Know your enemy

Find the silent killers in hydraulic systems.

October 10, 2012   by Steve Gahbauer, contributing editor

Condition-based maintenance (CBM) is gaining traction, and for good reason. It’s based on machine condition rather than elapsed time or running hours, and reduces time and money spent for unnecessary preventative maintenance, such as time-based oil changes.

This is especially true of hydraulic systems maintenance, according to a presentation at the Toronto MainTrain maintenance conference presented by the Plant Engineering and Maintenance Association of Canada (PEMAC).

Kevin Eaton, responsible for OEM support services, warranty administration and failure analysis investigations at Bosch Rexroth Canada, said to get the most from CBM, establish what is “normal” for a piece of equipment or system. Every machine is different, so review its characteristics and history with the maintenance department and the original equipment manufacturer.

CBM of hydraulic systems is all about knowing your enemy, says Eaton. Fluid condition is critical. Monitor items that cause degradation or wear, follow cause and effect backwards to the first measurable factor, identify the “silent killers” and how they affect hydraulic components, and check whether they are measured reliably and what influences the measurements to cause false alarms.

The silent killers in hydraulic fluids are particle contamination, water content, and the relationship between operating temperature and fluid viscosity.

To determine the contamination level, use the ISO 4406 code, which defines cleanliness and provides guidelines for acceptable levels.

Water in hydraulic fluids accelerates the aging process of the oil, resulting in oxidation, additive depletion, reduced lubricant film strength, and corrosion. It also negatively affects bearing life.

Although there is always some water present in hydraulic systems, it’s a concern when it becomes free or emulsified.

Viscosity, the most significant factor in hydraulic systems, is defined as “the amount of resistance to flow.” It determines how thick or thin a fluid is at any given temperature. Low viscosity results in component damage. On the other hand, excessively high viscosity damages system components through cavitation, which results from the thick fluid’s inability to fill a void created by a moving part.

Through thick and thin
If too thick and too thin is bad, how do you determine the correct viscosity? Temperature. The target for a hydraulic system should be based on the optimum fluid viscosity for the most sensitive component in the system. Eaton suggests you address temperature-related issues before it’s too late, because damage throughout the system can be significant. ISO standard 10816-1 provides guidelines for velocity range limits and machine classes.

To optimize maintenance performance on hydraulic systems:
• use machine history and OEM specifications to establish machine “normal”;
• interpret condition by change in historical trends;
• use permanently mounted sensors to ensure measurement consistency;
• detect, then analyze; and
• don’t confuse condition monitoring with alarm monitoring.

Hydac International has introduced the CMU 1000, an electronic evaluation device for permanent online condition monitoring of machines and systems. Information recorded by sensors connected to the CMU 1000 is transferred via different ports or as an analogue value to other devices. A backlit LCD display and three coloured LEDs indicate status and display messages and values.

The CMU 1000, used on stationary and mobile machines, is operated onsite where data is added using a built-in keypad within the menu structure. Special features include eight input channels for HSI or SMART sensors, eight input channels for analogue sensors, four input channels for digital signals, two output channels for analogue signals, four relay switching outputs with changeover contacts, a USB slave port for PC connection, a USB master port for storing measured data on a standard memory stick, and an ethernet interface.

For more condition-based monitoring tools visit and

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