I Could Use Some Help Identifying This Substance That is Seizing Our Motors

Interesting suggestion. Let me speak with the bushing manufacturer and i'll report back.

Keep in mind that the failing bushing may be a counterfeit or honest mistake replacement. So the bearing manufacturer standards could be irrelevant.

You might also contact Mobil about a different lubricant that won't react.

I initially thought this as well. I had a chemical analysis conducted, and confirmed that the bushings were the correct composition.

I don't see any evidence of galling in your photos, where does it appear to you the jamming is actually taking place?

There is little to no galling, which is only adding to my confusion.

All of the motors that I recieved back from customers, the motor was locked in place. I had to put a pretty good effort into spinning the shaft to get them to move. After disassembling and cleaning the "gel" out of the bushings, I put the motor and unit back together and it spins freely like it should.

Another interesting note is that I have had complaints from customers that motors are jammed right out of the box...so it can't be linked to misuse. It is either happening during manufacturing and slipping through our QC and assemby (basically what Yahlasit mentioned), or its happening during shipping.

I mapped out all of our shipping variables such as time of year/ambient temp/shipping location/date of manufacture and I see no patterns pointing to any one cause...which leads me to believe Yahlasit has the right idea.

This motor is only used in a hotplate/stirrer. I feel like there isn't enough force being exerted on the shaft that would cause it to shift.

My solution at this point is to replace the bushings with bearings. Its an extra $1 per motor...which puts it right on the border of being cost effective compared to the volume that we anually produce.

If I can't identify a cheaper alternative, I may just have to have our assembly team do a quick check of each motor before it is installed in the unit to make sure a faulty one isn't being sent to the customer.

GA

Fully agree. My old US company put cheap fans deep in a machine we used to sell here, we replaced all the fans with high quality ones and we got no problems. The rest of the world had huge problems till they copied us.....

The factory would still not install the good fans, even when we said that we would pay extra, still no dice!!!

NIH problem!!

Fibers from the felt pad? That seems to be a problem....Possibly you need a felt pad with less recycled content and longer fibers....

Cardboard box fibers?

What does a chemical analysis of the goo tell you?

Unfortuntely I wasn't able to get the analysis done on the gel. I didn't think to get a chemical analysis conducted until after the bushings were cleaned and retested. D'oh.

I still think that Yahlasit nailed this on the first answer and that the mineral oil that reacts with the Mobil 626 lubricant is coming from the bearing. No amount of bearing cleaning will stop the embedded mineral oil from reacting if this is true. When you get another jammed motor, have the gel analyzed and proceed accordingly. If you don't get another seized motor, well then your problem has been solved.

Agreed. I ended up going with Mike K's suggestion, and i put some 626 on the bushing / felt pad / bushing boss / shaft. I have them in their own little control environments, so i'll wait a few days or weeks to see if any type of reaction occurs. I'll let this run its course while I gather more information from the manufacturer.

Thanks for the help redfred. Much appreciated!

just my little guess, the goo is something used in the assembly process as an assembly lubricant (the bushing shouldn't require it), the jamming problem is either on the motor end-bell or a faulty assembly, a simple "spin test by the assembler would identify bad motors before they get installed and ultimately shipped

You describe that "motors straight from the box" are also reported jammed.

Hypothesis is that there is a chemical reaction of some sort (not relating to rotation or temperature) that is turning your lubricant into a gel/stiff sheet that is then behaving like an adhesive/sealant instead.

Checking the chemical compatibility of teh associated components would be my suggestion.

You are correct. I will be setting up an experiment very similar to what Mike K describes a few posts down. I'll keep you posted.

Yahlasit may be right and this isn't my area of expertise, but . . .

It appears that particles on the inside of the bushing are being sheared off, leaving debris in the bore that jam the shaft. Is it possible that the shaft has a sharp (insufficient radius) end or shoulder that is causing this problem at assembly? Or (less likely, I think) that the bushings are received contaminated or the assembly area is contaminated?

Thats what it looks like to me also.

