Hydraulic Oil Viscosity

To approach the subject of “hydraulic oil viscosity,” it is worth first taking a look at the past. The word “viscosity” comes from “viscum,” the Latin name for mistletoe. The Romans made a very sticky glue from the juice of mistletoe berries, which they used to construct bird traps.

Viscosity describes the flow properties of a liquid. Liquids that spread and flow away quickly are referred to as “low-viscosity.” These include water and solvents such as gasoline or acetone. “High-viscosity” is exactly the opposite: it describes slow-spreading liquids such as tar or pitch.

Viscosity – the tasks of hydraulic oil

The oil in hydraulic systems has three important functions. Its primary task is to transmit hydraulic pressure and thus also power. The second important task of hydraulic oil is the lubrication of moving components. The third task is heat dissipation within hydraulic systems.

These three tasks are partially contradictory in terms of the properties of the lubricants. This is especially true in hydraulic systems that operate with very high pressures.

Degree of viscosity as a dependent variable

The degree of viscosity is heavily dependent on the ambient temperature. The warmer a high-viscosity liquid is, the thinner it becomes. The more a thin-bodied medium is cooled, the more high-viscosity it becomes. In order to be able to compare two media with each other, a reference temperature or a defined temperature range must therefore serve as the measurement basis.

Pressure also plays a role in assessing the flow behavior of liquids. This is particularly important for hydraulic oils. Pressures of many hundreds of bar often prevail in hydraulic systems. The higher a pressure rises, the more viscous the oil becomes. An approved hydraulic oil must have proven itself even under these harsh operating conditions.

Hydraulic oil viscosity: always consider holistically

Hydraulic oil is a highly stressed operating medium. When selecting the right oil, it is very important to know the operating conditions of the hydraulic system precisely. The viscosity of hydraulic oil can fluctuate significantly depending on the operating conditions. The careful selection of hydraulic oil makes the dimensioning of all relevant technical components much easier and thus more efficient and economical. These include:

• Cross-sections of lines
• Performance of hydraulic pumps
• Dimensioning of hydraulic oil coolers
• Change and inspection intervals for hydraulic oil
• … and much more

Condition triangle for hydraulic oil

The requirements for hydraulic oil must find an ideal operating point between

• Flowability
• Lubricity
• Heat dissipation

to find an ideal operating point.

Oil that is too viscous makes a hydraulic system inefficient. To build up the required pressure, a lot of energy must be used and the specified technical components must be selected accordingly large. However, if the oil is too thin, it loses its lubricity. High wear on the inner walls of all moving parts such as cylinders, pumps, or radial motors is then the result. Thin hydraulic oils also tend to cause cavitation, which further accelerates internal destruction.

Hydraulic oil viscosity: altered viscosity

In addition to the directly acting parameters of pressure and temperature, medium- and long-term influences also change the flow behavior of hydraulic oil. Typical factors are:

• Aging
• Formation of resins
• Contamination by abrasion
• Degradation of additives
• Errors during refilling (wrong oil)
• Oxidation
• Contamination by water

Each of these factors shifts the oil away from its intended technical properties. Filtering the oil can significantly extend its usability. Furthermore, a regularly performed oil analysis provides certainty as to when a hydraulic oil change is indicated.