Page 290 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

270 Major Process Equipment Maintenance and Repair

Viscosity Improvers
Viscosity improvers in gear drives should be used with great care.
These polymer additives make great textbook improvements in the vis-
cosity index and extend the operating temperature range of an oil. How-
ever, these polymers are non-Newtonian fluids, and the viscosity of these
fluids reduces with shearing. A gear drive is a very heavy shear applica-
tion, and as a result, the viscosity is reduced rapidly if too much polymer
is used. These lubricants are seldom recommended in long life gear
drives.

Methods of Supplying Lubrlcant

Several different techniques of supplying lubricating oil to the gears
and bearings in a gear unit are available to the gear manufacturer. The
three primary methods in use today are splash lubrication, forced-feed
lubrication, and intermittent lubrication. Each of these methods has iden-
tifying characteristics which are described in the following sections.

Splash Lubricatlon

Splash lubrication is the most common and foolproof method of gear
lubrication. In this type of system, the gear dips in oil and in turn distrib-
utes that oil to the pinion and the bearings. Distribution to the bearings is
usually obtained by throw off to an oil gallery or is taken off the sides of
gear by oil wipers (or scrapers) which deliver the oil to oil troughs.
When using the throw-off system, care must be taken that the operating
speed is high enough to lift and throw off the oil. In this system, the mini-
mum speed required may be determined using the following formula:

np = (70,440/d).5
np = Minimum speed, RPM
d = Pitch diameter, inches

Oil wiper systems can operate at much lower speeds, which are usually
determined by test or through experience.
The splash system can be used in gear units with up to 4,000 ft per
minute pitch line velocity. Higher speed gear units can be splash lubri-
cated with special care.
Forced-Feed Lubrication

Forced-feed lubrication is pressurized lubrication and is used on almost
all high speed gear drives, on spiral bevel drives, and on low speed