September 12, 2006 

'Inside Hydraulics' Newsletter

1. Strange but true
2. Number one in hydraulic maintenance
3. Read hydraulic schematics - like the experts
4. Content for your web site or e-zine
5. Help us spread the word
6. Tell us what you think


Strange but true

One of our readers, Doug Lien, founder and former owner of Cylinder City Inc., wrote to me recently with the following suggestion:

"Brendan, it could be fun to ask your readers to submit "Strange But True" troubleshooting stories. I have several stories that stumped the best engineers on staff. But I was able to come up with the answers"

Seems like a good idea to me, so I invited Doug to 'step up to the plate'. Here's a story that he's willing to bet no one will solve! The situation was as follows:

The end of the month was just a few days away when an OEM customer called and advised its hydraulic cylinder manufacturer that the boom extension cylinder on a rough terrain forklift was SQUEALING. This single-stage cylinder was 6" bore x 3" rod x 14' stroke. The OEM was in a terrible position because they had 80 machines that could not ship by the end of the month deadline due to the squealing - which could be heard blocks away. The OEM was demanding that the cylinder manufacturer send a crew of men to change out all 80 cylinders!

But what could the problem be? Seal stick and slip? Excessive stroke speed? Internal leakage? Load-control valve? What else? How would you approach this situation?

The outcome and root cause of the problem will be revealed in next month's issue of 'Inside Hydraulics'. In the meantime, if you have a quirky or unusual hydraulics troubleshooting or maintenance-related story you'd like to share with over 21,000 hydraulics users around the world, please send us your story:

13,000 hydraulics users can't be wrong...

Who else wants to get a LIFETIME of
hydraulics knowledge AND keep it?

Find out more...

2.   Number one in hydraulic maintenance

A couple of months ago, I presented a workshop on minimizing hydraulic equipment operating costs at a local University. 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.

"Brendan, Insider Secrets to Hydraulics is a great book.
I made two mistakes:
1. Waiting so long to get it, and
2. Not having you autograph it!"

Frank Wagoner
SMC Rig Repair, Casper WY

3.   Read hydraulic schematics - like the experts

A schematic diagram is a 'road map' of the hydraulic system. The ability to read and interpret one can save a lot of time and effort when troubleshooting hydraulic problems. How to Read Hydraulic Schematics was developed by JI Case to teach their technicians how to read and understand hydraulic circuit diagrams. Find out more

4. Content for your web site or e-zine

Need some fresh content for your web site or e-zine? You now have permission to reprint these 'Inside Hydraulics' articles on your web site or in your e-zine, provided:

1. Each article is printed in its full form with no changes.

2. You send an e-mail to to advise us where you'll be publishing them.

3. You include the following acknowledgement at the end of each article:
About the Author: Brendan Casey has more than 16 years experience in the maintenance, repair and overhaul of mobile and industrial hydraulic equipment. For more information on reducing the operating cost and increasing the uptime of your hydraulic equipment, visit his web site:

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6. Tell us what you think

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