Page 289 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Power Transmission Gears 269

A good gear EP oil should have a Timken OK load above 60 lbs. and
pass a minimum of 11 stages of the FZG test. The Timken OK test is
considered acceptable for determining whether a lubricant has extreme
pressure properties, but is considered questionable in evaluating levels of
extreme pressure capacity. The FZG test is widely used in Europe for
evaluating gear oils and is being used with increasing frequency in the
United States. This test uses spur gears in mesh under load, and the
amount of wear is determined by weight loss. The FZG test procedure is
very sensitive to scoring and is considered capable of evaluating the ex-
treme pressure properties of industrial lubricants.
Boron compounds as EP additives are being tested, and these products
show promise as extremely high load capacity lubricants. The com-
pounds being tested exhibit Timken OK loads greater than 100 lbs. and
pass 14 stages of the FZG test. This type of additive is nontoxic and
highly stable but sensitive to water.

Synthetic Lubricants

Synthetic lubricants are not usually recommended by gear manufactur-
ers for general gear applications due to high cost, limited availability, and
lack of knowledge of their properties. Nevertheless, they are used with
good success in applications with extremely high or low temperatures,
where fire protection is required, or where very high speeds or high wear
rates are encountered. The user must be careful when selecting these lu-
bricants since some of them remove paint and attack rubber seals. The
new synthesized hydrocarbons (SHC) have many desirable features such
as compatibility with mineral oils and excellent high and low temperature
properties. They are excellent selections when EP lubricants along with
high temperature operation are required.

Compounded Oils

Compounded oils are available with many different additives. The
most commonly available is a molybdenum disulfide compound that has
been successfully used in some gear applications. It is very difficult for a
gear manufacturer to recommend these oils at this time since some of
these additives have a tendency to separate from the base stock. In many
instances, however, compounded lubricants are the only solutions to gear
lubrication problems. These oils can be blended for extremely high load-
carrying capacity and high temperature operation. Most of these “super”
properties can be obtained, but sacrifices must be made in other lubricant
properties such as life or corrosion protection.