Page 187 - Improving Machinery Reliability

158 Improving Machinery Reliability

produce an almost insignificant preload of approximately 22 Ibs, whereas an interfer-
ence fit of 0.0007 in. would result in a mounted preload of 200 lbs. A much more
significant preload would result from temperature differences between inner and
outer bearing rings. Such differences could exist in pumps if heat migrated from high
temperature pumpage along the shaft, or if the pump design incorporated cooling
provisions which might artificially cool the outer ring and would thus prevent it from
expanding. By far the worst scenario would be for a pump operator to apply a stream
of cooling water to the bearing housing from a firehose. It is very discouraging to see
this done even today, but old habits are difficult to break, regardless how counterpro-
ductive they may be.
Several ball bearing manufacturers now recommend “flush ground” bearings or
bearings with different contact angles (e.g., 40” on the active and 15” on the inactive
side) for back-to-back installation in centrifugal pumps. During operation, these
bearings either become preloaded or remain preloaded. This prevents (or minimizes)
ball skidding problems discussed on page 172. Moreover, flush-ground bearings will
operate at a lower temperature than the initially preloaded pair. Lower temperatures
can improve lube oil oxidation stability and life. Lowering the total load (i.e., thrust
load plus radial load plus preload) would allow us to anticipate greater oil film thick-
ness and less metal-to-metal contact.
Specification Amendments Covering Pump Bearings. The “best-of-competi-
tion” plants in the processing industry use definitive component specifications when-
ever reliability engineering concepts are actively pursued. They somehow realize
that buying on price alone is rather naive and that a better product will have to com-
mand a better price.
A Dutch refinery with 11,000 pumps amends API 610 by asking that:

All rolling elements should have metal rolling element retainers,
Parallel roller bearings are preferred as pure radial bearings.
Roller bearings should have the roller retaining rim on the inner race.
Shielded or sealed bearings should not be used except for vertical in-line pumps up
to 22 kW.

A major refinery in Texas tacks a machinery component parts specification to
both inquiry and purchase documents (Table 3-13).
When the API 610 Seventh Edition became available in February of 1989, we
advised a large-scale user of centrifugal pump bearings to consider replacing para-
graph 2.9.1.5 with the replacement wording shown in Table 3-14.
Take your pick, or combine elements of the various specification amendments into
a docuinent that suits your particular needs. Seek out the world’s leading manufac-
turers of rolling element bearings and ask them if they have a business team, or
application engineering group dedicated to fluid machinery. Get their advice, request
their relevant literature-you may be pleasantly surprised to see the value of sound,
experience-based recommendations. But don’t expect to get better bearings, or any
other precision components, without making an effort to assemble a pertinent speci-
fication document.