oil bearing and grease bearing

I know this doesn't exactly answer your question, but for an exhaustive, in-depth look into bearing lubrication have a look at this download.

Page 20 gets into a little bit of what you are looking for, but not all of the issues.

It depends on the specific application and many other factors in oder to determine the "best" style of lubrication (oil or grease).

But let's take your case as an example: centrifugal pumps.

Oil Advantages:

• Dissipates heat fairly well.
• Level can be easily monitored - usually through "bull-eyes" (however, they can sometimes be clouded over)
• Moves wear particles away from bearings and seals (even better if you install a magnetic drain plug to capture metallic contaminants)
• Oil analysis can be performed to determine condition of lubricant
• It is easy to change out all of the oil, that is - it is hard to completely flush out all grease in a bearing, unless it is torn apart and repacked.

Grease Advantages

• Easy to carry around / cleaner
• Not as critical to keep containers clean of contamination
  
• It's late and I can't think of any others right now

Oil Disadvantages

• Must be very careful/diligent in storage/transfer of oil

Grease Disadvantages

• See exact opposite of Oil Advantages
• You must be VERY careful not to over fill/pressurize bearings as you will blow-out seals

There are probably other issues I am missing, but like I mentioned it is late and I am going to stop there.

The oil bearing design will have less pairisitic loss of power and a longer service life than the greased bearing type. The oil bath bearings will run at a higher rpm and transfer heat better to the housing for disapation than the grease will.

Under severe duty the oil can be circulated through coolers and filters if necessary. If this is just a general purpose installation the greased bearing type may have a satisfactory service life. Regardless of whitch type you use the proper choice of lubricant is critical to service life

Advantages of grease:

· Provides lubrication for longer periods of time without being changed or filled up.

· Requires only simple seals as its high viscosity helps it to stay in place.

· It provides a better anti‑rust film when the machine is not running.

Disadvantages of grease:

· It does not easily disperse heat.

· Old grease is difficult to remove.

· It is limited to low and moderate speeds.

Advantages of oil:

· It decreases fluid friction which leads to a cooler running bearing.

· It has a flushing action on bearing surfaces to wash off the dirt and abrasive material, and helps disperse the heat.

· It can easily be removed from the housing with a minimum of down time.