What's wrong with this bearing?

[swbluto]

Here's a description of the problem I noticed.

On my electric scooter with an HXT outrunner, It appears I "whollered" the original timing pulley when I was filing down the D-bore into a round one the shaft would fit in. So, I'm going to try to get a small-toothed pulley and enlarge the bore to 8mm (or whatever the diameter of the shaft is - I'll be getting a caliper soon to know); Further-more, I'm going to replace the shaft to make it longer so I can put the timing pulley on the shaft in the "right direction"; And, now that I'm going to make it longer, I'm thinking about adding an external bearing support to help take the load of the main motor bearings.

So in the process of taking apart the motor to find out what was causing this problem, I noticed that it was difficult to take the motor apart so I eventually evolved to a hammering it out using some rubber to distribute the pressure on the windings and a bench vise to hold it. It worked! The back bearings popped out to.

So, I thought maybe the magnets were rubbing against the bell - I looked at the magnets and they all seemed to be pretty scratch free. They were all still encased in the extra epoxy I used and none seemed to have "slipped", a common problem with the original epoxy (Apparently the newer versions of the Turnigy motors are more generous with the epoxy and they have better support). So I looked at the next culprit - The bearings. The back ones, the ones away from all the lovely pulley on shaft action seemed to roll as smooth as bearings. The one right next to the pulley, right where the scooter wheel flings mud and debris on the shaft, was slightly less smooth. Despite this, it wasn't clicking.

I then started checking the inner race using a small pick hook, and I noticed the pulley bearing's inner race was slightly wobbling around. I checked if the back bearings had this behavior and they didn't - they were as secure as could be. Uh oh. Place pressure on the front bearing started to cause a noticeable "click", so I thought I found the cuplrit.

Was it a small piece of dirt that inserted itself and wreaked havoc? It was an "ultra seal", which isn't exactly meant for off-roading, so maybe it was vulnerable to that damage. Or was is the fact it was closest to the point of belt tension and thus suffered the most stress? I don't know and I just want to find a solution (or set of solutions) that solves "everything", once and for all.

The place where this front bearing was at http://www.bocabearings.com/main1.aspx?p=product&id=4458&n=SMR6800C-YUU_NB2

Here's what the motor originally looked like before I removed the bearing.

And I tried to remove it by clamping it down and pounding on the inner race using a drift punch / drift pin, and the bearings went flying and the inner race thus separated from the outer race. Here they are.





Any suspicions or diagnosis? I did occasionally take the scooter down dirt trails and roads which did involve flinging dirt all over the front of the motor, so maybe it got too dirty? I don't know.

 

[liveforphysics]

De-grease it and take the best shots you can of the outside of the inside race, and the inside of the outside race. This can help us find what areas show heating and wear, and help understand what type of failure you are experiencing.

 

[fechter]

Any suspicions or diagnosis?

Made in China?

 

[lawsonuw]

De-grease it and take the best shots you can of the outside of the inside race, and the inside of the outside race. This can help us find what areas show heating and wear, and help understand what type of failure you are experiencing.

What he said, though it does look like the front bearing has a lot more sand in it.

Lawson

 

[swbluto]

Here's some pictures. It came apart when I was degreasing, so here's all the parts in its glory. I think the shield may have been bent by my drift pin. I don't know about the other side, though.











View attachment 4



View attachment 3

 

[swbluto]

And one more.

 

[swbluto]

Any suspicions or diagnosis?

Made in China?

That's probable. Even so, I hoped that Boca's hybrid bearings as recommended by drewjet would be good enough. I might have got the wrong kind, though, since I believe it read "these bearings are meant for relatively clean environments" and they had "yellow seals" meant for dirty environments. In the RC car world, I don't know what "relatively clean environments" are since it seems RC car races typically happen in the dirt.

But, here's some highlights.

-They're ABEC-1. Relevancy unknown.
-Dynamic load rated 380ish pounds.
-Pulley was attached to shaft about 1-1.5 cm away from the bearings (The pulley hub was closest to the bearing).
-Usually pulled 4-5 N.M. from start-up, but occasionally pulled 8 or so N.M.. Something "clicked" whenever I launched off with that much torque. Lately, it's just been chewing through belts when trying to launch off like that as the belt skips instead of just "launching off".
-Went through some rocky terrain at 15-20 mph.
-Belt tension made as tight as possible, once, although it didn't seem to get any more tense than "moderate". This happened after the sound went from clicking to rubbing.
-The timing pulley was typically around 170-180 degrees fahrenheit while the can was typically at 130-150 during the harder runs.

