Functions of Hydraulic Fluids | Specification of Oil

Functions of Hydraulic Fluids
A hydraulic fluid is the transmitting medium of a hydraulic system. It is, therefore, an essential part of the system and we must know enough about it to ensure that the hydraulic system works efficiently. The most common liquid that is used as a medium in fluid power systems is petroleum-based mineral oil.

In fluid power systems, a hydraulic fluid has to perform various functions such as the following:

1. Power transmission: To transmit power, which is the primary function.
2. Lubrication: To lubricate various parts, so as to avoid metal-to-metal contact and reduce friction, wear and heat generation.
3. Sealing: To seal the moving elements to avoid leakage.
4. Cooling: To carry away the heat generated in the system and to dissipate the heat through a reservoir or a heat exchanger.
5. Contaminant removal: To carry along the contaminations to the tank, where they can be removed through filters.

For a fluid to perform efficiently, it must possess certain properties. The various properties required for an ideal hydraulic fluid are as follows:
1. Ideal viscosity.
2. Good lubrication capability.
3. Demulsibility.
4. Good chemical and environmental stability.
5. Incompressibility.
6. Fire resistance.
7. Low flammability.
8. Foam resistance.
9. Low volatility.
10. Good heat dissipation.
11. Low density.
12. System compatibility.

It is almost impossible to achieve all these properties in a hydraulic fluid. Although we can select a good fluid with desirable properties, some of the characteristics of a fluid change with usage. For example, it is common for the temperature of a fluid to rise due to friction in the system, which reduces the viscosity of the fluid, which in turn increases leakage and reduces lubrication ability. A fluid gets oxidized and becomes acidic with usage. Certain additives are added to preserve the desirable properties and to make the fluid more stable.

Specification of Oil as Per ISO
Standardization of hydraulic oils has been done by the International Organization for Standardization. Table lists ISO VG for engine oils and Table lists ISO VG for industrial oil. Indian Oil Corporation markets oil as per ISO designation.

Other Properties Of Oil

Low Flammability

A fire-resistant fluid is one that can get ignited in the presence of an ignition source but does not support combustion when the source is removed. This characteristic is defined as flammability. It refers to the ease with which a fluid gets ignited and propagates the flame.
Hence, it is desirable to have a low flammability for a hydraulic fluid.

Foam Resistance
Air can be present in a hydraulic fluid in two forms: dissolved and entrained. For example, if the return line to the reservoir is not submerged, the jet of oil entering the liquid surface will carry air with it. This causes air bubbles to form in the oil. If these bubbles rise to the surface too slowly, they will be drawn into the pump intake. This can cause pump damage due to cavitation. Another adverse effect of entrained and dissolve air is a great reduction in the bulk modulus of the hydraulic fluid.

Low Volatility

A fluid should possess low vapor pressure or high boiling point. The vapor pressure of a fluid varies with temperature and hence the operating temperature range of the system is important in determining the stability of the fluid.

Good Heat Dissipation

A hydraulic fluid should have a high heat dissipation capability. The temperature of a fluid shoots up if its heat dissipation characteristics are poor. Too high fluid temperature can cause a system to malfunction.

If the fluid overheats, it may cause the following:
1. Give off vapor and cause cavitation of the pump.
2. Increase the rate of oxidation causing its rapid deterioration by producing sledges, varnishes, etc., thus shortening its useful life.
3. Reduce viscosity of the fluid resulting in increased leakage, both internal and external.
4. Cause thermal distortion in components.
5. Damage seals and packaging owing to embrittlement.
Hydraulic systems should be designed so that a heat balance occurs at a satisfactory operating temperature.

Low Density

The relative density of a mineral oil is 0.9 (the exact value depends on the base oil and the additive used). Synthetic fluids can have a relative density greater than 1. The relative density is important when designing the layout of pumps and reservoir