Page 286 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

266 Major Process Equipment Maintenance and Repair

problem, the manufacturer cannot assume the fault lies with the lubrica-
tion. Very often a gear problem may be due to unknown factors in the
system, installation errors, or poor maintenance, and yet it appears as a
lubrication problem. Also, poor manufacturing workmanship and engi-
neering design can initially resemble lubrication problems.

Lubricant Function

Lubricants in gear units have basically two functions: to separate the
tooth and bearing surfaces, and to cool these surfaces. On low speed gear
units, the primary function is lubrication; on high speed units, the pri-
mary function is cooling. This statement does not imply that both func-
tions are not important but rather refers to the relative quantity of oil re-
quired to perform each function.
On low speed gear units, the quantity of oil necessary is determined by
the amount required to keep the gear tooth and bearing surfaces wetted.
On high speed units, oil quantity required is generally determined by the
amount of heat loss (or inefficiency) in the bearings and mesh. As a gen-
eral rule, one gallon per minute must be circulated for each 100 horse-
power transmitted; this quantity would result in a temperature rise of ap-
proximately 25°F. Higher horsepower units use a 40 to 50°F temperature
rise and require 0.5 to 0.6 gallons per minute per 100 horsepower trans-
mitted. These figures are based on the assumption of 98 percent gear unit
efficiency.

Modes of Gear Tooth Lubrication
Three different lubrication conditions that can be present between the
teeth of two meshing gear elements are boundary lubrication, hydrody-
namic lubrication, and elastohydrodynamic lubrication. Depending on
load, speed, temperature, tooth design, and tooth finish, any or all of
these lubrication modes could exist in the same gear drive. The goal is to
have either hydrodynamic or elastohydrodynamic lubrication present be-
tween meshing gear teeth. Unfortunately, all too often boundary lubrica-
tion is present and damage to the gear teeth results. In other words, gear
life is determined by wear and consequently, by the mode of lubrication
present.

Boundary Lubrlcation
Boundary lubrication most often is found at slow to moderate speeds,
on heavily loaded gears, or on gears subject to high shock loads. This