Page 228 - Centrifugal Pumps Design and Application

202 Centrifugal Pumps: Design and Application

Helical gears introduce thrust which must be reacted by the bearing
system. Thrust can be eliminated by use of herringbone gear design, but
this option loses some packaging attraction with ground gears in that a
wide central groove must be provided between the gear working halves
for grinding wheel runout. Piecing helical gears back to back to provide a
herringbone design detracts from already stringent precision require-
ments. Helical gears are generally (but not universally) believed by gear
authorities to offer the advantage of lower noise, but here again precision
looms more important than the spur-versus-helical choice per se. In any
event, hydraulic noise in high-speed pumps has been found usually to
overwhelm gear noise, divorcing noise consideration as a factor in gear-
type selection.
For RE. pumps, where speed is always tailored to a given application,
desired speed is provided by simple gear size selection of standardized
gears to fit within standardized gearboxes. Rarely do power and speed
combine to require the maximum gearbox rating, so most often an added
design margin exists at the rated power in a given application.
Bearings. Ball bearings have evolved to a high state of perfection and are
attractive from the standpoints of low friction loss and modest lube sys-
tem demands. Roller bearings have higher capacity than ball bearings,
but are not well suited for high speeds due to a tendency of the rollers to
skew in operation.
In general industrial equipment, the API guidelines are sometimes
viewed as being unnecessarily conservative, and are modified in the in-
terest of simplicity and low cost. Life projections should be tempered by
foil realization that only contact stress is considered, and that the quality
of lubrication and many other practical aspects of bearing application are
not addressed. In any event, high-speed/high-power design imposes
bearing demands which soon outrun any realistic expectations of design
adequacy with rolling contact bearings.
Hydrodynamic bearings possess capability to operate for indefinite pe-
riods at high load levels and high speed. This bearing type is self-acting
with a film of lubricant separating the bearing elements in steady-state
operation, precluding metallic contact and thus providing zero wear. The
term "thick film" is used to describe these bearings, but this description
must be taken in context since "thick" usually implies film heights of
only a few ten-thousandths of an inch. Metallic contact cannot be toler-
ated in high-speed bearings, so the need for precise alignment is obvious.
Materials selection is important largely because boundary lubrication or
rubbing contact exists during start-stop cycles where full fluid-film sepa-
ration cannot be achieved.
Plain journal bearings have excellent capacity and are nearly always
suitable at pump speeds. For extreme speeds and powers, tilting pad jour-