Page 185 - Improving Machinery Reliability

156 Intproving Machinery Reliability

Significant Differences In Bearings and Bearing Housings

The eighth edition of API 610 continues to require axially preloaded 40" angular
contact thrust bearings. This requirement stems from three observations that pump
users have made over the decades: Thrust bearings fail relatively often; 40" angular
contact bearings have higher allowable thrust load ratings than bearings with 15" or
29" contact angles; and many failed bearings exhibit ball skid marks in the race areas.
Axial preloading greatly reduces the risk of incurring bearing distress due to skid-
ding of rolling elements. Details on this failure mode can be found in many publica-
tions from major bearing manufacturers. A typical analogy to skidding can be seen
in aircraft landings. Upon initial touchdown, the wheels will skid until their peripher-
al speed has caught up with the forward speed of the plane. Just as skidding would
cause accelerated wear of tires, it would result in potentially severe metal-to-metal
contact in a rolling element bearing.
Axial preloading can ensure that the bearing will always be loaded. With pairs of
angular contact bearings, axial preloading may be necessary.
Preloading or flush-grinding of thrust bearing sets will also prevent axial oscillato-
ry movement of pump rotors. This motion is quite prevalent in pumps that experi-
ence cavitation or low-flow induced internal recirculation. The resulting instanta-
neous acceleration forces must be absorbed by the rolling element bearings. Again,
bearing defects are much more likely to develop with bearings that operate with
axial looseness than with preloaded or flush-ground bearings that operate without
looseness in the axial direction.
In most applications, properly installed and lubricated axial preload bearings have
extended the mean time between pump repairs. This is why many pump manufactur-
ers, and especially the overseas manufactures of centrifugal pumps, do not take issue
with the API requirement. However, there are instances where preloaded 40" angular
contact bearings have been unable to solve problems or have made a problem worse.
While we might have assumed that bearings with larger contact angles create more
frictional heat, research by the FAG Bearing Corporation demonstrated 40' angular
contact bearings generate less heat than thrust bearings with less ang~larity.~~ Never-
theless, by its very nature, preloading adds to the heat load, and using an interference
fit between shaft and inner ring compounds the problem.
Most ball bearing manufacturers consider bearings to be preloaded if an offset, or
predetermined gap, exists between the inner ring and outer ring faces while a light
nominal load, often called a gauge load, is applied to the inner and outer ring thrust
faces (see Figure 3-46). When axially clamped in a back-to-back or face-to-face
fashion with a second equivalent bearing, the balls and races are forced to deflect,
thus creating an internal load or preload. Preload can also be created by interference
fits between the shaft and bearing bore, or the housing and bearing outside diameter,
and by temperature differentials between inner and outer rings-most often due to
the inner ring running warmer than the outer ring. Interference fits and temperature
differentials decrease internal clearance; this will either create or cause an increase in
preload. It is important to realize that shaft interference fits for back-to-back mount-
ed angular contact bearings must be near the minimum of the range normally