Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

526 Centrifugal Pumps: Design and Application
lubricant must dissipate heat rapidly, grease should not be used. In many
cases, associated machine elements that are oil lubricated dictate the use
of oil for rolling element bearings. Listed below are some of the advantages
and disadvantages of grease lubrication.
Advantages
• Simpler housing designs are possible; piping is greatly reduced or
eliminated.
• Maintenance is greatly reduced since oil levels do not have to be maintained.
• Being a solid when not under shear, grease forms an effective collar at
bearing edges to help seal against dirt and water.
• With grease lubrication, leakage is minimized where contamination of
products must be avoided.
• During start-up periods, the bearing is instantly lubricated whereas
with pressure or splash oil systems, there can be a time interval during
which the bearing may operate before oil flow reaches the bearing,
Disadvantages
• Extreme loads at low speed or moderate loads at high speed may create
sufficient heat in the bearing to make grease lubrication unsatisfactory.
• Oil may flush debris out of the bearing. Grease will not.
» The correct amount of lubricant is not as easily controlled as with oil.
Oil Characteristics
The ability of any oil to meet the requirements of specific operating
conditions depends upon certain physical and chemical properties. In
cold ambients, the lube oil must have a low enough pour point to ensure
that the oil remains in the liquid condition, whereas in hot ambients, the
lube oil must have a high enough viscosity to ensure that the rolling elements
are coated with an oil film of sufficient thickness to prevent metalto-metal
contact.
Viscosity
The single most important property of oil is viscosity. It is the relative
resistance to flow. A high viscosity oil will flow less readily than a thinner,
low viscosity oil.
Viscosity can be measured by any of a number of different instruments
that are collectively called viscosimeters. In the United States, the viscos-