Viscosity/temperature diagram

V-T Diagram

For choosing the fluid of the hydraulic system the temperature and viscosity range is deduce from V-T diagram. The SI unit is the CENTISTOKE – cSt or mm²/s.

This article is a continuation of Selection of Hydrostatic Drive Transmission. As I mentioned a crane should be able to slew or rotate, lifting and hoisting a safe working load whenever is need it. Therefore to do that a certain aspect should be fulfilling. One of this is the supply of fluid into the crane hydraulic system.

Nowadays I revise my old article, service report and I remember my experiences on board commercial or offshore vessels as hydraulic troubleshooter

The parameter ranges – minimum and maximum permissible – for temperature and viscosity of different hydraulic component required is stated in catalogue of different manufacturers. E.g. gear pump fixed displacement a fluid range temperature between -15 to 80°C and viscosity range 10 to 300 mm²/s. Another important aspect if is utilize it a variable displacement pump, the conditions of use with respect to viscosity-temperature would change considerably.

About viscosity

The most important parameter when selecting a fluid is the viscosity. It is the behavior of a fluid at a reference of temperature. Fluids must not become thicker or thinner when the temperature varies otherwise the flows at constriction points change – change in a velocity of the actuator. The best index for a fluid is indicate by the flattest curve in a V-T diagram.

Viscosity is measured using standardized methods such as:

A constant temperature 40°C fluid in a time frame – 1minute –
- – to flow through a capillary tube with fixed diameter.
- – to flow through an orifice and fill a small wide cup/container.

Pressure increase in a hydraulic system

The viscosity of fluids changes as pressure increases, practically as high pressure occur the oil becomes thicker.This characteristic must be taken into account when planning hydraulic system that use pressure more than 200 bar. By increases the pressure like 400 bar the viscosity already been doubled.

V-T Diagram

V-T diagram use for choosing the right supply of fluid into the hydraulic system;
Permissible viscosity when selecting hydraulic parts is in the manufacturer documentation;
The best viscosity index for a fluid is indicate by the flattest curve in a V-T diagram;
The system pressure increases the fluidity of the oil changes.

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