Oil vs Grease, max roller bearing speeds?
Oil vs Grease, max roller bearing speeds?
(OP)
Working on a bearing assembly, the bearing mfr defines max speed limits for oil and lower ones for grease lubrication. Is there a generic formula for this derate or is each bearing evaluated on geometry, size and load rating? Since the published value is given for a certain set of operating conditions, what's the best way to derate published speeds for higher axial or radial loads? Thanks!
RE: Oil vs Grease, max roller bearing speeds?
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.
RE: Oil vs Grease, max roller bearing speeds?
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!
RE: Oil vs Grease, max roller bearing speeds?
Best regards,
Will Miller
NOVA Engineering & Consulting
novaeng@tampabay.rr.com
RE: Oil vs Grease, max roller bearing speeds?
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.
RE: Oil vs Grease, max roller bearing speeds?
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.
RE: Oil vs Grease, max roller bearing speeds?
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
RE: Oil vs Grease, max roller bearing speeds?
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
RE: Oil vs Grease, max roller bearing speeds?
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.
RE: Oil vs Grease, max roller bearing speeds?
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
RE: Oil vs Grease, max roller bearing speeds?
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.