The ideal method of lubricating enclosed
gear sets would be to place the
proper amount and quality
of gear lubricant is a
sealed case and make no
renewal of the oil during
the life of the mechanism. Manufacturers of
automobiles have this in
mind and approach
such a solution
of gear lubrication by recommending
no drain periods and prolonged use of gear oils. Manufacturers of other
equipment have the same thought in mind and the trend will no doubt increase.
For illustration a side entering mixer description states: “Lubricant is sealed
in the gear case at the factory and is designed to last five years.” Naturally,
any statement as to the life of gear lubricants should be modified with an
expression as to operating condition, environment, etc. However, it is
questionable if the present gear oils are everlasting even under the best of
conditions. Therefore, from both a
service and economy standpoint, used
gear oils should
be removed from the gear cases and replaced with fresh lubricant either when the oil
has deteriorated or become contaminated
or at stated intervals.
AGMA, which is interested in
insuring log and uninterrupted service from gear sets, recommends the
following: “The oil in a new unit should be drained at the end of two weeks
operation and the case thoroughly flushed with light flushing oil. After this,
a change of oil every 2500 hours of operation or every six months, whichever
occurs first, is recommended for the units operating under favorable conditions.
Where operating conditions
are severe such
as rapid rise or fall
in temperature of the gear case
with accompanied sweating
of the inside walls resulting
in a formation of sludge, or
where operation is in
moist or dusty atmosphere or in the
presence of chemical fumes, it may be
necessary to change the oil at
intervals of one to three months.”
It is also pointed out that gear
sets are usually treated by the manufacturer with rust preventive s before
shipment. Such materials may have an adverse effect upon the gear lubricant
and, therefore, should be removed before gear oil is added. Petroleum solvents
are best for this purpose, and such fluids will also help to remove
contaminants, such as metal chips. Immediately
upon removal of such solvents a
low viscosity flushing oil should
be sprayed on the
gears; otherwise, the metal surfaces may
rust in a few minutes. Also the gears should not be
operated, even for a short time, in a dry condition.
In enclosed gear
cases which do not have a drain or a
circulating system, the used gear
oil or flushing oil should be removed by suction. Use of pressure to force
out such oil may damage seals and in the case of
automotive equipment will
probably force oil onto the brake
bands. When considering oil changes
for reduction gears, Forbes et al.^20 mention that
the first month of
operation is the most critical in
the life of gears; therefore, they suggest
a change or careful filtration of the lubricants within two weeks
after the unit is put in service. It is pointed
out that fine metal
particles resulting from the run in period act as catalysts
for oxidation if left in the gear
case. Rather than setting an arbitrary
period of change, the above authors^20
think that periodic oil samples should be taken to determine if the oil is in a
usable condition. Such samples
can be checked for presence of dirt, metals and water. Also the acid number,
viscosity and interfacial tension can be determined. Certain fleet owners
operating heavy trucks have a rule that the oil in transmissions or rear axles
must be changed when the viscosity has increased by fifty per cent.
Where gears are lubricated
by circulating systems, Forbes et al.^20 states: “ the charge may often be used for several years without change, particularly when
adequate filtering equipment is employed”. However, cleaning such
a system when changing oil requires more effort than in splash systems.
Flushing oils are available
which, due to either additives or the
particular solvents used, will
remove most of the deposits from
the oil reservoir and the piping
as well as the gear case. However, a final
cleaning with dry rags free from lint may be necessary.
The greatest care should be
used in cleaning
gear cases and auxiliary
equipment since any used gear
oil left in a case and mixed with
fresh oil will
tend to act as a catalyst to
promote deterioration of the new
lubricant. With the flushing oil on the gear set an inspection is possible.
If this shows rust, the gear manufacturer should be consulted before operating.
If the gears appear to be in satisfactory condition, they should be coated with
the gear oil as soon as possible. In the
case of a circulating system this can
be done without
operating the gears. If a splash system
is employed the gears can be sprayed or wiped with the lubricant before
operating.
Where meshing gears are of dissimilar
metals as in most worm gear sets, the importance of a change of oil and
inspection of the mechanism is very necessary. As the gears seat themselves
bronze particles may become attached to the worm. Such particles
adhere to the steel and
cause a rough surface which
will score the gear. Removal of
the used lubricant and cleaning of the worm threads after a short period of use
will often prevent further wear.
When oils are used on a once through
basis flushing may not be possible. In the case of lubrication by oil fog, only
fresh oil is supplied, and the tight system is under some pressure so that contaminants
should not enter. However, in the case of open gearing, cleaning at intervals
is advisable. Not only does the residuum
type of gear oils pick up
dirt which in turn
may act as abrasives, but also
some of the heavy material packs in the
roots of the gear teeth. If such build ups continue shafts may
be thrown out of line. Such deposits can usually be softened by kerosene or
some other solvent provided there is not a fire risk. If drip pans are provided
for open gears these should also be cleaned at regular intervals.
