14 replies to this topic

[Robert BG]

I've got an ancient strap can motor that the mags fell out of from just getting old. I have no intent on racing this and just want to get it running but to be honest it's not worth spending the money for the tools to do an oddball arm size.
 
Having said that is there any way I can get this things mags back in good enough for it to run around the track again? The goal for the car is a fun, sort of fast car for the kids to thrash and that is all.
 
I figure there has to be some sort of way to do it without spending three times what the thing's worth to do it right.
 
Thanks.

[Mike Patterson]

You can always take the arm and wrap some tape around it. Regular clear adhesive tape is around 0.004", IIRC. Either use multiple wraps, or a thicker tape to get the size you need. Not REAL precise, but should work OK for a fun motor.

[Robert BG]

That is what I was thinking to do as I was staring at a roll of blue 3M painters tape. It's a .470" setup and I've got two-three arms left for it that I just can't stand to toss and it should do well in a old Zap wire chassis I have. This way I'll have a wing car to lend out and not worry about getting a bent race car back.

[Bill from NH]

In the days before I had a lathe to make my own slugs, I used to wrap an arm with tan masking tape to get nearly the final diameter I needed. Then I fine-tuned the size with several wraps of Scotch tape, which was thinner. This method worked pretty well, whenever I used it. The  blue painter's tape wasn't available in those days, but it's use should work well too.

[zipper]

I've always used Scotch Magic tape, two or three wraps around an old arm. That tape even tolerates some heat upto nearly 300°F unlike an ordinary clear tape so you can warm your epoxy in an oven.

[Don Weaver]

Robert,

 

Just be sure that you put the start and finish of the wrap in the gap between the magnet tips to keep the thickness the same on each side.

 

Don

[Rob Voska]

What are you going to do when this "old slug" is one of your fastest motors? Then you have to ask do I risk taking it apart and gluing it right or just hope? 

Any job worth doing is worth doing right.

[Cheater]

I used tape-wrapped arms as slugs for many years and later wrapped tape around slugs to set the air gap.

Recommend you use Scotch Magic Tape, as it is very consistently about 2.5 thou thick, so one full wrap equals almost exactly 5 thou.

Have you read this THREAD?

[havlicek]

What Greg and Don said. Tape works very well indeed. The only downside is that if the tape-wrapped-arm is a tight fit, you can either bunch-up or otherwise mangle the tape which can both screw-up the gap you're setting, as well as force the magnets towards the bushing end of the can as you insert the arm. I have some slugs here in the common sizes, and I use them when I can because they're easier to work with, but I also use the tape method without too many problems at all. I've also found that:

1) Using a heavy-bodied epoxy to set magnets means you need to actually figure on a little extra for the thickness of the epoxy joint, and be a little more careful as you "ease" the arm or the slug into the can.

2) If you let the tape hang just a bit off the end of the arm and then work it to form a little bit of a "chamfer", that can help getting the arm into a tight can. You can also do just the opposite. If you're using a few wraps of tape, do one wrap at a time and stagger them back slightly towards the com side of the arm.  This will also form a "chamfer" or tapered edge, similar to what many slugs have.

3) There are "teflon" tapes used for masking when powder coating that are good to very high temps. I've used them up to 400F and above. These are a very accurate/consistent thickness... you just need to avoid having greasy/oily fingers when using them as this can defeat their adhesive properties. Here are some:

High Temperature Resistant Green Powder-Coating Masking Tape
 
... and the stuff I use which I keep for powder coating arms:

High-Temp Blue Masking Tape
 
-john

[havlicek]

Not applicable, Phil. Parallels exert even pressure on magnets to keep them "against" the can and setting compound until it cures. They don't set an air gap.

 

-john

[havlicek]

Perhaps I don't understand what's involved with a strap motor, but it seems to me, and I could be wrong, that all he really wants is to get the magnets back in the can in a reasonable way.

"Having said that is there any way I can get this things mags back in good enough for it to run around the track again? The goal for the car is a fun, sort of fast car for the kids to thrash and that is all."

 
Well, the title of his thread is:
 

Setting the air gap without the proper slug?

Parallels of course won't do anything at all about setting an airgap, and he already suggested that the tape method was something he considered when it was first suggested. The procedure is not really different regardless of whether it's a strap motor or a more conventional can type motor. With either a slug or an arm wrapped in tape, you can set the airgap and... with a heavier-bodied adhesive, you can set an airgap closer than it would be by just squeezing the magnets against the inside of the can. If he just wanted to glue the magnets in, the "arm-wrapped-in-tape" method would also work perfectly and he wouldn't have to buy anything at all!  
 
-john

[Robert BG]

Thanks guys,I got one done and the other I'm going to hold off on till later when I get some tools built up again.

[LindsayB]

if the magnets are cobalt and they are singles, just clean the magnets and can. Then re-glue. Skinners glue is best. You should use wrightway parallels to stop the magnets moving while in the oven. Not many people use slugs these days glueing cobalts. Slugs are used to centre the bearing. Air gaps are set using hones.