Hydraulic Systems: A Survival Guide
Hydraulic system failure: Where do I start?
Pencil and paper are more effective than a full toolbox in fixing fluid power system problems.
By Tom Nash, Product Manager - Hydraulics, Applied Industrial Technologies
|One of the worst sounds in an industrial setting is a fluid power system with serious problems. |
Even worse is the eerie silence when everything shuts down due to a failure.
|When a fluid power system fails, the first impulse is to grab a tool kit and start tearing the system apart to find the problem. A better approach is to pick up a pencil and paper. A few minutes spent analyzing the situation up front will save big dollars later. Troubleshooting fluid power systems is as much an art as a science.|
The starting point in troubleshooting a failed system is to list every component that could have generated the symptom. An entire system usually does not fail. Rather, a single component or a few related components fail and shut down a system. Keep in mind that the symptom could be different than the cause of the failure.
|Once a list of suspect components has been assembled, they can be eliminated systematically. First, talk with the system operators. Most likely they will not have noticed a slow failure in a system until the machine could not keep up with production. They certainly would have noticed a sudden failure. Write down all pertinent information and use it to help validate the list of suspects. This process may add more components to the list.|
Now, begin asking “what if?” This is where the art and science blend. The art portion requires understanding a few of the more logical places to start. The science validates any suspicions. Test each suspected component mentally by asking, “If that part failed, what would have happened to the rest of the system?” If a particular component appears to be a logical suspect, start by testing that component and work through the other possibilities.
Typically, by this time, the “parts changer” mechanic would have already torn apart the system and changed two or three components. He or she might even have been lucky enough to find the one component that caused the symptom and might have the system back on line. However, while this approach might provide a short-term fix, the root problem still exists. Examining some general symptoms of system failure will help eliminate some components from the list or add others.
Heat provides an important clue to system failure. Heat in a hydraulic system results from input energy not being converted to useful work due to normal friction or a component failure. If a system has been idle for a while, heat may not be a useful tool for finding the problem. If the system will at least start without damaging anything else, start it and check every component with a heat gun or by touch. Highlight any component on the list of suspects that exhibits an elevated temperature.
Noise is a prime indicator of a problem with a particular component. For the “tools first” repair person who dives in immediately with wrenches, a noisy component is usually the first thing to be changed. The problem with this approach is that there is no system analysis to consult if the replacement component makes the same noise. Noise is a good failure indicator only when maintenance personnel know how and when to check the component.
|Time for Tools|
Once the paper analysis is complete, it is time to pull out the tool kit as well as some diagnostic tools. Any system problem can be solved with three essential parts:
• Pressure gauges
• Flow meter
• A way to create a false load on the system
Not all troubleshooting operations need these tools but they should be close at hand. The challenge is tapping into the system quickly to test a component. It takes a bit of art blended with science to know where to install test components first.
|For example, a pressure gauge is showing low system pressure. The symptom is the cylinder not building enough force to operate the machine. However, a number of components could cause this condition, including the pump, relief valve set too low, valve bypass or cylinder bypass. To track a problem, the general rule in system design and troubleshooting is to start at the end and work back to the pump and prime mover.|
Cylinders – The basic design of most cylinders allows for fluid to enter one end to extend or retract the rod, and for exhaust flow to escape the other end. The two key things to check with cylinders are:
1. Does the exhaust flow have somewhere to go? A simple gauge installed at both ends of the cylinder is the best indicator of this. When the cylinder stops moving, one gauge should read system pressure and the other zero or return line pressure if other things are happening in the system. Either way, there should be a difference.
2. If a cylinder stalls either mid-stroke or at the end of the stroke, flow from the other end should stop. If flow continues, fluid is bypassing the piston seals and pressure will not be able to build.
|Motors – Motors can be tested the same way as cylinders. Sometimes, the challenge is to apply enough load on the motor to cause a stall. Dropping system pressure is usually the best way.|
|Directional Valves –A basic understanding of valve function is required to troubleshoot these components effectively. Most valves have two or three positions. Typically, the valve includes a schematic on the nameplate or on its body to show flow patterns. The question to ask is, “In each position, where should there be flow and how much?” The challenge is to isolate each valve and test them individually. When several valves are mounted on a manifold, either install blanking plates or note the results from the test for each valve. The test results will indicate if there is a problem with one or two valves.|
To test a directional valve, install a flow meter and a pressure gauge in the inlet line. Remove the actuator hoses and plug the ports. Again, this modifies the system so be sure to understand what will happen when it starts. Draw a diagram of the valves with enough room to note flow meter and pressure gauge readings.
With the valve in its normal rest position, record the flow rate and pressure reading. If the valve normally is ported to allow flow from the pressure line to the tank port, there should be full flow and very little pressure. If the valve is ported to be blocked in that position or to have flow to one of the actuator ports, pressure should be system pressure and flow should be zero. Keep in mind that all fluid power components may have a minor amount of internal leakage. Check the manufacturer’s specifications for that value.
Pressure Relief Valves – In a properly designed system, the relief valve never operates because it is a safety device designed to relieve pressure at a given value. If the system uses the relief valve to limit pressure for extended periods of time, a tremendous amount of heat could be generated.
To determine if a relief valve is bypassing fluid at low pressure, adjust the pressure setting to its lowest value, start the system and slowly increase the setting. If the relief valve opens at too a low a setting and pressure cannot build, replace the valve and check the new one for proper operation.
Pumps – The only job of the pump in the system is to create flow. To check a pump, install a flow meter and pressure gauge as the first components after the pump. Be sure to include a pressure relief valve as a safety component. The next component after the relief valve should be a flow control valve. Ensure that the flow control valve is completely open, and start the pump with flow routed back to the reservoir in a secure line. The flow meter should read full pump capacity. If not, stop the test and replace the pump.
If the pump is operating at full capacity, record the flow meter and pressure gauge readings. Begin to close the flow control valve causing a restriction in the pressure line. As resistance to flow builds, pressure should begin to rise. The flow differential between full flow at low pressure and flow at high pressure should not be more than 10 to 20%, depending on the pump type, manufacturer and configuration.
For variable volume pumps, if flow decreases to zero prior to reaching the pressure compensator setting, the pump cannot keep up with internal leakage. It should be rebuilt or replaced.
Once the faulty component has been isolated and replaced, the job is only half done. The final steps are to determine why the component failed. Was the cause contamination or heat? Did the component exceed its expected service life? Is the fluid in good condition? It is much easier at this point to isolate the true root cause than to return in a few weeks to replace the same component.
|Plan to Fail – The System Will|
The natural response to the procedure outlined above is that it seems easy but finding the hoses, gauges and flow meters to test each component separately can be difficult. Make it easier in the future by installing test points in the system during routine maintenance. A single flow meter placed after the pump and relief valve, but before any other components, is relatively inexpensive compared to system downtime.
Tom Nash is a hydraulic product manager at Applied Industrial Technologies.
Published 2008, Plant Engineering