Ing. mecanico

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HYDRAULIC FLUIDS

When selecting a hydraulic fluid for a particular application, a number of properties may needto be considered:
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viscosity
lubricity and wear reduction
oxidation stability
corrosion protection
air and water separation characteristics
bulk modulus (resistance tocompression)
pour point
fire resistance

Viscosity
In the majority of systems, where hydrocarbon-based hydraulic fluids are used, kinematic viscosity is generally the
most important property toconsider when selecting the hydraulic fluid.

In order to determine the viscosity grade required for an application, it is necessary to consider: the starting viscosity
at ambient temperature, theoptimum operating viscosity range for the system’s components and the system’s
expected operating temperature.

Manufacturers publish permissible and optimal viscosity values for hydrauliccomponents, which vary according to
the type and construction of the component.

For example, consider an application where the ambient temperature is 30°C, system operating temperature is 75°C
and theoptimum viscosity range for the system’s components is between 16 and 36 centiStokes (cSt). From the
viscosity/temperature diagram on page three it can be seen that if an ISO46 oil was chosen, theviscosity would be 80
cSt at 30°C and 12 cSt at 75°C. Clearly, the viscosity at operating temperature falls outside of optimum for the
system’s components. In this application, ISO68 oil would be moresuitable, giving a starting viscosity of 110 cSt at
30°C and an operating viscosity of 18 cSt at 75°C, which falls within the optimum range for the system’s
components.

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