Page 229 - Centrifugal Pumps Design and Application

High Speed Pumps 203

nal bearings are sometimes used to take advantage of their excellent sta-
bility characteristics. Plain thrust bearings are inexpensive, and are gen-
erally used up to their load limits of 100 to 150 psi. More severe thrust
loads require use of tilting pad thrust bearings with about 500 psi unit
load capacity.
Bearings are obviously important elements in high-speed design, but
the temptation to oversize bearings in the interest of unwarranted design
conservatism should be suppressed because hydrodynamic bearing para-
sitic losses are not negligible.
Lube Systems. In small units equipped with ball bearings, lubrication
needs can often be met with simple splash systems. Higher power units
equipped with hydrodynamic bearings generally require a pressure lube
system, including a lube pump, over-pressure relief valve, filter, and
heat exchanger.
Free-standing lube pumps are sometimes used, but a pump driven from
the gearbox input shaft is preferable, because lubricant is supplied during
coastdown from high speed in the event of a power failure. Auxiliary
lube pumps are sometimes required when start-up demands are severe.
An example of this is an application with very high suction pressure act-
ing over the shaft seal area producing high thrust at start-up. The thrust
bearing must be copiously lubricated at start-up in order to survive the
short-term boundary lubrication conditions existing until sufficient speed
is achieved to provide lift-off to full film separation. Large machines and
machines with very high stand-by suction pressure are often equipped
with auxiliary lube pumps to provide full lubricant flow and start-up.
Shaft Dynamics. Shaft dynamics is a rather complex discipline that has
evolved substantially over the years to ever higher levels of sophistica-
tion along with other engineering sciences. The advent of modern com-
puter technology has raised analytic prowess to heights which would be
otherwise impractical if not impossible.
The dynamic behavior of a shaft is strongly influenced by the charac-
teristics of the bearings upon which it is invariably mounted; the impor-
tant bearing characteristics being the spring rate and the damping coeffi-
cient. Rolling contact bearings have high, but finite, spring rates as
opposed to relatively low spring rates in fluid-film bearings. Critical
shaft speeds decrease with decreasing bearing spring rates. The high
spring rates of rolling contact bearings usually vary over only a narrow
range, so past experimental spring rate information generally suffices in
shaft dynamics analyses. For hydrodynamic bearings, the relationship
between load and film height are well established, and the spring rate is
calculated by taking the first derivative of the W/h relationship, dW/dh.
It must be recognized that the hydrodynamic spring rate will vary with