Priority number one in hydraulics maintenance

I presented a workshop on minimizing hydraulic equipment operating costs at a local University recently. During that presentation, I shared with attendees what I consider to be THE most important proactive maintenance routine for hydraulic equipment.

No, it's not contamination control. These days, best-practice contamination control is an accepted precondition for reliability. And given contemporary advances in technology for excluding and removing contaminants, it could be said that failure to control contamination is a failure of machine design - rather than a failure of maintenance.

The maintenance routine that I believe ranks above contamination control in order of importance these days - largely due to its neglect, is: maintaining fluid temperature and viscosity within optimum limits. This involves:

1. Defining an appropriate fluid operating temperature and viscosity range for the ambient temperature conditions in which the hydraulic machine operates;
2. Selecting a hydraulic oil with a suitable viscosity grade and additive package; and
3. Ensuring that both fluid temperature and viscosity are maintained within the limits defined.

In order to determine the correct fluid viscosity grade for a particular application, it is necessary to consider:

• starting viscosity at minimum ambient temperature;
• maximum expected operating temperature, which is influenced by system efficiency, installed cooling capacity and maximum ambient temperature; and
• permissible and optimum viscosity range for individual components in a system.

For example, consider an application where the minimum ambient temperature is 15°C. Maximum operating temperature is 75°C. The optimum viscosity range for the system's components is between 36 and 16 centistokes. And the permissible, intermittent viscosity range is between 1000 and 10 centistokes.

From the temperature/viscosity diagram, exhibit 1, it can be seen that to maintain viscosity above the minimum, optimum value of 16 centistokes at 75°C, an ISO VG68 hydraulic oil is required. At a starting temperature of 15°C, the viscosity of VG68 oil is 300 centistokes, which is within the maximum permissible limit of 1000 centistokes at start up.

Having established the correct oil viscosity grade, the next step is to define the fluid temperature equivalents of the optimum and permissible viscosity values for the system's components.

By referring back to the temperature/viscosity curve for VG68 fluid shown in exhibit 1, it can be seen that the optimum viscosity range of between 36 and 16 centistokes will be achieved with a fluid temperature range of between 55°C and 78°C. The minimum viscosity for optimum bearing life of 25 centistokes will be achieved at a temperature of 65°C. The permissible, intermittent viscosity limits of 1000 and 10 centistokes equate to fluid temperatures of 2°C and 95°C, respectively (see exhibit 2).

Exhibit 2. Correlation of typical operating viscosity values for a piston pump with oil temperature, based on oil viscosity grade.

Going back to our example, this means that with an ISO VG68 hydraulic oil with a viscosity index similar to that shown in exhibit 1 in the system, the optimum operating temperature is 65°C. Maximum operating efficiency will be achieved by maintaining fluid temperature in the range of 55°C to 78°C. And if cold start conditions at or below 2°C are expected, it will be necessary to pre-heat the oil to avoid damage to system components. Intermittent oil temperature in the hottest part of the system, which is usually the pump case, must not exceed 95°C.

Having defined the parameters shown in exhibit 2 for a specific piece of hydraulic equipment, damage caused by high or low oil temperature (low or high oil viscosity) can be prevented, and recurring PM tasks in respect of this routine can be virtually eliminated, by installing fluid temperature monitoring instrumentation with alarms and shutdowns.

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