 

[amberwolf]

Best solution I can recommend is that when you replace the bearing, use one of the ones with the plastic seal rings, and then, if clearance allows, fully clean the bearing's outside surfaces of any oils and do this:

1) With a toothpick as a dabber, very carefully add just the tiniest bit of light oil to the inner race but not on the inner ring for the axle hole.

2) Using another toothpick, dab on swaths of high-temperature silicone sealant to make a full cap over the bearing on each side, from the outer ring to the inner ring, ensuring that on each of those rings it is actually sticking to the metal there.

It's time consuming, but it will create a seal that dust and dirt cannot get into for a long while (years, potentially). As long as the silicone used can handle the temperatures the bearing will be reaching, it won't break down and come unstuck, either. Stuff meant to seal engine blocks is probably best on a motor.

The cleaning step is really important, because if there is any kind of residue on the surface of the metal, especially on the edges of the inner and outer rings, the silicone won't adhere properly, and it'll just peel away later, leaving gaps where dirt can get in.

EDIT: I just realized that the way I describe doing it will not quite work for you, because the seal would then be glued across two counter-rotating surfaces and would just tear. So instead, DO put the light oil also on that inner ring and a bit on the shaft near it, BUT when you put the silicone on, make sure it spreads out down around the shaft to make a larger-surface-area seal, harder for dirt to migrate into.

I used this method to seal out dirt on some crappy roller-skate bearings I used on my ebike's remote steering (since I didn't have any rose/heim joints), and it worked well there, just as described including gluing to the inner ring, because they don't rotate, just swivel a few degrees, and the silicone can stretch that much just fine.

The problem that destroyed the bearings ended up being the forces on the thinwalled stamped outer shells, instead. So I replaced them with bearings that look like yours in style (from skateboard wheels), and they already had a plastic seal. Since they're a couple feet off the road I decided not to do the silicone seal to see how well they work on their own, and so far it seems to be enough, but if I had them where dirt would be spattered on them a lot I'd still do the silicone.

 

[John in CR]

Did you hammer the bearings to put them in to begin with? If so, maybe damage occurred right off the bat.

John

 

[swbluto]

Did you hammer the bearings to put them in to begin with? If so, maybe damage occurred right off the bat.

John

I did use a socket around the outer race to hammer it into place. I don't think that would be the problem, though, because it(timing pulley + motor mount) started rubbing long after it was installed (Instead it annoyingly clicked), and I think the bearing's "wobbliness" is what exaggerated the timing pulley's wobble that permitted it to rub.

Maybe that could've been a problem. I'll be sure to check for this behavior after installation of the replacement bearing.

 

[amberwolf]

What did you use to push against the *inner* ring, as it slid onto the shaft? If it is not a simple slip-fit onto the shaft, then that means the outer race was being forced against the bearings and then against the inner race, to get it onto the shaft.

That could definitely damage a bearing seal, or even damage the bearing holder ring, if not the races themselves.

If the bearing is being pressed onto the shaft (the most common way they're done) then you should use a pipe that is just a bit larger than the shaft diameter, but no more than the inner ring of the bearing, to press against that part of the bearing *only*, and nowhere else.

If the bearing is being pressed into the housing, and the shaft will *loosely* fit into the bearing's inner ring, then use a pipe that is just a bit smaller than the housing diameter, but no less than the outer ring of the bearing, to press against that part of the bearing *only* and nowhere else.

In either case you also need to support the part you're pushing against, too, so it is not warped by the installation.

If that's the motor shaft, don't press against the housing with the bearings on the other end of the shaft, place the end of the shaft itself against a hard surface, so you are only putting pressure on the shaft and on the inner ring of the bearing.

If it's the motor housing, make sure you are not putting pressure against the motor shaft.

If you do put pressure on another part then you're pushing the *other* end's bearing against itself, which could be damaging *it* or it's seal.

It might not have any effect, depending on the force involved and the material strengths, but it's still not a good idea.

 

[liveforphysics]

You started with a sh*t bearing. Doesn't appear to have even got hot, it was just junk from the get-go. Normally a bearing failure will have heat marks in the areas the balls run in the races. By seeing where those marks are, and if there is pitting etc, you can tell what caused the bearing failure.

Shoot for ABEC 3 at the minimum, hopefully 4-5. If you want to never have a problem again, go full ceramic, but then you must have your shaft surface flawless and correctly clearanced.

 

[liveforphysics]

Failure analysis is something you can spend a lifetime learning.

