Lubricating Guidelines for Enclosed Gear Drives
 No
single factor affects the life of a gear drive more than lube oil.
To complicate matters, today's marketplace offers more types of
high quality lubricant than ever before, and users are faced with
important decisions on which product is best to install in their
drives. In many cases, plant engineers simply don't have the information
necessary to make the right selection for their application. Of
course, one should rely on lube suppliers or the drive's manufacturer
for advice; but for general background, the following guidelines
can be followed to help select and maintain the proper lubricant
for enclosed gear drives.
Lubricant's Basic Function
To determine which lubricant you need, it is necessary to understand the oil's basic function. Lube
oil prevents metal to metal contact between all sliding and rolling surfaces in a gear drive. In other
words, it reduces friction to extend life of the drive. As a secondary function, lubricants also carry
away wear particles and contaminates to prevent premature wear.
How to Select the Proper Gear Box Lubrication
Selected lubricants must match manufacturer specifications, usually inscribed on the drive's name
plate. These specs are designed to balance the needs of the bearings (generally requiring a light
lube) and gears (usually needing a heavy lube). This balance can only be achieved through proper
viscosity selection. Viscosity determines film thickness and strength, which in turn, determines drive
performance and life. Viscosity is also affected by ambient temperature, drive loads, operating
speed, and the final operating temperature of your unit. When selecting lube oils, always determine
its "viscosity range" because it has direct correlation with the different lube types, i.e.: extreme
pressure (EP) compounds, rust and oxidation inhibited (R & O) mineral oils, synthetic oils, etc.
For reference on which viscosity range to select, the American Gear Manufacturers Association
(AGMA) has adopted the international standards organization (ISO) system for classifying
viscosities. Most gear manufacturers recommend using the AGMA 250.01" Lubrication of Industrial
Enclosed Gear Drives" as a standard. Generally, when operating a unit in cold weather, they advise
the use of lighter oils (e.g. AGMA 4/ISO grade 150) that can circulate freely at low temperatures.
During hot summer months, use a higher viscosity oil (e.g. AGMA 6/ISO grade 320) that will not
lose its film/strength thickness.
Gear Box Lubrication
In most gear drives, a mineral oil with rust and oxidation inhibitors is sufficient when the proper
viscosity range is maintained. Still, some operators insist on using higher price doils with special
additives to increase performance. Generally speaking, this isn't necessary. When selecting special
lubricants, the best oil is one that meets your needs in viscosity, consistency, and overall life.
Basic Lube Types
There are four basic lubricant types for enclosed gear drives: Single grade R & O lube - A proven
performer widely recommended by most gear manufacturers. These lubes are generally low priced
and won't break down due to oxidation of additives. However, they have limited ambient
temperature changes and may require seasonal changes to match viscosity to anticipated
temperatures. Multi grade R & O oil - This lube type offers a wide temperature range, but generally
isn't recommended for gears in the mid to high horsepower range. In addition, it offers limited life.
EP lube - Used to provide more protection from metal-to-metal contact, there by increasing gear
life in marginal (heavily loaded) Synthetic hydrocarbons The "high price" option. On the positive
side, their wide operating temperature may eliminate the need for seasonal changes. Plus, they can
reduce friction levels and increase operating efficiency (although difficult to quantify.)
On the negative side, synthetic lubricants can be sensitive to contamination. Even though these
lubricants are said to be compatible with R & O mineral oils, it's best to flush the gear unit at least
once when changing to synthetics.
Lube Maintenance
Once you've selected the proper lubricant, proper maintenance becomes the next critical factor
affecting your gear drive. Lubricants wear out just like other drive components, and their lubricating
qualities can change with time and temperatures.
To prevent sludging and metal to metal contact, the lube should be either changed or periodically
qualified by sampling. Lubricant samples should undergo a complete oil and ferrographic analysis
conducted either by an independent laboratory or the lubricant supplier. Such an analysis can reveal
the presence of small and large foreign particles. One should also check for changes in lubricant
viscosity and water content due to condensation or leaking of a cooler. Contamination from
internally generated wear particles and outside debris should also be monitored.
In addition, because today's gear drives may operate near 200 operating within the lube life is more
critical than ever before. Guidelines from lube suppliers or equipment manufacturers should be
followed to insure lube breakdown will not occur.
Lubrication problems need not be common at all. The best way to insure your equipment's life is not
shortened by a lubrication problem is to select the proper lube for your drive...then make sure that
the lube is maintained in good condition at all times.
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