why the C3 bearing is commonly used in MOTOR 1

HELLO"
MY QUESTION IS THAT WHY C3 bearing is used in High or low voltage induction motors. Why not normal or C1 or any else.
thanks

 

[electricpete]

As you know C3 is a designation for internal radial clearance. C2 is normal, and C3 is looser than normal.

I do know that SKF considers C3 to be the standard internal radial clearance for electric motors. Their bearing part number sometimes includes a suffix “EM” for electric motor. When you ask them what that really means they tell you it is the same as adding suffix C3 for any ball bearing (ie 6313EM=6313C3).

As you probably know some bearings have manusfactur’s part number written on them which includes suffixes like C3 (for instance SKF), but some only include a 4-digit number on bearing with no suffixes (like FAG). So it can be difficult to figure out what was the fit of the bearing you pulled out without contacting the OEM. In my limited experience when I have been able to figure out the fit it has been C3.

Now the question why. I can only offer my GUESS:
Internal clearance must be loose enough to avoid clearance-induced overheating but tight enough to avoid skidding.

Things that will make you more prone to overheating (push you towards selecting a looser internal clearance);
- tight interference fits to shaft and housing which will squeeze the bearing and reduce internal clearances further.
- thermal expansion which will squeeze the bearing and reduce internal clearances further.
- high speed bearing
- oil lubrication (produces more heat than grease lubrication)
- - high loading

Things that will make you more prone to skidding: (push you towards selecting a tighter internal clearance)
light load.
roller bearings

It is possible that the range of bearing applications include many applications which are not exposed to high temperatures such as may be encountered in a motor (and even higher in a high-temperature fluid pump). Perphaps the “normal” is defined for those applications and anything exposed to elevated temperatures (like motor shaft) is more likely to require a looser fit.

If we limit ourselves to ball (vs roller) bearing applications which includes almost all small non-belt-drive motors, then skidding is not a concern.

Slightly related topics discussed recently at thread238-24826.

 

[electricpete]

C3 may not be appropriate for angular contact bearings used as vertical motor thrust bearings. There are many variations and considerations for this type design.

 

thanks
but in norrmal bearing the clearence is less so how we select that this motor need c3 or normal bearing.
few days ago we have a high temperature problem in our ht motor whose rating is 45kw and driving a compressor, in its design the bearing used were "normal", when replaced the same then there happen no change in temperature, the temperature go upto 90c,then again we replaced the same bearing with c3 , but the result is same ,now we are in the dilema that what to do.
thANKS

 

[electricpete]

It sounds like you had a bearing that had high temperature 3 times. First time on the originally installed bearing, 2nd time on a replacement c2 bearing, 3rd time on a replacement c3 bearing.

There can be a large number of factors responsible for bearing high temperature.

The very first one that comes to mind is "wear-in" of the grease lubrication. Many times a bearing will run hot for a few days until it properly redistributes the grease. We have that happen many times at our plant, on one group of machines in particular (I'm not sure why it happens on one and not the others). What we do is start and run until temperature is too high, then shut down and cool off and try starting again. We let it get up to perhaps 95C on the external bearing housing, provided that we are monitoring it and ready to shut it down. repeat for 3-4 starts and the bearing settles out around 45C.

I haven't tried it but I think it would be OK to just let it run for a few days, even if it gets up to 110C-120C. I think most greases can withstand that temperature for short-term operation.

Also perhaps you can have a vibration analysis done to look for machine anomalies that may be causing a problem. Most likely would be misalignment... which of course causes high 1x, 2x, and axial vibs.

It is probably well-worth your time to check for out-of-round condition on the shaft and housing. Take 3 diameter readings 60 degrees apart and compare. Also these diameter readings can be compared to the bearing ID and OD readings to check for the proper shaft and bearing fits (too much interference could cause a problem). Also a little bit of debris or roughness on shaft or housing can squeeze the bearing.

If your guys are lubing the bearing (you may want to check that they pack the bearing full and then fill the cavity about 1/2 full. If they fill the cavity completley that will cause overheating. If it's a double-shielded or double-sealed bearing then my recommendation is don't even bother lubing it (although some other folks will tell you to lub a double-shielded bearing).

Any conditions which might cause improper loading? If this is a belt drive check for proper belt tension.

 

[jbartos]

Suggestion: Visit

http://dirtbikestore.co.uk/acatalog/DirtBikeStore_com_Engine_Bearings_40.html

http://www.stieber.de/products/cskkkgb.html

http://www.dodge-pt.com/pdf/catalog/bearings/unified_nom.pdf

http://w1.910.telia.com/~u91008203/go-kart/reference.html

etc. for more info

 

[motorman]

Why do motor designers use C3? By customer demand – cost and availability issues. Since it is known that these bearings must be replaced every three to five years, customers demand, and write in their specs, that C3 bearings be used. They do not want motors with expensive or hard to get anti-friction bearings. Also, the C3 clearances are easy for removal and assembling. So C3 becomes the default.

Electricpete, after you add new grease, do you leave the drain plug out during the run-in period until grease quits draining? that seems like an awful long time. Pete, after you add new grease, do you leave the drain plug out during the run-in period until grease quits draining? Also, I do not recommend filling the cavity by more than 1/3 the capacity.

 

[electricpete]

motorman - I believe that your line of questioning is aimed at uncovering why I have observed some bearings to run hot on initial startup after bearing replacement.

During bearing replacement the bearing itself is of course full of grease and we pack the cavity 1/2 full. We don't leave the grease drain plug open after bearing replacement... that would only be in case of periodic relubrication (do you agree?).

This behavior has been repeatedly observed on one set of machines. Not on any others in our plant to my knowledge. I'm not sure what is different about these machines, but we talked to SKF and they told us it is common for newly-replaced bearings to require a run-in period for redistribution of the grease, after which temperature goes down. Our observations on these machines confirm that the temperatures will go down over the first few days of run-time and even faster if the machine is started/stopped several times. I think it is prudent for anyone that repeatedly experiences hot bearing upon replacement to watch the temperature over time and consider the grease redistribution period as a possible cause (relative harmless and easy to cope with).

 

[jbartos]

Suggestion: Beside technical parameters, there are other factors involved too, e.g. price, availability, life cycle, reliability, material, etc.