I am currently in contact with the bushing manufacturer, so I should have some more information on their processes sometime today. But I can speak for our assembly area...and you could eat off of any surface in there.

As far as the shaft have some kind of sharp edge which would scrape the inside of the bushing: It definitely looks like the "gel" has some bronze powder in it. That was my initial thought too. But there are no signs of galling in the bushings and the motors are failing before the customer even has a chance to turn them on.

I noticed in your 2nd picture that the bushing is not centered. Since it is a self aligning bushing, it has rotated in it's housing causing the shaft to be mis-aligned/freezing up. The fiber material may be getting in between the bushing and housing.

I intentionally mis-aligned the bushing for those photos to give some type of reference point. It was harder to interpret the pictures looking straight down through the bushing.

I don't buy the "seizing in the box" scenario. Lubricants are tested as far as I know to the extent that no mixture would make an "epoxy glue". Have you verified that motor temperature curve is as expected and disassembled the motor in a way to verify if seizing has any favorite side, like the one nearer to heat? The worst case is device plugged-off while element is at highest temp and no "load" to dissipate it. Then, depending on design specifics many things can go wrong. S.M.

The motor had the exact same gel on both the heater side of the motor and non-heater side. I have tested 50+ of these motors measuring operating temperature at various points including the bushing, bushing boss, windings, etc. I saw no evidence that my data was wrong.

The thermal fuse on the motor is also rated for a temperature that is lower than the spec point on the 626SHC lubricant (I account for the fact that the winding temperature will be significantly lower than the point of contact temperature between the shaft and bushing)

What if the bushings are defective ?

If the bore of the bushings is not correctly heat treated.

The other remote possibility is a circulating current, due to the winding design. This destroys many roller bearing generators (typically) and some motors usually above 500 hp.

Nitric acid from corona discharge creates a white powder and if the bearing is shedding a lubricant it could combine.

Are you using a digital drive ? Variable motor drives can produce very high voltage spikes that can damage winding insulation.

Its possible that the bushing was not correctly heat treated, but could particles detach from the bushing even when the motor isn't running? The customers are seeing this problem before they can even run the unit.

One more thought: is the bearing subject to a heat-soak condition when the motor is off, thus not providing air cooling? Obviously wouldn't account for "out-of-box" failures, though.

It is subject to heat-soak, but again, I ran thermal tests on multiple test points and so no indication of the temperature hitting the operating temp limit of the lubricant.

During normal applications, the motor will only hit ~50°C.

Under extremely rare, extremely high load applications (such as viscosities higher than 200 cSt paired with rare earth magenet stir bars), the bushings will reach ~130°C max before the thermal fuse cuts off.

Small stirrers may be speed controlled by an SCR dimmer. Bigger ones may well be VFD drives. The motors may have arrived with their rotor out of center somewhat, as #6 suggested. In that case, a slow or not at all moving motor may well hammer the bearing on the same spot, squeezing out the oil film and effectively run dry. Do not forget, to build a cushioning oil film, a minimum speed is required for every mechanical load. This you can simulate well and fast yourself, with sideloads applied to the shaft.

A roller bearing may help by not needing a sliding motion. Or, on the other end of the spectrum a self lubricating plastic bearing may be helpful.

I happened to have the second one at home, working in rude conditions as a sump pump in the basement. The water has a noticeable clay (polishing and clogging if it can) content. The shaft is polished stainless steel. The open bearing is a nylon like plastic. Runs dry or wet for the better part of 10 years, as needed. No noticeable bearing wear. It is anecdotal obviously, meant to be a pointed finger. If applicable, it may be cheaper, than the sintered one.

What do you know about the environment?

Is the oilite bearing leaching something that's incompatable?

What do you do for cleaning?

I hate it when the only thing you have to do to make something successful is to clean it properly. A semiconductor probe manufacturer never clean their probe properly so they corroded under high temperature. Boiling the probes in baking soda and electroless plating solved that until they soldered with 60/40 solder instead of brazed. They were able to solder with Sn96 so I was happy again.

