Viscosity

Basic factors which must be considered in selecting a fluid for an industrial hydraulic system include viscosity, viscosity index and pour point. Other properties such as gravity, flash point, and carbon residue and neutralization number have little significance in determining the initial suitability of oil, but are useful in checking on the condition of used oil to determine its suitability for continued service.

Viscosity is a measure of resistance to flow. It is the most important property of a hydraulic fluid.

Pumps are the most critical part of hydraulic systems with regard to viscosity. Most pump manufacturers specify the viscosity required. If the exact viscosity requirement for a pump is not known, the approximate viscosity requirements for various types of pumps and motors can be determined from the Table.

There are numerous accepted methods for determining the relative viscosity of oils, but the one most commonly employed for industrial hydraulic oils is the Saybolt Universal method. The unit of measurement is time in seconds (Saybolt seconds, Universal or SSU) required for 60ml of the oil to flow through a standard orifice under a standard head and at a given temperature normally 100 or 210 F.

A second method in fairly common use, for determining and reporting viscosity, is the kinematic method. Time is measured for a fixed volume of sample to flow through a standardized capillary under an accurately reproducible head of liquid and at closely controlled temperature. Kinematic viscosity in centistokes is calculated from the measured time.

A third unit, absolute viscosity, is independent of specific gravity. The unit of absolute viscosity is the centistokes and can be calculated by multiplying the kinematic viscosity in centistokes by the specific gravity of the oil.

For any specific application viscosity of oil chosen should represent a balance between requirements of power transmission and those of lubrication and sealing. Low viscosity oils transmit power more efficiently. On the other hand, lubrication and sealing require that the hydraulic oil be of comparatively high viscosity. However the oil must not be so viscous that exceeds the suction capability of the pump.

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1. What properties of fluid are useful to determine its suitability for continued service?

2. Why do most pump manufacturers specify the viscosity required?

3. What are the methods for determining the relative viscosity of oil and what is the difference between them?

4. What requirement should oil meet as regard to viscosity?

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WHAT IS HYGRAULIC FLUID?

Hydraulic fluids are a very large class of materials that are used in machines and equipment to transfer pressure from one point to another. They are used in many ways including all fluids for car automatic transmissions, brakes and power steering. Hydraulic fluids are also used in many machines like tractors and other farm equipment, forklift trucks, bulldozers and other construction equipment, and airplanes. In industry hydraulic fluids are used in machines that push, lift, pull, turn and hold things. This profile covers only three of the many types of hydraulic fluids: ventral oil, organophosphate ester and polyalphaolefin. These types are among the most commonly used today.

Some hydraulic fluids have a bland oily smell, while others have no smell. Mineral oil and polyalphaolefin hydraulic fluids are mixtures that have oil in them and will burn. Oil-in-water hydraulic fluids (a special type of mineral oil hydraulic fluid) do not burn because they contain water. Organophosphate ester hydraulic fluids are mostly made without oil and will not burn unless there is a flame directly on them; once the flame is removed, these fluids will stop burning. Because they do not burn, organophosphate ester hydraulic fluids are used in airplanes and other places where fires are very undesirable.

Mineral oil hydraulic fluids are produced from crude oil. Organophosphate ester and polyalphaolefin hydraulic fluids are manufactured. All hydraulic fluids contain many ingredients which reduce wear, make the fluid flow better, and make it thinner when it is cold. More than 200 million gallons of hydraulic fluids are sold each year in the United States.

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