November 12, 2003 

'Inside Hydraulics' Newsletter

1. Reduce operating costs and downtime with this inventory of maintenance tips
2. Determining hydraulic pump condition using volumetric efficiency
3. Keep up to date with maintenance and reliability issues
4. Content for your web site or e-zine
5. Help us spread the word
6. Tell us what you think


Reduce operating costs and downtime with this inventory of maintenance tips

As our Inside Hydraulics newsletter approaches the end of its second year, I thought it would be worthwhile to review some of the articles from our early issues. Take a second look at each of these articles and make sure you have incorporated the lessons learnt into your hydraulic equipment maintenance practices:

In Issue 01, we explained the 'diesel effect', how it damages hydraulic cylinders and how to prevent it from happening.

In Issue 02, we looked at the consequences of dry-starts, a major cause of premature failure in hydraulic components.

In Issue 03, we discussed cavitation, how it occurs and how it can damage just about any component in a system - not just the pump.

In Issue 04, we examined a pump failure caused by particle contamination and explained how it could easily have been prevented.

For a complete list of back issues, visit:

2.   Determining hydraulic pump condition using volumetric efficiency

I was recently asked to give a second opinion on the condition of a variable displacement pump. My client had been advised that its volumetric efficiency was down to 80%. Based on this advice, he was considering having the unit overhauled.

What is volumetric efficiency?

Volumetric efficiency is the percentage of theoretical pump flow available to do useful work. In other words, it is a measure of a pump's volumetric losses through internal leakage. It is calculated by dividing the pump's actual output in liters or gallons per minute by its theoretical output, expressed as a percentage. Actual output is determined using a flow-tester to load the pump and measure its flow rate.

Because internal leakage increases as operating pressure increases and fluid viscosity decreases, these variables should be stated when stating volumetric efficiency. For example, a pump with a theoretical output of 100 GPM, and an actual output of 94 GPM at 5000 PSI and 120 SUS is said to have a volumetric efficiency of 94% at 5000 PSI and 120 SUS. In practice, fluid viscosity is established by noting the fluid temperature at which actual output is measured and reading the viscosity off the temperature/viscosity graph for the grade of fluid in the system.

What is the significance of volumetric efficiency?

As a pump wears in service, internal leakage increases and therefore the percentage of theoretical flow available to do useful work (volumetric efficiency) decreases. If volumetric efficiency falls below a level considered acceptable for the application, the pump will need to be overhauled.

Calculating the volumetric efficiency of variable pumps

The pump in question had a theoretical output of 1,000 liters per minute at full displacement and maximum rpm. Its actual output was 920 liters per minute at 4,350 PSI and 100 SUS. When I advised my client that the pump's volumetric efficiency was in fact 92%, he was alarmed by the conflicting results. To try and explain the disparity, I asked to see the first technician's test report.

After reviewing this test report, I realized that the results actually concurred with mine, but had been interpreted incorrectly. The test had been conducted to the same operating pressure and at a fluid temperature within one degree of my own test, but at reduced displacement. The technician had limited the pump's displacement to give an output of 400 liters per minute at maximum rpm and no load (presumably the maximum capacity of his flow-tester). At 4,350 PSI the recorded output was 320 liters per minute. From these results, volumetric efficiency had been calculated to be 80% (320/400 x 100 = 80).

To help understand why this interpretation is incorrect, think of the various leakage paths within the pump as fixed orifices. The rate of flow through an orifice is dependant on the diameter (and shape) of the orifice, the pressure drop across it and fluid viscosity. This means that if these variables remain constant, the rate of internal leakage remains constant, independent of the pump's displacement.

Note that in the above example, the internal leakage in both tests was 80 liters per minute. If the same test were conducted with pump displacement set to 100 liters per minute at no load, pump output would be at 20 liters per minute at 4,350 PSI - all other things being equal. This means that this pump has a volumetric efficiency of 20% at 10% displacement, 80% at 40% displacement and 92% at 100% displacement. As you can see, if actual pump output is measured at less than full displacement (or maximum rpm) an adjustment needs to be made when calculating volumetric efficiency.

Time for an overhaul?

In considering whether it is necessary to have this pump overhauled, the important number is volumetric efficiency at 100% displacement, which is within acceptable limits. If my client had based their decision on volumetric efficiency at 40% displacement, they would have paid thousands of dollars for unnecessary repairs.

"Brendan Casey has spent 15 years working in the hydraulics industry and his experience and expertise show in this book. Packed with practical suggestions and tips, and backed up by the author's knowledge of the subject matter, Insider Secrets to Hydraulics has the potential to save many organizations lots of m0ney". Find out more

The Editors of
Hydraulics & Pneumatics Magazine

3.   Keep up to date with maintenance and reliability issues

If you're involved in the maintenance of hydraulic or any other type of rotating machinery, increasing uptime and reducing operating costs are likely to be two of your primary objectives. A resource that I have found to be valuable in keeping up to date with maintenance and reliability issues is 'Machinery Lubrication' magazine. This magazine contains practical articles on lubricant selection and application, contamination control, filtration and condition-monitoring of hydraulics, engines, transmissions, gearboxes, compressors and other types of rotating machinery.
To receive a complimentary subscription to this informative magazine (US and Canada only) go to:

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 let us know where you'll be publishing them.

3. You include the following credit 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:

5. Help us spread the word

If you've found our 'Inside Hydraulics' newsletter interesting and informative, then chances are you have a friend or colleague who would too. Help spread the word about 'Inside Hydraulics' by forwarding this issue to a friend or colleague. If they share your interest in hydraulics, then they will surely appreciate being told about this newsletter.

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6. Tell us what you think

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