I also remember using Asco valves which would buzz. Taking them apart and ultrasonically cleaning them worked. We were using dryair and the dust was causing them to buzz.

Excellent photos. The gel seems to indicate incompatible lubricants. Dis-assemble one of the bearings and mix your Mobil 626 with the felt and see if any gel forms. Then mix the bearing alone with the 626, warm it up to release the impregnated oil from the bearing to see if any gel forms.

If either test makes gel, change your oil and re-test.

Heated stir plates are by no means cheap. I would think a test of each completed unit would have shown the problem by now. Maybe you should stop trying to figure out all the possibles and just go with another motor.

I don't think anyone has commented on the bearing clearances on the shaft. I've seen these kind of motors on fans seize up, displaying similar problems as yours. Opening up the bearing clearances fixed the problem.

Have you looked at the motor shaft tolerances and the bearing bore tolerances? It would seem to me that you might be seeing tolerances that are too tight at one end of the statistical distribution curve. (IE, a shaft at the upper end of its allowable diameter and a bushing at the low end of its allowable bore.) A mix of gummy lubricant, a few fibers, and some bearing casting flash may all add up to taking that minimal clearance down to nothing and jamming up your motor. This combination of guck "disappears" once the motor is pulled apart and the jamming will be hard to replicate.

You might need to talk to the manufacturers of the respective parts and review the allowable tolerances. A looser tolerance might be all you need.

I've seen this on plastic bearings, too. This was a bit of a different animal, though, since some plastics can swell with oil or atmospheric humidity.

Hope this helps,

Jon.

I dimensioned all of the failed bushings/shafts and they were all well within tolerance (not at the upper or lower limits), so that doesn't seem to be the problem.

Does the motor shaft end up with some lacquer on it when the insulation is coated?

Just have to ask - is this motor being run by a VFD?

From Mobil's web site the 626 SHC is compatible with mineral oils. There is another possible source. The felt. They use soaps to help the fibers intertwine. They may not be rinsing the soaps from the fibers afterward. There also could be a binder in the felt that the lubricant is dissolving.

"While Mobil SHC 600 Series are compatible with mineral oil based products, admixture may detract from their performance." -From Mobil's website

So there is still the chance that the combination of mineral oil + synthetic oil + elevated temperature could cause some congealing.

I will speak with the felt pad manufacturer and find out thir rinsing method as well as find data sheets for any binders used.

There's another thing you might wish to do. I'm a big fan of measuring attributes when you do not know what is happening. Take a working motor and measure the locked rotor torque. (Don't go by the manufacturer's typical values.) Whatever condition is binding your motor must be able to resist this torque. Your goo may be a red herring and not the root cause. Your goo could also be the result of a locked rotor heating and not the cause.

Many interesting ones. I am still partial to #19, #6.

The off center rotor / SCR contoller / Slow movement / side load caused galling by displacing the oil film is an independent mechanism, easy to test.

All it takes, is: A large bearing + weight for side load. SCR control to run the motor Oh-so-slowly to disrupt oil film buildup. If there is anything to it in your case, a few days running of a returned motor gives you answer one way or the other.

At the same time, delrin (I think) plastic bearing machined to the same size, will give you answer in the same time. Under the same conditions.

No extra time wasted.

Are you suggesting that the particles sheared off from the galling are congealing with the oil to make the "gel"?

Or are you suggesting that the side load/disrupted oil film that caused the galling now allows for the rotor to slightly shift and mechanically lock up with the stator.

The only thing that doesn't lead me to believe the rotor is contacting the stator is that I can take apart the failed motors, wipe down the bushings/bosses/housings, reassemble the motor, and run them again with 0 problems.

Have you done any mechanical vibration / impact testing to see if the motors are actually being jammed and the gel is caused by the excess friction and heat resulting?

This is not your problem. You should go back to the manufacturer of the motor. You just inherited his problem. This is something he has to correct. Maybe the motor manufacturer changed specs and didn't tell you.