I'd have to ask first, in what type of application do you intend to use these bearings?  Most bearings are designed for particular applications and appropriate bearing lubricants specified for the mechanical application.  

The NLGI, National Lubricating Grease Institute, publishes several handbooks which may be of help, but I would first determine the following questions:

1. Is the bearing used for high-speed or slow speed applications?

2. Is the bearing used in a specific unusually high heat situation (examples: coffee roasters, boilers, incinerators)?

3. What does the manufacturer of the end-product specify for a lubricant?

Finally, with the info above, you might look at some of the high technology synthetic greases out there, many of which far exceed petroleum-based pressure applications,etc.

For oiled bearings, you might want to consider a top-line synthetic oil as well.  
 

We are the manufacturer - large pumps with spherical roller and thrust bearings.  Speed varies on each size and higher speed units get synthetic oil.
Some customers specify grease - usually on very old bids that just get resent each time - and some 'old school' engineers have not seen the advantages of synthetics. I'm trying to determine the max safe speed for grease.

Thanks for your help!

Sprint, if you'd like, I can recommend a manufacturer of top-line synthetic oils & greases for your application. This company has 30 years experience in the synthetics business and in addition to a wide variety of synthetic oils, it also manufactures a wide line of synthetic greases specifically for high speed or low speed bearings applications, and for extreme pressure, extreme temperature, and marine or water-soaked applications.

Best regards,

Will Miller
NOVA Engineering & Consulting
novaeng@tampabay.rr.com

Generally speaking grease can be assumed as the "better" lubricant for a bearing. Better because it stays where you put it and the change of leaking away is very small with a good design. It is also capable to avoid contamination of the bearing from outside sources because it can create an entrance barrier. At high speeds greases may give too much friction and therefore generate heat. These speeds (and loads) are usually determined by the bearing supplier. Although there do exist some general "rules of thumb" for the speed criterium, it seems that each bearing manufacturer has its own set of criteria. This may be related to the material used, the surface roughness etc. I would advise to adhere strictly to the suggestions made by the bearing manufacturer and in doubt consult their technical support staaf.

Under certain conditions grease can not be used. Especially when the lubricant is needed to carry away heat generated elsewhere (heat generation in the bearing itself should not be a problem, unless you overfill the bearing) greases are unsuitable because they cannot act as a heattransportation medium.

Apart from the speed and load conditions there is also the quality of the lubricant itself, be it oil or grease. Under certain conditions (high temperature, low temperature, oxidative gases surrounding the bearing etc) synthetic products (both oil and grease) may be advantageous. Synthetic products usually last longer and this can also be an advantage when it is difficult to relubricate the bearing or when no skilled maintenance staff is available.

Finally, you should realize that most of the time it is the oil that is dissipated in the grease structure that actually lubricates, and not the grease structure itself. The greasestructure more or less acts like a container for the lubricant. It may be regarded as a sponge filled with a lubricant - under pressure the lubricant is temporarily made available and as soon as the pressure is relieved, the sponge/grease structure "soaks back" the lubricant.

romke, just to balance a bit the picture in favour of liquid lubricants, it's worthy to remember one service that greases are not supposed to provide is cooling by convection as flowing oils do. Except at low sliding speeds where some greases may be superior to oils, their general lubricating performance is inferior to mineral oils. Besides, they cannot be used at speeds as high as oils because frictional drag would cause overheating.

>> large pumps with spherical roller and thrust bearings.  >>Speed varies on each size and higher speed units get >>synthetic oil. >>snip<< I'm trying to determine the max >>safe speed for grease.

I think you need to find your way to the bearings engineering tech dpeartment.  I have a few contacts if you need help.

Grease limits a thermally stable upper rpm limit.  It's not a bearing/grease/cage material problem that the bearing is running at 140 F, the problem may be the inner ring is 40 degrees warmer than the outer, and the internal clearance has gone to zero.

Things like shaft and housing abutment shoulder squareness geometry and bore and shaft parallelism can add to the heat generation.

Too Light loads can increase heat generation by allowing roller skidding.

A greased bearing may be able to run a very high speed and be ok temperature wise, but the grease life can still be short because of shearing.
We make precision spindles for machine tools.  Those bearings can run at shockingly high speeds, but the grease life can be a few hundred hours.

If you want to go real fast for a long time, you may have to use some exotic $100 grease.  We do.

Do your bearings squeak or squeal during break in?  You may be using the wrong grease
 

Thanks guys!
We build both grease and oil lube assemblies, using the Bearing Mfr limits on speed. What I needed (and what you supplied) was a few 3rd party comments on the application.
DanT raised an excellent point on housing alignment, and this is a constant battle to make sure the parts are within the spec. I prefer oil in all but small, low speed applications since it does a better job of heat/contaminant removal. We've got a great synthetic oil and warranty claims have been almost non-existent since we switched to it. However, there are still a few 'old school' folks who think grease is the only answer since they have not seen the latest lubes on the market. Man - if I even suggested $100 grease around here, I'd be packing my desk!!

Keep the wheels on the ground
Bob
showshine@aol.com

The fact is oil is a better lubricant than grease, unfortunately oil cannot be used in all circumstances. Area in particular are poor sealing, virtical shafts,etc.
NORIA have a table which lists maximum DN values for applications using grease and those for oil.
DN is calclated by taking the speed of the bearing (N) the inner diameter of the bearing and the outer ( Di) (Do) and using the following,
DN = N(Di=Do)/2. DN factors are then used to determine the optimum oil viscosity in both grease and oil applications for the temperature at which the bearing is operating.

Bearing                 Oil        Grease
Cylindrical Bearing    500000      300000
Spherical bearing      290000      145000
Thrust ball & roller   280000      140000.

This clearly shows that grease has a lesser ability to lubricate than oil.    

When I ran a bearing distribution company years ago we ensured that we sold a range of greases backed by one of the "world majors" in oils and lubricants. The synergy of the two engineering departments was excellent and we solved many customers problems.

I would recommend that you speak to your bearing supplier, preferably one that can get you access to the manufacturer's themselves (i.e. manufacturer direct or major distributor), who will be able to advise you for your specific application. Remember that speed limits in catalogues are for the 'average user' and cannot cover all possible applications.

Oil is the better lubricant, but brings about its own problems, usually sealing related.

Knowing the ndm of your application (rpmx average diameter) a good start as some lube manufacturers can offer 'limiting ndm factors' for their lubricants.

Some good advice in this thread.

Lester Milton
Telford, Shropshire, UK

Greases are non-Newtonian materials. One useful propety of greases is that in the high shear region within bearings and gears the viscosity of the grease approaches that of its base oil, whereas in the covers and housings, where the shear stress is low, grease tends to act as a seal.

It is an accepted view that the only satisfactory method of predicting grease performance in rolling bearings is to undertake simulated service trials under controlled conditions in a lab.

Standardized emthods for this purpose are: ASTM D 1741 in which 30-mm ball bearings are used and D 3336 in which 20-mm bearings are run at elevated temperatures until excessive torque indicates inadequate lubrication.

The IP 168 method assesses the quality of greases by measuring grease weight loss, wear of bearing elements and changes in consistency after a 500-hour test, in a 40-mm bearing. The test may be run at its natural operating temperature or with higher temperatures raised by additional heating and speeds in the range of 1000 to 10000 rpm.  

Many bearing manufacturers test greases under a variety of loads, speeds, and temperatures in apparatus of their own design.