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    The lowdown on hydraulic oil viscosity

    Viscosity temperature chart

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    Hydraulic oil viscosity is vital when choosing the right hydraulic oil for your system.

    In hydraulics, there’s a direct relationship between hydraulic oil viscosity and its temperature. For instance, if the temperature of the oil increases then the viscosity decreases. Similarly, when you put cooking oil into a pan and heat it up, the oil moves faster the longer it is heated. A temperature drop makes the oil move less easily and it becomes more viscous.
    Hydraulic oil viscosity needs to be able to work effectively to enable smooth operation no matter the temperature. This is particularly important as applications become more demanding.

    Viscosity explained

    The hydraulic oil viscosity depends on how smoothly it flows. For example, if viscosity increases, then it will take longer for the oil to pass through the hydraulic system. Moreover, high viscosity means the oil is thicker and is more difficult to transport through a system. On the other hand, lower viscosity means the oil is easier to pass through the system. The measurement of viscosity in hydraulic oil is taken in Centistokes (cSt) and most commonly at temperatures of 40°C and 100°C.

    Viscosity index

    The viscosity index helps us to measure the change in temperature in hydraulic oil. A lower viscosity index means that the oil may be more susceptible to change than if it were a higher viscosity index. Moreover, if the viscosity is high, the oil will be better suited to an application which is used in harsher environments, exposed to a variety of operating temperatures.
    The viscosity must be accurate for your application and the temperature. This is true even if the oil has properties include anti-wear, anti-oxidisation or anti-corrosion. An incorrect viscosity could result in damage to hydraulic equipment, problems during operation and reduced service life.
    The table below shows the classifications of viscosity against different temperature ranges:

    Viscosity IndexClassification
    110°C and greaterVery High

    Pumps and viscosity requirements

    Hydraulic Oil ViscosityDifferent pumps require different viscosities. There are three main types of pump that can be found within a hydraulic system: vane pumps, piston pumps and (internal and external) gear pumps. Each pump is utilised in different ways for different applications.

    Vane Pumps

    There are rotors inside vane pumps that include slots attached to a shaft that is rotating erratically to a cam ring. When in operation, the vanes can wear out, due to the constant contact between two surfaces. Because of this, vane pumps are usually less cost-effective when it comes to maintenance, however they are useful for keeping a stable flow. Vane pumps work best with viscosities ranging between 14 to 160 centistokes (cSt) at normal operating temperatures.

    Piston pumps

    Piston pumps are more suited to harsh conditions. They are manufactured for durability and are able to operate at higher pressures, usually up to 6,000 psi. Piston pumps work best with viscosities ranging between 10 to 160 centistokes (cSt) at normal operating temperatures.

    Gear Pumps

    Although gear pumps are classed as the most inefficient out of all of the pumps, they are still able to function through a large amount of contamination. They work by expelling fluid once it has been pressurised between the meshing teeth in the pump. Gear pumps come in two variations: internal and external.

    Internal gear pumps are usually available in a variety of viscosities, ranging up to 2,200 cSt. With high efficiency and reduced noise, these pumps typically operate under pressure from 3,000 – 3,500 psi.

    Although external gear pumps are less proficient than there counterpart, they still offer easy maintenance, stable flow and are cost-effective when it comes to purchasing and repairs. Similarly to the internal gear pumps, external gear pumps produce pressure between 3,000 – 3,500 psi – however, their viscosity range only reaches 300 cSt.

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