High hydraulic fluid temperature - how it causes premature failures

I was asked recently to conduct failure analysis on two radial-piston hydraulic motors that had failed well short of their expected service life. Inspection revealed that these motors had failed through inadequate lubrication, as a result of low oil viscosity caused by excessive operating temperature.

How does this happen?

As the temperature of petroleum-based hydraulic oil increases, its viscosity decreases. If fluid temperature increases to the point where viscosity falls below the level required to maintain a lubricating film between the internal parts of the component, damage will result.

The temperature at which this occurs depends on the viscosity grade of the fluid in the system. Hydraulic oil temperatures above 180°F (82°C) damage seals and reduce the service life of the oil. But depending on the grade of fluid, viscosity can fall to critical levels well below this temperature.

How can this type of failure be prevented?

The above example highlights the importance of not allowing fluid temperature to exceed the point at which viscosity falls below the optimum level for the system's components.

Continuing to operate a hydraulic system when the fluid is over-temperature is similar to operating an internal-combustion engine with high coolant temperature. Damage is pretty much guaranteed.

Therefore, whenever a hydraulic system starts to overheat, shut down the system, find the cause of the problem and fix it!

Editor's note: for more information on hydraulic failures and how to prevent them, read Preventing Hydraulic Failures.

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