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Motor bearings vs oilites


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#1 gatormark

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Posted 18 December 2016 - 05:52 PM

Running my stock Pro Slot yesterday and a friend was checking it out and pointed out to the pinion bushing being worn and sloppy. He said if you're going to pull the motor down you might as well put ball bearings on the pinion end and the endbell and be done with it.

 

My question is do ball bearings improve speed to the point I should be using ball bearings in all of my motors?


Mark Conner




#2 Taylor Davis

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Posted 18 December 2016 - 06:03 PM

Bearings create less resistance, which reduces heat, and less heat means faster for a longer period of time. How much will depend on the motor.

 

Plus they don't wear out as often as long as you take care of them.



#3 Ramcatlarry

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Posted 18 December 2016 - 08:23 PM

But some races and race series limit whether you can compete with a ball bearing modified motor.  For instance, the USRA Nats this coming April allow can bearings for 16D and both can and endbell ball bearings for C-cans.


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#4 Mike Patterson

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Posted 18 December 2016 - 09:48 PM

Generally, you will get the most benefit from having a ball bearing on whichever end of the motor has the pinion gear on it, usually the can. A BB will greatly reduce rebuilds due to bushing wear, and "dubious quality" armature shaft material.


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#5 havlicek

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Posted 19 December 2016 - 07:11 AM

Running my stock Pro Slot yesterday and a friend was checking it out and pointed out to the pinion bushing being worn and sloppy. He said if you're going to pull the motor down you might as well put ball bearings on the pinion end and the endbell and be done with it.

 
Hi Mark,
 
If you look at the way you framed your question, you were really pointing to a "wear" issue, but asking about a "speed" issue. Certainly, a decent quality ball bearing will provide a longer service life than a sintered metal bushing. The bearing is made (generally) out of steel, the bushing is made out of powdered (usually) softer metal and has a lot of holes or spaces so it can absorb and hold oil like a hard "sponge."

- Cheap bearings (and there are a lot of them around) will have more "slop" than the better ones, but all bearings will have at least some because of their nature. The best bearings have very little. That "slop" can allow the armature to move laterally ("vibrate") while rotating, which could cause arcing. In reality, and with the slower/low cost motors, it's probably not an issue and is probably less than the wobble that happens when either an oilite or even a solid bronze bushing has very slightly worn.
 
- If you could set up a tightly controlled experiment where you took one armature and installed it in a set-up where the bore of the can and endbell were slightly off from each other, you would find that it was easier to insert the armature if there were two bearings, than if there were two oilites. The reason is that ball bearings by nature have this slight "slop" (the inner race is riding on... balls!, and can rotate a bit). Of course, the above experiment would be difficult to set up because the bore of the oilite and the ID of the bearing would have to be precisely the same, which they most often aren't). I would guess that "needle" or small "roller" bearings can have less of this kind of ability to wobble than ball bearings, but engineers would know for sure. There was a brief time when I was a kid that (I think) Cox sold needle bearings for slot car axles, but they fell out of favor.

- When an oilite starts to wear, the amount of slop can quickly become way more than even a cheap bearing has to begin with, and that wear will accelerate. The more wear, the faster it will wear. The more "slop," the faster other motor parts will be affected and the shorter the motor life... plus degrading performance (slower, more heat, and current draw from arcing, poorer gear mesh at the driven end of the motor) before the motor fails.
 
- The difference in "speed" between a fresh tight oilite (or even a solid bronze bushing) and a ball bearing is probably not that great, but the bearing will survive much better over time, and especially be able to better handle a too-tight gear mesh on the driven end of the motor. All other things being equal, the bearing will provide better service over the long haul, and most likely better speeds. In slower classes of racing that allow you to use either, you'd have to balance the cost of bearings with the cost of the whole motor. It might not make "economic" sense, when the whole motor can be replaced, but I still think the faster guys will use bearings. It may be also that inexpensive bearings (at something like $6 or $7 a pop) are perfectly fine. At the high end where a motor can cost $400 or so, I think people will opt for the best bearings because they're spending so much on the motor anyway, and a bearing not rated to handle those kinds of speeds can fail in a very sudden and bad way.

 

... so, bearings are "better" for the most part, but not necessary in many cases. As was mentioned above, a bearing only on the driven end of the motor would be a good "half measure." If cost isn't a factor, the bearings on both ends sure isn't ever going to be a "bad thing," and might even be "the best thing."

 

I hope this makes things as clear as mud!   :)
 
-john


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#6 Samiam

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Posted 19 December 2016 - 07:36 AM

Most rules that allow American arms also allow a bearing on the can end. This helps reduce wear on the shaft.


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#7 havlicek

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Posted 19 December 2016 - 08:14 AM


Hi Sam,
 
The cheaper motors do sometimes have oilites that are magnetic, so I have to assume there is some ferrous material in there, and iron content would make the oilite more prone to wearing the shaft. Since these shafts are made from a mild steel (as opposed to the hardened steel used on better armatues), they would be more prone to wear from a hard oilite than from an oilite made from softer materials (brass, bronze or other non-ferrous metals/alloys).  

It's also possible for a steel shaft to wear an oilite, even when it is made from harder materials, and I've seen a lot of this (oilite wear) on these motors, probably as much as shaft wear.

 

 In either case, a bearing would be a good way to avoid wear on either the shaft or the bushing.


John Havlicek

#8 zipper

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Posted 19 December 2016 - 08:50 AM

It's also possible for a steel shaft to wear an oilite, even when it is made from harder materials, and I've seen a lot of this (oilite wear) on these motors, probably as much as shaft wear.


I remember in early '70s I had a very good Steube S25 arm in a can with bushings. During the race it got very slow. I checked it and the endbell bushing had seized and had a giant slop. The hole was figure eight! And the arm shaft had a deep gouge. I had to build an external bearing holder still to be able to use that arm.

Probably just a bad bushing as lubricating them was a norm and that was the only fail ever.
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#9 havlicek

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Posted 19 December 2016 - 08:28 PM

Yeah, I would think that a bad bushing might have been the case there, Pekka. I think motor brushes are also sintered, and I have personally seen some bad ones make a motor run like poop. Makes sense that some bad oilites happen from time to time.  

Shame it had to happen with (of all things) a Steube 25.
 
-john
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#10 gatormark

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Posted 21 December 2016 - 06:02 AM

I also noticed on my Pro Slot motor the end play on the arm was tight. I was told by some that a arm spacer or even two would need to be removed to allow the arm to run in its natural path between the magnets. I checked it and the motor did seem a bit tight.
Mark Conner

#11 havlicek

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Posted 21 December 2016 - 06:37 AM

That's probably a good thing, Mark. Carefully tap the endbell end of the shaft on something hard/unforgiving like a piece of steel or even thick glass. Do this holding the motor perfectly vertical. Try sort of lightly at first, and if it's still tight, a little harder. This should (surprisingly) easily give you a bit more endplay. You might be surprised to find out just how easily it is to deflect the bushing strap on the can end outwards.  

Sometimes (rarely it seems) an endbell bushing isn't fully seated, and tapping on the can end of the shaft will pop it in, but anything more than that is no bueno for the endbell.
 
-john
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#12 gatormark

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Posted 22 December 2016 - 07:09 PM

John what your saying is the roller bearings should not make the arm tighter? maybe the tightness comes from the dis assembly of the motor?


Mark Conner

#13 havlicek

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Posted 23 December 2016 - 06:38 AM

Hi Mark.  I'm not sure what you mean.  I only remember roller bearings for axles marketed by (I think) Cox.  I don't think there were ever roller bearings for motors.


John Havlicek





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