Here is a very basic little chart to give you an idea of what things to look for to better understand bearing failures.

http://www.koyousa.com/brochures/pdfs/cat2000ex%20-%20Failure%20Chart.pdf

 

[swbluto]

You started with a sh*t bearing. Doesn't appear to have even got hot, it was just junk from the get-go. Normally a bearing failure will have heat marks in the areas the balls run in the races. By seeing where those marks are, and if there is pitting etc, you can tell what caused the bearing failure.

That sounds saddening to hear. Given the reputation of Boca, though, I'm thinking there's likely an element of "user" error than the bearing's fault. Maybe the shaft wasn't properly fitting? It seems the bearing popped right into its housing just fine by tappingly hammering on the outside race, so I don't think it'd be the housing. The shaft, however, just measured to be somewhere between 9.6-9.7mm (I'll properly measure it as soon as I get my digital calipers) and the bearings are supposed to fit on a "10 mm" shaft. Would .3 mm clearance cause problem with the dynamic load?

I hope it's user error because if it's junk, it's rather high priced! (Being $12 each for ABEC-1; $19 each for ABEC-7. I got these Koyo rubber seals that are a bit bigger for $5.50 from a local shop - they're normal metal balls, though.). They only seem to have either "ABEC 1" seals and "ABEC 7" seals for their 6800 rubber seal versions. It seems they're specifically targeting the most economic and the best performance-wise.

Thanks Luke for linking to the document. It seems the bearing parts' surface(s) most matches the "less interference fit with shaft" caused one. Any idea what that means?

 

[liveforphysics]

ABEC-1 bearings are used for things like making quiet door hinges. IMO, they aren't even supposed to spin. lol They normally feel like the inner race is wobblely and about to fall out right off the shelf.

 

[swbluto]

Well, the other bearings don't wobble. :/

Anyhoo, I was searching bocabearings and it appears the only thing that's "sealed" and "not abec 5" and higher at the same time are orange seals.

http://www.bocabearings.com/main1.aspx?p=product&id=18016&n=SMR6800C-2OS_#7_LD

However, they cost $19 each. If it turns out this is something I'll have to replace every 400 miles (Due to who knows what reason), I don't want to be paying that much for each one!

So I'm looking around. What do you think of non-stainless sealed chrome-steel bearings operating in moist conditions? Would they be likely to last longer than 400 miles / 2 months assuming no problems other than moisture-related / dirt problems?

Here's the kind I'm looking at: http://www.4ballbearing.com/10-Ball-Bearing-Lot-6800-RS-Sealed.htm

I can't seem to find any 6800 stainless sealed bearings other than ceramic. Wait a minute... they don't even mention ABEC. Is it so hard to find Sealed 6800 with an ABEC rating? I'm thinking let's forget about stainless.

BTW, I just received my digital calipers and it appears the shaft's diameter, despite all the sanding I've done, is 10mm dead on. In the parts I sanded, it only thinned it by .03 mm. I thought sanding was more effective in decreasing the diameter and now I think I'll have to come up with another plan to thin the diameter to accompany an imperial unit-based pulley.

 

[liveforphysics]

I've liked VBX bearing. They are paypal friendly, and always seem to have my order to me in 1-2 days. Prices are fair.


If you want a permanent solution, ceramic would be the way to go. Here is a little info about ceramic bearings:
http://www.howcroft.co.uk/ceramicbearings/

Like with any bearing you wish to last, you will need to have a perfectly round shaft and recess for them to mount upon.

If you want the best:
http://www.vxb.com/page/bearings/PROD/Kit8226

If you wana get enough to change them out whenever they make noise:
http://www.vxb.com/page/bearings/PROD/kit804

 

[swbluto]

I've liked VBX bearing. They are paypal friendly, and always seem to have my order to me in 1-2 days. Prices are fair.


If you want a permanent solution, ceramic would be the way to go. Here is a little info about ceramic bearings:
http://www.howcroft.co.uk/ceramicbearings/

Like with any bearing you wish to last, you will need to have a perfectly round shaft and recess for them to mount upon.

If you want the best:
http://www.vxb.com/page/bearings/PROD/Kit8226

If you wana get enough to change them out whenever they make noise:
http://www.vxb.com/page/bearings/PROD/kit804

Thanks for the hand holding. That made my purchasing decision a whole lot easier!

I don't honestly know if the dynamic load is what's killed it, but if that happens to be the case, I'd rather constantly kill cheap bearings than expensive ones. I don't get the impression that full ceramics have mystical load properties (Although, for the ceramics, the balls themselves wouldn't fail, would they? So it'd just be a matter of replacing the races which would be relatively cheap, I guess. That's assuming the balls just don't start spilling like they did once out of my rear wheel. )