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 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 fluid is required. At a starting temperature of 15?C, 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 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).

Viscosity ValuecStTemperature (VG68)
Min. Permissible1095?C
Min. Optimum1678?C
Opt. Bearing Life2565?C
Max. Optimum3655?C
Max. Permissible10002?C

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 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 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 95?C.

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.

If you enjoyed this article, you'll love Brendan Casey's Inside Hydraulics newsletter. It gives you real-life, how-to-do-it, nuts-and-bolts, hydraulics know-how ? information you can use today. Listen to what a few of his subscribers have to say:

Can't Put It Down
?I get magazines and e-mails like this all the time. I never find time to read them. I decided to read Issue #30 and I couldn't put it down. I'll make time from now on.?

Richard A. Shade, CFPS
Project Engineer (Hydraulic Design)
JLG Industries Inc.

So Valuable It Earned Me A Raise
?The knowledge I've gained from this newsletter has been so valuable it has earned me a raise!?

Jack Bergstrom
Heavy Equipment Mechanic
Sharpe Equipment Inc.

Love It - Keep Them Coming
?I just love this newsletter. As a Hydraulics Instructor for Eaton, I make copies and distribute them to my students as I address various topics... Keep 'em coming.?

Michael S Lawrence
Hydraulics Instructor
Eaton Hydraulics Inc.

Here's a sample of what's covered in this powerful newsletter: troubleshooting, contamination control, component repair and testing, preventative maintenance, failure analysis, and much, much more!

To get a FREE subscription to the Inside Hydraulics newsletter, fill out this form - don't forget to capitalize the first letter of your name - and hit 'SUBSCRIBE NOW!'

First Name *
Email *

This is a private mailing list that will never be sold or given away for any reason.
You can also unsubscribe at anytime.


Copyright © 2002 - 2013 Brendan Casey; Insider Secrets to Hydraulics