Hydraulic vane pump

The anatomy of hydraulic vane pump failure

One of our readers wrote to me recently regarding the following problem:

"Recently, we bought a used hydraulic power unit (15HP electric motor directly coupled to a vane pump). A high pitched, clicking noise is generated when the unit runs. We have checked the following:

  1. We thought it was a motor bearing, so we detached pump from motor, no noise heard.
  2. Pressure line was connected to tank line (to simulate low pressure < 100 psi), very little noise heard.
  3. As pressure is increased, noise gets louder and louder, very intolerable.
  4. Measured current draw of motor - no overload.
What do you think could be causing the excessive noise?"

Given that the symptoms described above are consistent with a restriction at the pump inlet, I inquired if there was a suction filter in the circuit. Our reader replied:

"The system has a 40 micron suction filter but I haven't checked it because I have to drain the oil and take off the access hatch to get to the filter."

The restriction caused by a suction filter, which increases at low fluid temperatures (high viscosity) and as the element clogs, increases the chances of a partial vacuum developing at the pump inlet. Excessive vacuum at the pump inlet causes cavitation erosion and mechanical damage.

Cavitation erosion

When a partial vacuum develops in the pump intake line, the decrease in absolute pressure results in the formation of gas and/or vapor bubbles within the fluid. When these bubbles are exposed to elevated pressures at the pump outlet they implode violently. When bubbles collapse in proximity to a metal surface, erosion occurs. Cavitation erosion contaminates the hydraulic fluid and damages critical surfaces.

Mechanical damage

When a partial vacuum develops at the pump inlet, the mechanical forces induced by the vacuum itself can cause catastrophic failure. In vane pump designs, the vanes must extend from their retracted position in the rotor during inlet. As this happens, fluid from the pump inlet fills the void in the rotor created by the extending vane. If excessive vacuum exists at the pump inlet - it will act at the base of the vane. This causes the vanes to lose contact with the cam ring during inlet, and they are then hammered back onto the cam ring as pressurized fluid acts on the base of the vane during outlet (figure 1). The impact damages the vane tips and cam ring, leading rapidly to catastrophic failure.

vane pump section

Figure 1. Vane pump section (Bosch Rexroth Corp).

The intolerable noise our reader is referring to is symptomatic of cavitation bubble collapse and the vanes being hammered against the cam ring. Both of these conditions are intensified by increasing system pressure.

The solution to our reader's problem is simple: replace the suction filter or better still, discard it completely. If suction filtration must be installed, follow these precautions to prevent pump damage:

  • A filter located outside of the reservoir is preferable to a suction strainer. The inconvenience of servicing a filter located inside the reservoir is a common reason why suction strainers go unserviced - until after the pump fails.
  • If a suction strainer is installed, opt for 250 microns rather than the more common 150 microns.
  • The filter should be grossly oversized for the pump's flow rate to ensure that pressure drop is minimized, even under the most adverse conditions.
  • Regardless of the type of filter employed, it must incorporate a bypass valve to prevent the element from creating a pressure drop that exceeds the safe vacuum limit of the pump.
  • A gauge or transducer should be installed downstream of the filter to enable continuous monitoring of absolute pressure at the pump inlet.

Editor's note: for more information on hydraulic failures and how to prevent them, read Preventing Hydraulic Failures.

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