hydraulic oil


Hydraulic Oil Viscosity

Different oil types are used in hydraulic systems for various functions such as lubrication and heat transfer, power transmission, sealing, and other duties. Hydraulic fluid properties such as viscosity and compressibility, demulsibility and fire resistance are all important to ensure a functioning hydraulic system. Hydraulic oil viscosity, which is the most important property to reduce machine wear and improve accuracy, is essential.


We can discuss more on hydraulic oil viscosity here.

Viscosity is the fluid’s internal resistance to flow. The hydraulic oil viscosity determines how well every hydraulic system performs. Viscosity determines the oil that should be used for which application. Higher viscosity fluids will cause friction, pressure drop and heat generation. Higher temperatures will also cause internal leakages due to lower viscosity. It is crucial to select a fluid with sufficient viscosity. It will negatively affect the hydraulic system’s lifetime.


Hydraulic oil viscosity is a measure of the fluid’s film thickness and strength in machines. Fluids with a lower viscosity flow more quickly, while fluids with a higher viscosity flow slower. Hydraulic oil viscosity is classified as dynamic (absolute), or kinematic (dynamic) viscosity. On-site viscometers measure dynamic viscosity, while oil analysis laboratories report kinematic viscosity.


What is hydraulic viscosity index?

It refers to the temperature-dependent change in hydraulic oil viscosity. The viscosity index can be divided into four categories: low (up to 35), medium (35-80), high (110-110) or very high (1110). Low viscosity indicates that the viscosity of the material changes with temperature. High viscosity is the opposite. To be more precise, viscosity increases with heat and decreases with cold.


For viscosity index calculation, you can use the formula:

VI= 100(L-U/L-H)

Here, “L” is the oil having a viscosity of zero at 40degC, “H” is the oil that has a viscosity Index of 100 at 40degC and “U” is the kinematics oil viscosity at 40degC.

The viscosity index formula can help you choose the right hydraulic oil for your application. Highly desirable hydraulic oils must have a viscosity range greater than 95. A low viscosity fluid should be used for applications that require low temperatures. The fluid will freeze and resist fluid movement through the system.

The viscosity of synthetic oils will be greater than that of natural oil. Normally used mineral oils will have around 100 viscosity index.


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Importance of Hydraulic Oil Viscosity

Hydraulics can be used indoors and outdoors, in extreme cold temperatures to desert heat. The temperature can affect hydraulic oil viscosity, as we have already mentioned. Lower temperatures will cause the fluid to solidify, making it more difficult to pump and flow. Higher temperatures will cause the fluid to become thinner, which reduces its lubricating ability.

Low hydraulic oil viscosity can lead to internal leakages, and lower volumetric efficiency for motors and pumps. A fluid with a higher viscosity will also reduce mechanical efficiency. This can lead to issues such as high energy consumption, heat generation and poor air release.

Low or high hydraulic fluid viscosity could cause damage to hydraulic system components such as pumps, valves and filters. Fluids with high viscosity are more likely to flow from the reservoir to their pump. It will also make it difficult for fluid to flow through the filters and valves. Low hydraulic oil viscosity can also cause damage to the sealing and wear of the metal parts.

Hydraulic oil viscosity is the most important factor to consider when choosing an oil for your hydraulic system. Fluids with sufficient viscosity provide the best performance, maximum life expectancy, and lower maintenance.

The hydraulic oil viscosity will decrease if the hydraulic system is used for longer periods. Viscosity index enhancers can be added to the fluid to overcome this problem. VI enhancers are polymeric additives that reduce the dependence of viscosity on temperature. Low temperatures cause polymeric molecules to contract, while high temperatures cause them to relax. The system will be more fuel efficient and last longer if it has proper VI enhancers.