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The principle task of a hydraulic oil is to transmit power; it also has to lubricate the internal components of pumps and motors within the system. Other important considerations for hydraulic systems will be the ambient temperatures over which they will be required to work and the length and diameter of pipes through which the hydraulic fluid will pass.

ISO 6743/4 ‘Classification of hydraulic fluids’ identifies no less than 15 different categories for hydraulic oils that are intended to cover every possible type of industrial hydraulic system applicable.



Above: Hydraulic Particles Test Kit

Info Centre >> Fuel and Lube Oil Technical Manual >> 7. Lube Oil - Test Results >> 7.6 Hydraulic Oil

7.6 Hydraulic Oil

Hydraulic Oil

The choice of hydraulic oil to be used requires careful consideration. As a generalisation, the best hydraulic oil is the thinnest that will do the work effectively as this minimises the pressure drops and pipe size/strength.

Where the hydraulic system requires the use of mineral oil, it will be necessary to allow for the change of viscosity that occurs with change of oil temperature (e.g. arctic/tropical).

Hydraulic Fluid Viscosity

Low temperature viscosity will need to be low enough to enable the system to be started without creating unacceptably high initial system pressures. At the other extreme, the hydraulic fluid’s viscosity at the highest working temperature at which it will operate will need to be high enough to provide effective lubrication of the critical moving parts within pumps and motors. Within modern commercial applications, there are a wide range of hydraulic systems which will have varying requirements in respect of hydraulic fluid performance and viscometric characteristics.

The hydraulic oils can be ‘monograde oils’ with improved rust oxidation and antiwear properties or ‘multigrade’ which include the additional feature of improved viscosity index. These hydraulic fluids will either meet, or approximate to, the requirements of ISO 6743/4 Code HM or HV hydraulic fluids.

An additional requirement for high V.I. hydraulic fluids which use polymer additives to achieve enhanced V.I. is that the fluid should be shear stable.

For the more critical item of hydraulic equipment, a structured programme for sampling and onboard analysis will be appropriate. No hard and fast rules can be laid down on the required frequency of testing.

In many cases, recommendations may be available from equipment and oil suppliers. Items of equipment that have suffered previous operating problems probably warrant more attention than other equipment, at least until it becomes clear that problems have been rectified. In the absence of any other information on the equipment, the safe approach will be to initially use relatively short monitoring intervals to get an initial profile of the trends in the system and then adjust the interval in line with experience. It is important to appreciate that filling the wrong oil into a system and water leakage at coolers is not a progressive event and these types of problem will be missed if the sampling and monitoring interval is too long.