Page 287 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Power Transmission Gears 267

mode of lubrication exists when the oil film is not thick enough to pre-
vent some metal-to-metal contact. This condition usually shuws up as
early wear and pitting on the teeth due to irregularities in the tooth sur-
faces. When boundary lubrication is encountered, extreme pressure oils
should be used to minimize wear and possible scuffing.
Hydrodynamic Lubrication

Hydrodynamic lubrication occurs when two sliding surfaces develop
an oil film thick enough to prevent metal-to-metal contact. This type of
lubrication usually only exists on higher speed gearing with very little
shock loading.
Elastohydrodynamic (EHL) Lubrication

Elastohydrodynamic theory of lubrication is now accepted as very
common in gear teeth. The formation of EHL films depends on the hy-
drodynamic properties of the fluid and deformation of the contact zone.
This flattening of the contact area under load forms a pocket that traps oil
so that the oil does not have time to escape and results in an increase in oil
viscosity. This increase makes possible the use of light oils in high speed
drives and usually only occurs above 12,000 feet per minute pitch-line
velocity.

Lubricant Selection

Six factors affecting lubricant selection for gear units are listed in Table
6-3 along with the lubricant properties that should be considered in rela-
tion to each. Viscosity is probably the single most important element in
lubricant selection and is determined by load, speed, and temperature
variations. All of these factors should be reviewed and evaluated to deter-
mine the exact lubricant properties necessary for satisfactory gear per-
formance. Final selection of the lubrication oil for the gear unit should be
based on the best combination of all of the required lubricant properties.

Lubricant Types

A good rule to follow when evaluating the type of lubricant to use is to
consider the least expensive one available that will perform well in that
situation. If a specially blended type of oil is to be tried, determine its
stability by selective use before making major changes. Lubricant fail-
ures are expensive!