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Pump Bearings

7.5 The total bearing arrangement

To ensure that these bearings provide long-lasting, trouble-free service,
it must be recognized that they are only a part of the total bearing
arrangement. Other important aspects include the support and the
protection of these bearings. Such support includes a strong shaft and
housing to minimize the effect of any externally induced stresses or
vibration. It also requires accurate machining of the housing and the
shaft as well as the correct fits for the bearings.
Protection of the bearings is supplied by the lubricant which is required
to separate the rolling elements and the raceway contact surfaces, to
minimize the effect of friction, and to prevent corrosion. The selection
of the lubricant is a consideration of it's viscosity, and depends on the
operating temperature, the bearing size and it's rotational speed. The
bearing manufacturer can identify the minimum viscosity required for
all these conditions, and the chosen lubricant should provide a higher ~
viscosity than the minimum identified. This reduces the friction losses
in the bearing and extends the operating life.

7.6 Oil lubrication

ISO Viscosity Grade denotes the oil Viscosity at 40~ In other words
an ISO VG 100 oil has a viscosity of 100 centistokes at 40~ As the
Viscosity Grade of the oil increases, these oils will provide satisfactory
lubrication to the same bearing, but at increasing temperature
limitations. Most pump applications will require either an ISO VG 68,
or an ISO VG 100 oil to provide a greater safety margin to the bearing
lubrication.

Quality mineral oils oxidize at a continuous operating temperature of
100~ and should be changed every three months or 2000 hours,
whichever is the sooner. Synthetic oils however, tend to be more
resistant to temperature effects and therefore require less frequent
change. As a general guideline, the lubricant should be changed when
the maximum contaminant levels of 0.2% of either solids or water are
identified.

7.6.1 Static oil lubrication
In a horizontal pump, the oil level should be set at the center of
the lowest ball in race. Oil must be able to enter the bearing from
both sides, particularly in the case of double row or paired single
row bearings where a pumping action from the bearings may repel the
oil.