Page 174 - Know and Understand Centrifugal Pumps

Bearings

specifications. The pump manufacturer installs bearings suited to the
pump’s design and general utilization. Based on this logistic, it’s always
best to use and replace the same type of bearing in a maintenance
function.

Bearing lubrication

Oil lubrication
Oil is a popular pump bearing lubricating fluid. There are two
classifications of lubricating oils: synthetic based and petroleum based.
Synthetic lube oils were developed principally to improve their high
temperature stability and overall temperature range. Petroleum based
oils are lower in cost and have excellent lubricating properties. Animal
and vegetable based lube oils are rarely considered for bearings because
they deteriorate, spoil and form corrosive acids.
As a lubricating fluid, lube oil can be circulated, filtered, cooled,
heated, pumped, atomized, etc. For these reasons it’s more versatile
than grease. It’s convenient in severe applications involving extreme
temperatures and high velocities. The oil level and flow through high-
speed bearings is critical and these parameters should be monitored
appropriately. Because oil is a liquid, it is more difficult to seal and
contain inside the bearing chamber than grease.
Viscosity is the term used to indicate oil’s thickness or consistency. The
viscosity test is a measure of the fluid’s film strength. Excessive viscosity
can lead to the rolling elements skidding inside the bearing races. This
leads to raceway damage and overheating of the oil. Inadequate oil
viscosity can lead to premature bearing seizure from metal to metal
contact. Other properties to consider in selecting and using oil are:
corrosion protection additives, the pour point, the flash point, the
viscosity index, carbon residue, and the neutralization number.
Consider the loads imposed and operating temperatures of the bearings
when selecting the oil and its viscosity. Correct selection is essential for
long bearing life. The lubricant manufacturer publishes a data sheet
detailing the characteristics of their products. You should get this sheet,
study and understand the properties of the lube oil you’re considering
for use in your pump bearings.
Among oil’s advantages are:
Excellent friction reduction at high speeds.
Provides good lubrication at very high or low temperatures.
Good stability.
Heat transfer coefficient is 0.5 BTU/pound/degree Fahrenheit.

157