Troubleshooting electro-hydraulic pumps: a 12-step program

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By Scott Follett, Robert Bosch Fluid Power

PlantServices.com

Keywords: "fluid handling"

A logical, reasoned approach to diagnosing production problems is a key to effective maintenance practices.

The art of troubleshooting is not really an art at all. Instead, it is more of a science. When a system malfunctions, you gather information and evidence, form a hypothesis about what caused the problem, test the hypothesis, fix the problem, then continuously verify repair. Modern industry demands tremendous control of pressure and flow during very complicated, dynamic processes. Many hydraulic pump manufacturers meet this demand by integrating advanced electronic technology with the latest hydraulic technology. As a result, manufacturers enjoy shorter response times, precise pump control, and increased pump efficiency. However, when something goes wrong, troubleshooting the pump can be a challenge. There are few maintenance people with the experience and training in both electronics <I>and<I> hydraulics to diagnose and repair electro-hydraulic pumps. The following twelve steps provide a foundation for your troubleshooting.

Step 1: Make safety your first priority

Hydraulic pressure and electricity are inherently dangerous. I personally saw a mechanic maim a finger as he was running his hand along a high pressure pipe looking for a pinhole leak. He found it. Use common sense. Unless you are absolutely positive the system is "dead", locked out, and tagged, assume the system is under pressure and electrical components are live. Do not take shortcuts if it means sacrificing safety.

Step 2: Understand the system

The electro-hydraulic pump is only one component in a larger system. It is important to know how the pump and every other component interact with each other. Review and understand the system schematics, read manufacturer specifications, know the capabilities of every component; understand the purpose of each component and how it contributes to the function of the entire system.

Your system is only as good as its weakest component. For example, if you use a pressure transducer with a tolerance 5 percent, the best tolerance you can expect your system pressure to have is 5 percent. Five percent of hydraulic pressure means a lot of psi in the deadband.

Step 3: Talk to the operator

If the system has been in operation, talk to the operator. Ask what the symptoms are, when the problem occurs, where the problem manifests itself, and the magnitude of the problem in the system. In addition to describing symptoms, the operator can often give you some history. It is not unusual for the operator to remember how a previous maintenance crew fixed the same problem before.

Step 4: Test the system

Run the system yourself to get the full experience of the malfunction. If you are not familiar wnough with it to safely operate it yourself, observe as someone else runs the system. It is usually a good idea to have the operator <I>show<I> you the problem. By doing this step, you do not have to rely on other people's opinions.

Step 5: Verify that the components are properly connected

Spread out the system schematic and compare the drawing to the actual system. Follow the flow path through the circuit. Insure that the electrical connections are correct. Look for discrepancies between the schematic and the actual system. Focus on those discrepancies.

As just one example, contamination caused two pump shaft seal failures during a distributor's pump test. Initially, we thought the shaft seal was overly sensitive to contamination and we asked for a redesign of the seal. However, after asking some very pointed questions, the distributor discovered the operator used a thirty micron filter instead of the three micron filter specified for the system.

Step 6: Compare the problem machine with one that is working

If you have the luxury of another machine with a similar pump, look for differences between the machine that works and the one that doesn't. Look for differences in components, location of transducers, the age of the system, temperatures, the size of the pump, pressure and flow differences, and anything else that helps to answer the question, "Why does one system work, while the other one doesn't?"

For example, we had a valve test stand experiencing severe flow oscillations. We were sure the pump was broken. After ordering a replacement pump, an alert maintenance mechanic pointed out that this system had about seven gallons less fluid in the reservoir than systems with similar sized reservoirs. Before changing the pump, he added fluid to the reservoir. The problem went away as soon as the pump saw a flooded suction.

Step 7: Brainstorm possible causes of the malfunction

Consider everything that could cause the malfunction. Use maintenance manuals and textbooks. Ask co-workers who previously used or maintained the machine. Pump vendors have troubleshooting guides; use them. A little time spent during this step can save a lot of time later.

You may want to consider forming an impromptu team to try to determine what went wrong. If you want to see a good, effective team in action, watch the movie Apollo 13.

Step 8: Test the pump electronics

By now you should be reasonably sure that the pump is causing or contributing to the problem. It's time to focus on finding out what's wrong with it. Use a multimeter to check the continuity of the pump leads. Check the wiring diagram and make sure the pump is wired correctly. Check the voltage from the transducers to ensure the transducers are working. If your pump has DIP or DIL switches, double check the position of each switch. Measure the voltage output of the electronics portion of the pump.