Viscosity is considered as a very important physical characteristic of a hydraulic oil as it influences the capacity of the oil to flow and to lubricate the moving components of the system.

Viscosity is measured on the basis of the fluidity (the instability or easeness to flow) of the fluid in addition to the dynamic lubrication that maintain the contact of service parts.  Viscosity should a have maximum capacity to resist the extreme operating temperature of the system.

Absolute Viscosity:

The extent to which a fluid resist a tendency to flow on the surface of another material is the basis for determining the absolute viscosity.

The smaller absolute viscosity unit is centipoise, and 1 poise = 100 centipoise or 1 centipoise = 0.01 poise.

In application, selecting oil with a certain viscosity is very much recommended and accepted.  In many cases, selecting an oil has been determined.  Therefore, selecting an oil with a certain specification can meet the designed properties and charateristics of a hydraulic assembly.  However, the viscosity of a hydraulic oil have certain effects.

Extremely-low viscosity can cause:

  • increased leakages on the valves and the acatuators, leading to a decrease in the central system’s accuracy and loss of power.
  • external leakages on the gasket, seal, or mechanical joints.
  • higer possibility of slip in the pump, leading to decreased pressure and volumetric efficiency.
  • higher risk of wear to the moving parts, particularly the valves and the actuators.
  • increase in the fluid temperature and shorter fluid life span.

Extremely-high viscosity can cause:

  • higher decrease in pressure as a result of extremely-high resistance to flow, leading to a decrease in the entire system pressure.
  • fast operating system due to exteremely-thick fluid, particularly at low operating temperature.
  • decreased efficiency.
  • large vacuum in the pump’s inlet leading to an increase in air pressure toward the system and a decrease in the pump efficiency.
  • high temperature and shorter life span.
  • increased noise in the system.

Based on the the above-mentioned risks, it is recommended to select a hydraulic system with a proper capacity and characteristic or to use manufacturer’s manual.

Methods of Measuring Viscosity

For some practical purposes, it is important to identify the viscosity of a fluid.  Relative viscosity is determined by the time during which a quantity of fluid flows through a standard orifice at a certain temperature.

There ara some methods of measuring viscosity, including:

  • SUS =    Sybolt Universal Second = SUS
  • SAE =    Society automotive engineers (SAE number is determined by the Society of Automotive Engineers that determines oil SUS viscosity based on SAE test temperature).
  • Kinematic viscosity (for SI)
  • Euler Degrees.

The period of time required by an amount of fluid to flow through an orifice is measured by using a stopwatch.  The viscosity in Skybolt Universal Second (SUS) is equivalent to the time period that an amount of fluid requires to pass through a viscometer.

Based on Diagram 2-1, it can be concluded that a thick liquid flows slowly, and its SSU viscosity is higher than a lighter liquid that can flow faster because oil tends to be thicker at low temperatue dan will be lighter at hight temperature.

Viscosity Index

Viscosity index is defined as any viscosity measured on the basis of the fluid resistance to temperature changes.  Any fluid that has a constant viscosity on extreme temperature changes is considered as having a high viscosity index.  A fluid that is very thick at low temperature dan very light at high temperature is considered as having a low viscosity index.  Diagram 2-1 shows the comparison of SAE 50 oil and SAE 90 oil.  The difference in viscosity at three different temperature is showed in the diagram below.

Table 3-1 Viscosity index by SUS

Based on the above table, it is concluded that SAE 90 oil is ligher at 0°F and is thicker at 210°F.  Both oils, however, have the same viscosity at 100°F.

Resistance to Oxidation

Oxidation or chemical reaction with oxigen is a process that shortens the service life of fluids.  Petroleum oil is particularly sensitive to oxidation as oxygen can easily react with carbon and hydrogen in forming oil.  Most of oxidation products can form sludges, gums and varnishes.  Initially, the products that exist in oil is acid and can cause corrotion to the whole system as well as increased viscosity of the the oil.  The insoluble sludges, gums, and varnishes can cause clogged orifice, rapid wear and failed valves.


Catalisators always exist in any hydraulic system causing an acceleration of oxidation process.  Heat, pressure, contaminants, water, turbulence or metal surfaces can accelerate oxidation process when the process has already started.  Test results show that at 135ºF, oil oxidizes slowly.  However, any increase of 18ºF in temperature can double the speed of oxidation process (other chemical reactions).

Oil refiners usually add additives to hydraulic oils to limit oxidation.  As many systems work at high temperature, the properties of additives can:

  1. Stop oxidation
  2. Eliminate the causes of catalisators production.

Fluid Maintenance

As has been explained, dust and other contaminants are the big enemies of hydraulic systems.  Long term (non-stop) operation at high efficiency depends on the maintenance of the fluid.

Only the fluid recommended by the system manufacturer should be used.  The fluid should be maintained at the recommended level.  In addition, the filter used should be in good condition.  Replacement of the hydraulic fluid should be on the recommended schedule.

Modern hydraulic systems have actually been adjusted for operation with a certain hydraulic fluid although it is not so strict.  The users may use a hydraulic fluid other than that recommended by the system manufacturer.  As most of the modern hydraulic fluids have been well refined, filtered and supplemented with additives, they do not need to be flushed.  However, if necessary, flush should be done in accordance with the recommended procedure.

Storage and Handling

The following is a simple procedure for protecting fluids against contamination during maintenance and handling.

  1. Lay fluids containers or drums horizontally in a confined area, if necessary.
  2. Before opening a drum, clean the upper surface of the cover to prevent dirt from entering the fluid.
  3. Use a clean container to move the fluid from the drum to a reservoir. Using a pump fitted with flow filter 25 is very much recommended.

Maintaining a hydraulic fluid clean can lengthen its service life and prevent damages to the tightly-maounted hydraulic components caused by contamination.

Maintenance of Hydraulic Fluid During Operation

The procedure for maintaining hydraulic fluids during operation is as follows:

  1. Prevent contamination by maintaining the tightness of the system as well as using the recommended fluid and air filter.
  2. Establish the schedule of fluid replacement to maintain fresh fluid in the system. If necessary, conduct a laboratory test to the samples of the fluid.  This is to determine the fluid replacement schedule.
  3. The fluid tank should always be properly filled so that the heat-reducing capacity can be utilized to prevent condensation on the inner surface of the tank wall.
  4. Any leaky tank must be repaired. In case the oil level in the tank is lower than normal level, it is recommended not to add another oil into the tank.  It is better for you to flush the tank as, principally, the hydraulic oil level in the reservoir should not be too low before and after operation.


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