Hydraulic Oil Selection and Properties Required in Hydraulic Oils

Hydraulic Oil Selection

In a hydraulic system, effective transmission of power is achieved with a fluid that has the following properties.

Low Compressibility; so that pressure and power can be transmitted immediately and efficiently;

Air-Release Property; so that compressibility does not increase due to air bubbles remaining in the fluid;
Anti-foaming; so that foam does not enter the hydraulic system;

Appropriate Viscosity; so that it is viscous enough to provide the necessary lubrication while still circulating easily through the system.

Hydraulic oil selection is made by taking valves and hydraulic pumps into account, because these are the hydraulic circuit components with the tightest tolerances. An oil that is too thin can lead to leakage and insufficient lubrication; one that is too thick creates excessive fluid friction and damages the pump.

In many industrial systems operating at normal temperatures, pumps require oil with a viscosity in the ISO VG 5-100 range. Piston pumps need oil with higher viscosity than vane pumps. Gear pumps operating at high working temperatures need even thicker oil.

The most commonly used viscosity grades are ISO VG 32 and 46.

ISO viscosity grades

Hydraulic oil selection and the pressure factor 

An increase in pressure leads to an increase in viscosity. The viscosity of a typical mineral oil doubles when pressure is raised from atmospheric pressure to 350 bar. At the relatively lower pressures at which most industrial systems operate, the effect of pressure on viscosity is not significant. However, specialized hydraulic equipment—such as extrusion equipment—generates pressures so high that mineral oils cannot be used in these machines. Special synthetic oils can be used instead.

The environmental factor in hydraulic oil selection

Hydraulic equipment expected to operate across a wide temperature range requires a fluid with a high viscosity index.

If the equipment will operate in a very cold environment—for example, a forklift application in a cold storage facility—the hydraulic oil must also have a low pour point. If the hydraulic equipment operates in an environmentally sensitive area, such as by a lake, riverbank, or in a forest, great care is taken to prevent the fluid from leaking. Biodegradable hydraulic oils should be used whenever possible.

The safety factor in hydraulic oil selection

Among the properties required in hydraulic oils, compliance with safety conditions and occupational safety rules is also important.

Mineral oils are not suitable for use in places with fire hazards, such as steel plants and coal mines.

In these cases, fire-resistant hydraulic oils are used. These oils resist the start of a fire and suppress its spread, but they are not required to extinguish it.

The characteristic of fire-resistant hydraulic oils is that they either contain water or are made from non-flammable synthetic oils. Fire-resistant oils that contain a lot of water are not suitable at high temperatures. They are generally weaker lubricants than mineral oils.

Properties Required in Hydraulic Oils

Hydraulic oils must have the following properties to meet the demands placed on them during operation:

1. They must be thermally stable. Otherwise, they lose their properties at high temperatures;
2. They must be resistant to oxidation; the working life of a hydraulic oil depends on its ability to resist oxidation without degrading.

3. It must be easily separable from water.
4. It must be easy to filter, so that small particles can be easily removed.

Related Questions