By Cameron MacNeil, Stauff Corp.
You hear a lot in the hydraulic field how a majority of hydraulic system failures can be attributed to some kind of contamination. In the beginning of my career, I spent a lot of time repairing hydraulic pumps and motors. From a repair point of view, close to 100 percent of all the failures looked at that were attributed to some kind of contamination. So, it’s really important that we find a way to limit all types of contamination in our hydraulic system.
An excellent first step for achieving contamination control is to come up with a contamination control or fluid contamination test plan. This plan should ensure cleanliness (which improves reliability and efficiency in a hydraulic system), the use of proper filtration and regular monitoring or testing of the fluid, to determine if the plan has been successful.
Contamination Types and Sources
Solid particulate contamination is generally measured in microns, which is a millionth of a meter — these are extremely small particles. When considering contaminants in a hydraulic system, we have to think about a more than just solid particulate, like dirt or wear metals. Contaminants can also be fluids or gases. But how to they get into the system?
One way that contaminants get into a hydraulic system is what is referred to as inbuilt. Those are the contaminants that are left over in the system from the actions of building the hydraulic system. You can have weld slag from welding on tanks or welding on pipes. You can have grind dust that’s left in a reservoir that hasn’t properly been cleaned out. You can even have things that were left in a reservoir, like sandwich bags! These are generally larger contaminants and are quite easy to filter or to get out of the system.
The next type is ingressed contaminants. These are the contaminants that comes into a hydraulic system during the action of a hydraulic system. Most systems have breathers. Contaminants can be pulled into the reservoir along with the air. Those are ingressed contaminants. Many machines have cylinders. When a cylinder extends, dirt or debris gets on the cylinder shaft, and when the shaft is retracted, contaminants can get by the scraper and enter the system. These are also considered ingressed contaminants. Again, those are relatively large and thereby somewhat easy to filter out of a system.
Finally, we have generated contaminants. These are the wear products of the movement of different surfaces against each other in a hydraulic system such as in a pump, motor, or valves. These contaminants can be smaller than three microns and can be created under extreme pressure and high temperatures, which makes them very hard. These are the contaminants that cause other contaminants or accelerate the wear process in a system.
Beyond these solid contaminants, liquid and air can negatively affect a system. The number one liquid that damages a hydraulic system is water. Water from condensation or humid air gets into a reservoir through a breather during the day when the machine is running. At night, when it is shut down, it condenses water vapor, and the droplets get into the hydraulic system. Over time, that can build up.
Liquids contamination can also be other hydraulic fluids. Do you have a process within your plant so that the proper fluids are going into the proper reservoirs? If you have a hydraulic reservoir and you put lubrication fluid in it, then it is going to affect the hydraulic fluid. The pumps aren't going to like it. The cylinders aren't going to like it.
In some industries like steel mills, they use water glycol, a water-based fluid. If you dump a petroleum-based hydraulic fluid into water glycol, you get a Jello-like substance that clogs all the filters and can really damage pumps.
Air can also be a contaminant, or at least something that damages the components in a hydraulic system. Air molecules and water molecules do not have the same load bearing characteristics that hydraulic fluids and lubrication fluids do. Under extreme pressure, those molecules implode and cause high temperatures, a process called cavitation. These mini implosions can damage hydraulic fluid as well as components.
Hydraulic Fluid and Wear
In training classes, I often ask the students: what is the most important component in a hydraulic system? Many people say it is the pump, motor or servo valves. I always make a case that the most important component is the hydraulic fluid. After all, the hydraulic fluid is the only thing that touches everything else in the system.
Hydraulic fluid is something that people put in and simply expect it to work. We have to be careful of chemical compound formation or acid buildup in fluids, which deplete the additives, and reduce the life of the hydraulic fluid.
Many hydraulic components have very small orifices that are essential to their operation. These small orifices can be blocked by small particles in the 1 to 5 µm range. This blockage will cause the hydraulic components to fail and the system to lose its ability to function. Wear in the system is also critical. There are different kinds of wear in a hydraulic component, including: abrasive wear, erosive wear, adhesive wear, and fatigue.
Abrasive wear is where you have two moving surfaces riding on a lubrication film and particles can get into that clearance. They act like a cutting tool and chip away and scratch things. A bearing might have a clearance of 1 to 3 microns; it’s not the larger 25-micron particles that cause a problem in here, because they can’t get in that clearance. They bounce off and get flushed away. It’s the extremely small particles that can really do damage where the small clearances are.
Erosive wear is caused by particles that “attack” the surface or “edge” of a component and remove material from that surface due to momentum effects. