High Temperatures and Bearing Breakdown

First of all let me accept that I am not in the furnace maintenance business

a) Bearing Endplay : I am not aware of the 2C but if it is C2 than it is class 2 clearance. This is normal. However all will depend on the design aspects hence may be a bit difficult to guess.
b) The Grease is OK for the application, However Molykote 41 is now available, which is a superior grease

c) You are saying thet it is an aging furnace. That means the bearing is the original one, not replaced ?

d) In case that is the fact, the above may be normal.

The greases are normally under a bit of stress in the closed bearings. As they keep on accumulating (you can not normally purge out the whole of the old grease and fill in new grease), these harden over time and may start behaving as you said.

Recently, we have opened one machine (Metal cutting) - for over hauling, the greased bearingshad quite a bit of muck, that was grease, and was almost on the point of getting jammed. Infact quite a few rolelrs were. We had to dip it in kerosene for a day and nrush it to remove all of it. Of course after that too, was found to be not useful, there were visual start of surface distress.

This is even after using the general maintenance guidelines.

The greases will oxidise over time and decompose. Especially the contact point temperature will be a bit high, but that is usally not enough to crack the grease. The selection of grease takes care of it. It is for general temperature and not the contact point, where the temperature will be just a bit higher, but not too much, since if the bearings (and endplay) are corerctly selected , the lube film will avoid the metal to metal contact and ensuring rolling motion.

The point of worry will be in case the bearings have prematuredly failed (furnace may be old but brgs new)

And you are not maintaining the proper greasing program/ method (there is a method to grease the bearing)

a) Bearing Endplay : I am not aware of the 2C but if it is C2 than it is class 2 clearance. This is normal. However all will depend on the design aspects hence may be a bit difficult to guess.

yes it is C2 but for what I know this isn't normally used in high temperature environments.

c) You are saying thet it is an aging furnace. That means the bearing is the original one, not replaced ?

it's an aging-furnace as in a furnace that is used for the aging process of aluminium profiles. the furnace itself is fairly new. the bearings are not the original ones, they're maybe half a year old.

And you are not maintaining the proper greasing program/ method (there is a method to grease the bearing)

the bearings in the furnace are greased every 2 weeks

so if we realy use the correct grease than there has to be something else that is wrong

When you grease the bearings, you have to ensure that no overgreasing takes place. Also in a demanding environment, check the grease condition inside and if required once in a while try to replace the grease. Push in a bit more, keep the safety valve open and check the grease coming out.
The signals are of the grease decomposition.
Also check whether any seal failure or things like that is there contaminating the grease (as I said, not in furnace business hence can only give general guidelines)
Is the bearing in hot zone? if not check any sort of heat getting transferred.
The bearing endplay- you have replaced the bearing what was the initial bearing clearance grade C2 or C3 ?
The initial endplays are decided based on the final endplay conditions hence it is difficult to say that it should be C2 or C3?
Is it having taper bore+adaptor sleeve? or it is shrunk on the shaft?

Ensure proper fitment in case of the adaptor sleeve, since one tends to tighten the locknut too much and land up in lower endplay- especially in these small bearings. These reduced endplay will result in more heating and the grease decomposition and the seizing are the next step.

But if you have not observed any damage/ polishing/ surface distress/seizing to the bearing then likely the clearance(endplay) is not the culprit. Since in the lower end of endplay, you are bould to have sliding action with the normal rolling and you will see atleast the wear if not worst.

Most probable is the grease decomposing, and not being replaced by the top up. Now the grease will decompose only when the conditions are worse than the 250 deg

(BTW, you are in belgium, SKF is nearby, ask their service men, now so many varieties of High temp bearings are available- ceramics etc)

Further to sb's comment regarding ceramic bearings: Koyo do a bearing range called EXSEV which is a bearing type for extreme conditions. It can consist of any combination of special bearing steels or ceramics together with special lubricants (i.e. solid lubricants!). One bearing specifically considered suitable for temperatures up to 500C is lubricated by graphite.

that condition sounds like those bearings haven't been greased in a very long time...

imho: ..the mfg's settings are probably ok..aging furnace..chances are good that those bearings have been superseded by newer types of bearings..

so .. are there " zerk " fittings to allow for a maint sched to grease those bearings .. ?

my 2 cents worth.. hope it's helpful..

Cool the bearings. Cool the shaft. Stick on a aluminum finned heat sink to the main shaft ends. Pump room temperature 30W low viscosity oil with a tea spoon of anti seize compound mixed in, through the bearing - in a simple circulatory system. Anything, as long as it is simple, reliable and easy.

These 'guidance' rollers wouldn't happen to be a conveyor type system, but on the sides or are these bearings on a door of some sort? Your right about the C2 thing with the heat, I would be more inclined to a C4. I don't understand why a 222 type on rollers unless the side loading on the rollers is extreme, in line with the rollers and not normal to them. How about a picture or two with some temperature readings the gurus would love to tie into it then. (not me)

this guidance rollers are between 2 transfer chains and support the baskets that roll into the furnace. a stack of baskets can easily weigh over 3000kg.

thanks for the pdf, it's realy interesting. but i think we'll just keep it all the way it is. if we would realy lubricate the bearings at the interval i calculated, then it would be more in the off-mode cooling down for maintenance than actualy working.

in that case it would be a lot cheaper to buy a second set of rollers and replace the bearings every half year and switch the rollers.

I found an interesting read after I posted a comment. The last bit of the temperature of the bearing and the grease interval, regardless of the kind that is used. I figured it out to be 12 times the normal interval for greasing, once every 12 days, but only two shots each time. Don't grease until old grease is pushed out as it will compromise the seals, as some manufactures suggest.

Oh and here's the post. If you can't get it, call back, I'll try another way.

http://www.nke.at/fileadmin/user_upload/material/articles_eng/Industrial_Lubrication_CA_Winter08.pdf