Viscosity is the measure of fluidity. The oil must have sufficient body to provide adequate sealing effect
between working parts of pumps, valving, cylinders, etc., but not enough to cause pump intake cavitation,
sluggish valve action, or in extreme cases, resistance to flow. Viscosity recommendations must at best be a
compromise, which takes into consideration the working temperature range, the type of hydraulic equipment
used, and the class of service. Refer to table of oil viscosity recommendations below.
The viscosity index is a measure of the rate at which temperature changes cause a change in oil viscosity. It is
very desirable that the oil viscosity remain as nearly constant as possible under the wide range of temperature
conditions encountered in operating mobile and construction machinery. The viscosity index (V.I.) of hydraulic
oil should not be less than 90 for this type of service.
Research has developed a number of additive agents which materially improve various characteristics of oils
for hydraulic systems. They may be selected for compounding with a view toward reducing wear, increasing
chemical stability, inhibiting corrosion, depressing pour point, and improving the antifoam characteristics.
Proper use of additive agents requires specialized knowledge, and they should be incorporated by the oil
manufacturer only, as serious trouble may otherwise result.
Most oil companies have several brands of crankcase oils of somewhat varying formulation that will meet the
API service classification of MS. The more desirable of these oils for hydraulic service will contain higher
amounts of the type of compounding that avoids scuffing and wear of cam lobes and valve lifters. These oils
will also be formulated to be stable under oxidative conditions and when in contact with small amounts of
moisture. There should also be reasonable protection against rust to any ferrous materials submerged in the oil
or covered by the oil's film.
Thorough precautions should be taken to filter the oil in the entire hydraulic system prior to its initial use to re-
move paint, metal chips, welding shot, lint, etc. If this is not done, damage to the hydraulic system will
probably result. In addition, continuing filtration is required to remove sludge and products of wear and
corrosion, throughout the life of the system.
Precautions should be taken in the de- sign of hydraulic circuits to assure that a means is provided to keep the
oil clean. This can best be accomplished by the use of a 25-micron full- flow filter or a 10-micron bypass filter
(not a strainer) and a micronic air breather or sealed reservoir.
Pump Inlet Conditions
Use of an improper grade of oil or restrictive inlet piping may result in inlet vacuum conditions exceeding the
recommended maximum 5 inches of mercury, and will reduce the life expectancy of the hydraulic equipment.
Where vacuum exceeds 5 inches of mercury, and it is not caused by improper oil selection, the Vickers Mobile
Hydraulic Division is to be consulted for recommendations.
Operation in excess of 180 results in increased wear of the system components and causes more rapid