What is determine the type of the oil viscosity of the bearing:RPM or FPM?

Hello.

As far as i know in general:low speed bearing are lubricated with high viscosity oil while higher speed bearing are lubricated with lower viscosity oil.
The issue that i don't understand is: what is,exactly,determine that the bearing is high speed,medium speed or low speed.? the RPM of the bearing or the FPM of the bearing.
For example,there are two rolling bearingne has small diameter while the other has much bigger diameter and them both have the same FPM.The small one will,obviously,have much higher rpm than the bigger one that could have relative much less rpm.
So,do i apply the same viscosity of grease(oil) to them both,since them both has the same FPM? or the small bearing which has high rpm should be applied with different viscosity(lower)oil than the other ,inspite the fact that they are moving in the same speed(but different rpm)?

Thanks in advance.

 

The speed of bearing which determines whether boundary or hydrodynamic lubrication will prevail is fpm, as it is the relative speed between the moving elements and stationary part of the bearing.

 

Hi inlavati.

So,when i see a grease that declare that it is for low speed bearing,i understand that it doesn't refer to low rpm speed but it refer to the FPM.Is it right?
It could be 100RPM bearing or 10,000 RPM bearing,and both could be considered as to be low speed bearing and them both should be lubricate with the grease i mentioned above.as long them both the same FPM.Am i right?
That is to say for example:that a small bearing which is spinning at 10,000 RPM will need the same oil viscosity as a bigger bearing which is spinning at 100 RPM,if them both the same FPM?
But doesn't the very high spin doesn't influence the oil in the aspect of the rising temperature compare to the low RPM?
Doesn't the power centrifugal has any influence on the higher rpm bearing oil,inspite that both of the bearing do the same distance in a minute(FPM)?

 

Friction is caused by sliding motion between surfaces, and sliding motion is measured in fpm. Ultimately fpm and rpm are related, but for tribology analysis sliding motion is more relevant. For bearings with similar specs under similar load conditions, a higher rpm translates to a higher temperature rise, so rpm is important. But when the bearings are different, and are under different load conditions, its not just the rpm that counts, other variables should also be considered.

 

Okay.But,practical,what about my main question?
If i see a grease in the store that says"for high speed bearing",is it refer to high RPM or high FPM?
For what measurement do they mean?to FPM or RPM?
While i have low RPM bearing with high FPM value,should i lubricate it with low or high speed bearing grease?
while i have high RPM bearing with low FPM value,should i lubricate it with low or high speed bearing grease?

 

Does anybody know if box of grease,which write on it"for high speed bearing",is it mean for high RPM or for high FPM?
I mean if you get in the store and see on the shelf some types of grease which some says"for high speed"some says for"low speed bearing"
Do they refer to RPM or FPM?
I didn't get an answer to my main target,which is to know what the low or high speed bearing sentences that are printed on the grease in the store refer to.RPM OR FPM.
I will very appreciate if you may answer directly to my question an not around it.
Thanks in advance.

 

Nobody's answering your question because we don't know what the grease manufacturer considers "high speed" or "low speed". Also what is high or low speed depends on the bearing... 10,000 rpm might be high speed for a large bearing, but low speed for a small bearing.

Your best bet is to use whatever the bearing manufacturer recommends.

 

Hi xchui,

Dana is perfectly correct in the comment that your question is not being answered appropriately, owing to all of the undefined ambiguities in it...
Things to consider are;
Where is the grease being applied - the I.D. or the O.D. of the bearing?
What is the tolerance between the bearing and the shaft, or the tolerance between the inner and outer race - this is relevant when considering the viscosity of the grease.
What type of bearing is it - there are many different types - thrust race, roller thrust, tapered roller etc.
And to be honest, there is no such thing as 'high speed' relative to 'low speed' grease. As Dana said, you should have a manufacturer specific data sheet that will specify the correct lubricant for type, duty, operating temperature, environment and load.

 

What is the application to which the bearings are being used?

Have you thought about the load?

Surely they can't be operating at the same speed if the RPM is different to each bearing can they?

 

Thanks for all the responses.
If you were reading the catalog(datasheet) of a company that makes greases(like:molykote),you could see that they mention each type of grease(among other specs) as it for high,medium or low speed bearing.
So,i try to figure out if it is refer to the number of the bearing's spins for a minute(RPM) or to the speed of the bearing for a minute(FPM).
Maybe it is refer to the FPM,like ihlavati explained,it seems reasonable.

Thanks