November 17, 2004 

'Inside Hydraulics' Newsletter

1. New, high performance hydraulic pump unit from Enerpac
2. Case drains - filter with caution
3. Determining hydraulic pump condition using volumetric efficiency
4. Content for your web site or e-zine
5. Help us spread the word
6. Tell us what you think


New, high performance hydraulic pump unit from Enerpac

Outstanding hydraulic and electrical performance, minimal size and weight, and real user-friendliness characterize Enerpac's new ZU4 electric hydraulic pumps. These units are an ideal power source for foundation repair or post tensioning work and for powering medium to large cylinders or hydraulic tools. ZU4 pumps have been designed from the ground up to deliver ease of handling and use, high output with modest electrical draw, and dependable operation at outdoor jobsites. Features of the ZU4 include:

  • Small size and light weight - 1.7 hp universal motor draws 18% less current than comparable pumps.
  • New motor brush design increases brush life by a factor of four.
  • Auto-stop feature stops the motor to prevent commutator damage when the brushes wear out - brushes are easily accessible and can be replaced in the field.
  • Two-stage, oil-immersed pump has oversized bearings and features replaceable check valves.
  • Ergonomic, thermally insulated handle - no need to wrap a rag around a hot handle with this machine.
  • Optional tubular skid frame makes for easy two-person carrying.
  • High strength FRP composite shroud protects components from damage, rain, and sun.

ZU4 hydraulic pump unit.

The ZU4 offers a variety of valving arrangements, pressure gauge ports, and an adjustable relief valve that allows the operator to easily set the working pressure for each job. For more information visit

2.   Case drains - filter with caution

One of our readers wrote to me recently with the following question: "We have recently been involved in designing and building a hydraulic machine. The system has three, separate circuits each with an axial-piston pump and a common reservoir. Case drain filtration was included to reduce the possibility of cross contamination if a failure occurs. After contacting the pump manufacturer I was led to believe that it isn't the norm, but if the pressure drop across the filter is kept to less than 30 PSI it will be OK. This just forces filter maintenance. What filter beta or micron rating should be used?"

Installing filters on piston pump and motor case drain lines can result in excessive case pressure, which causes seal failure and mechanical damage.

Seal failure

High case pressure results in excessive load on the lip of the shaft seal. This causes the seal lip to wear a groove in the shaft, which eventually results in leakage past the seal. If case pressure exceeds the shaft seal's design limits, instantaneous failure can occur. The subsequent loss of oil from the case can result in damage through inadequate lubrication.

Mechanical damage

The effect of high case pressure on axial piston pumps is the same as excessive vacuum at the pump inlet. Both conditions put the piston-ball and slipper-pad socket in tension during inlet (see below). This can cause buckling of the piston retaining plate and/or separation of the slipper from the piston, resulting in catastrophic failure.

piston separated from its slipper

High case pressure can cause the pistons of radial piston motors to be lifted off the cam. This can occur in operation during the outlet cycle. The pistons are then hammered back onto the cam during inlet, destroying the motor. If residual case pressure remains high when the motor is stopped, loss of contact between the pistons and cam can allow the motor to freewheel, resulting in uncontrolled machine movement.

To avoid these problems, pump and motor case drain lines should be returned to the reservoir through dedicated penetrations. These penetrations must be higher than the unit's case port and be connected to a drop-pipe inside the reservoir that extends below minimum fluid level. For the reasons outlined above, filters are not recommended on case drain lines. While this does allow a small percentage of fluid to return to the reservoir unfiltered, in most applications the contamination risk is low and can be effectively managed using oil analysis and other condition-based maintenance practices.

Filter with caution

If a filter is fitted to a pump or motor drain line, I recommend a 125-micron screen, grossly oversized for the maximum expected flow rate. The filter housing must incorporate a bypass valve with an opening pressure lower than the maximum, allowable case pressure for the particular component (typically 5-15 PSIG). Installing a gauge or transducer upstream of the filter for monitoring case pressure is also advisable. To learn more about the failure modes of hydraulic components and how to avoid them, read Preventing Hydraulic Failures available here.

"Thanks for the great work on the two publications, Insider Secrets to Hydraulics and Preventing Hydraulic Failures. I have been in the hydraulics business for the past 20 years and it is very difficult to find any decent material on hydraulic maintenance, troubleshooting and failure analysis. These two books cover it all in easy to understand language... I conduct hydraulic training courses and plan to purchase copies to distribute to my students to share your practical approach to understanding a not so understandable subject."

Paul W. Craven, Certified Fluid Power Specialist
Motion Industries, Inc.

3.   Determining hydraulic pump condition using volumetric efficiency

To learn how to correctly determine the condition of a hydraulic pump using volumetric efficiency, read Brendan Casey's article in the September-October Issue of Machinery Lubrication magazine, available here. 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 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:

5. Help us spread the word

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

New subscribers can get the newsletter by completing the form at

6. Tell us what you think

We would love to hear what you think of this issue of our 'Inside Hydraulics' newsletter. And of course, if you have any suggestions for future issues, please send us those too.

Just e-mail the editor at:

Copyright 2000-2004 by All rights reserved.

This message has been sent to the following e-mail address:

If you got this mailing in error, or do not wish to get any further newsletter mailings from us, send a message with "unsubscribe inside hydraulics" in the subject and [EMAIL] in the body.

To subscribe to our Inside Hydraulics Newsletter, go to the following URL:


Inside Hydraulics is published by: Pty Ltd
PO Box 1029
West Perth WA 6872