paint pressure pot for $100 that I could do the pressure cure with. Not sure I want to spend the money. Not sure if it makes enough of a difference.

 

Also, I need to knock out the old bearing in the housing. If I did pull vacuum on the assembly, the new bearing couldn't be installed yet as the internal lube of the bearing would likely be affected.

[BurtB 88]

Okay. So I've got 3.4ohms on this coil. That's 3.5 amps. I used an entire 1lbs spool of 20ga magnet wire. Supposed to be good to 155celsius / 311farrenheit.

 

I think I'm going to go with this. I don't know a ton about coils. I think this means I end up with a little stronger field. The 20ga is smaller than 18ga so a higher resistance is probably a good thing to prevent overheating.

 

The potting epoxy I've got is probably superior to the sand potting used on the old coil when it comes to heat transfer.

 

This coil has a smaller OD than my previous attempt.

 

I'm going to knock out the old bearing. Also going to see if I can find a cheap piece of polycarb to make a vacuum chamber out of my smaller old pressure cooker.

[WHX?? 48,815]

School me on the vacuum/pressure thing for the potting Burt. 

I woulda thought you just pour it in and let it set. 

[BurtB 88]

So, epoxy resins tend to be thicker than water. Even if you're careful, bubbles get in when you mix it. Those bubbles act as insulation and reduce its heat transfer ability. Also, more importantly, the wires in the coils trap air in between the windings. The surface tension of the epoxy makes it where those bubbles don't come out.

 

So, magnet wire has really thin insulation. Basically it's just painted on. If you have air gaps inside your windings, vibration can cause those windings to rub. The insulation gets scraped off and your coil shorts out internally.

 

You take your mixed epoxy and throw it in a vacuum chamber and it pulls the bubbles out. It looks like it's boiling. Use a container that has extra head room.

 

Then you pour it after degas.

 

For something like a coil that has trapped air, you vacuum it again after pouring. Often, you'll have to take it out and pour again because the windings fill with epoxy once you let atmosphere back in. The bubbles grew and popped, and the air pressure actually shoves epoxy into the winding gaps.

 

The step after that, if you really want to make sure you don't have gaps, is to let the epoxy set under pressure.

 

You put the assembly in a pressure vessel, and that pressure compresses any bubbles down to a microscopic size. The epoxy sets and those tiny bubbles are trapped.

 

I don't think I'm going to fool with the pressure set. I think vacuum will be good enough.

 

Potting compound and bubbles also have effects on capacitance and high frequency stuff that I don't know much about. It doesn't really apply to this use case.

[oliver2-44 9,743]

Many medium and high voltage motors and generators have pre-formed coils that are made with the vacuum/pressure process.  Large motors with form wound wire coils may be  dipped with the vacuum pressure process.  

[BurtB 88]

Quick update.

 

Vacuum chamber has held at 25" for a couple of hours. Will see how it does overnight. 29" would be nice, but 25" will probably get it done.

 

The thinner super glue actually wasn't as good as the gel stuff. Pulled off first side of bobbin and had some loose wire. Currently glued and clamped. Will probably have to do it again for the other side.

[BurtB 88]

The coil is potted!

 

Vacuum chamber worked really well, you could see bubbling from the center where the coil was. Had to add more epoxy after the first coil assembly vacuum as it sucked it into the coils.

 

It was wild! Love me some garage magic/advanced red-neckery.

 

Coil and housing. Did some die grinding inside of the housing to get the worst of the rust out and then cleaned with 70% isopropyl. Coil is taped, leads soldered in, heat shrink on the ends, fiber shims under the tape to hold it off the bottom.

 

Coil in the housing and dammed with duct tape. That Gorilla Duct Tape is great stuff, BTW.

 

 

Degassing mixed resin:

 

Getting the air out of the coil:

 

Leads on the assembly measure 3.4ohms. Now we let it cure for 24hrs.

[WHX?? 48,815]

2 hours ago, BurtB said:

advanced red-neckery

WAY above ... 

[BurtB 88]

Epoxy set up pretty as you please. It will need a little grinding where the surface tension cupped it up against the tape dam. Need to knock the bearing in and pack the pulley bearing.

 

May get to try it out today.

[BurtB 88]

Clutch works.

 

Off:

 

On:

 

Little hard to see, but the pulley smokes the wood I was using as a brake when it's turned on.

 

Had to remove clutch after first try and drive the bearing a little deeper as the face of the coil was rubbing when it was turned on.

 

Doesn't seem to be getting too hot. Now I just need to actually attach something to it and really work it.

 

Need to get a belt for the mowing deck and tiller, finish installing front mule drive, rewire, rebuild engine, rebuild transmission.