Hydraulic oil viscosity

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 the 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 fluid 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 15C, maximum operating temperature is 75C, 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 75C, an ISO VG68 fluid is required. At a starting temperature of 15C, the viscosity of VG68 fluid is 300 centistokes, which is within the maximum permissible limit of 1000 centistokes at start up.

Having established the correct fluid 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 55C and 78C. The minimum viscosity for optimum bearing life of 25 centistokes will be achieved at a temperature of 65C. The permissible, intermittent viscosity limits of 1000 and 10 centistokes equate to fluid temperatures of 2C and 95C, respectively (see exhibit 2).

Viscosity ValuecStTemperature (VG68)
Min. Permissible1095C
Min. Optimum1678C
Opt. Bearing Life2565C
Max. Optimum3655C
Max. Permissible10002C

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

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

Having defined the parameters shown in exhibit 2 for a specific piece of hydraulic equipment, damage caused by high or low fluid temperature (low or high fluid 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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