How to Check the Condition of Hydraulic Oil

A vertical machine's hydraulic drive transmits power and signals through pressurized oil, so the condition of the oil and seals directly affects system stability and, indirectly, part quality. About half of hydraulic failures are caused by incorrect use or maintenance of the oil, so good maintenance habits are the most effective way to prevent them.

Key properties

The most important properties of hydraulic oil are viscosity and cleanliness, along with resistance to oxidation, wear, foaming and corrosion, and good lubricity.

Viscosity is expressed as the average kinematic viscosity at 40 °C. Commonly used ISO VG 46 anti-wear hydraulic oil has an average kinematic viscosity of 46 cSt at 40 °C. Viscosity falls as temperature rises and increases as pressure rises.

Test regularly

Contaminated or degraded oil shortens pump life and can cause failures, so test the oil periodically and use the results to decide when to change it. Three quick checks:

• Appearance test. Put samples of new and used oil into separate test tubes and compare color, clarity, floating particles and any water at the bottom.
• Blotter test. Drop used oil onto filter paper and examine it after one hour. Dirty or degraded oil shows contaminants clearly.
• Finger test. Rub used oil between your fingers. If viscosity has dropped, it feels rough rather than slick, and drips off freely without a sticky, elastic feel.