Hydraulic Oil Choice Of Oil; Properties That Determine Which Hydraulic Oil Should Be Used; Viscosity; Viscosity Index - Viscosity Class - Kramp MPP Manuel D'utilisation

6.0 Hydraulic oil
6.1 Choice of oil
Energy transfer is the main function of the oil used in the system. In addition, it also lubricates the components and should be
able to carry impurities, wear particles and heat out of the system.
Required oil properties:
• good lubricating properties
• good dirt-absorbing properties
• appropriate viscosity for its purpose
• good antifoam additive
• good air-separating properties
• good water-separating properties
In the end, the conditions of use determine the oil to be selected. There are three basic types of oil to choose from:
• mineral oil (most common type of oil)
• synthetic oil
• organic oil
With synthetic oil, particular attention should be paid to whether it is phosphate-ester based, in which case special gaskets
are required. Organic oil should only be used under certain conditions as it can be highly hygroscopic (= water absorbent) and
may have a short life-span, depending on the conditions.
In selecting a type of oil, consult with the oil supplier and the supplier of the hydraulic system. Once a decision has been
taken, the type and brand of oil should be clearly indicated on the hydraulic system. The end user should also be informed
as different types and brands of oil cannot simply be mixed. The various additives used by the different brands may start to
react with one another, leading to the loss of the required properties of the oil. Contact your oil supplier if you have any doubts
about replenishing oil. Under normal operating conditions (38 to 50°C), the viscosity for gear pumps and plunger pumps
should be 32 cSt. Before using oil under extreme conditions, it is sensible to consult your oil supplier about the right choice.
6.2

Properties that determine which hydraulic oil should be used

When selecting the right type of hydraulic oil, the following important properties should be taken into account:

• viscosity

• viscosity index VI and/or viscosity class VG (viscosity at 40°C)
• pour point
The properties of the hydraulic oil should be appropriate for each specific usage and environment.
6.2. Viscosity
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Hydraulic oil has a low viscosity when it is thin (liquid) and a high viscosity when it is viscous. Viscosity is proportional to
temperature: when the temperature rises, viscosity decreases and when the temperature drops, viscosity increases.
Hydraulic installations, especially mobile vehicles, operate under extreme temperature fluctuations. Consequently, the
viscosity range is essential. The hydraulic oil must be liquid enough to flow through filters, suction pipes, return pipes and the
various components without a great deal of resistance. However, the hydraulic oil should not be too thin. If this is the case, the
(lubricating) oil film will break, which will cause internal mechanical damage.
6.2. Viscosity index – viscosity class
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The viscosity index (VI) is used to express the relationship between the temperature and viscosity of hydraulic oil.
Viscosity-temperature diagrams show the operational temperature range of hydraulic oil at various viscosity indices. The
temperature range is limited by a given maximum and minimum viscosity index. Most types of hydraulic oil have a VI between
90 and 110. Hydraulic oil with a VI between 130 and 200 is not very sensitive to temperature changes and is characterised by
good start-up properties and minimal loss of power at low temperatures. For high temperatures, hydraulic oil with a high
viscosity index can be used to ensure effective sealing and less wear. The high load-bearing capacity of hydraulic oil with a
high viscosity index prevents damage and machine downtime, lowers operating costs and increases the life span of the
installation.
6.2.

Pour point

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Hydraulic oil remains liquid when the pour point is reached at a low temperature. The lowest operating temperature
permitted during start-up of the installation should be well above the pour point temperature. In other words, the minimum
start-up viscosity should be compliant with the instructions of the pump manufacturer. The minimum start-up temperature of
the installation can be deduced from this.
6.3

Conditions for the use of hydraulic oil

Hydraulic oil must be free from impurities as these negatively affect the operation, life span and reliability of the system.
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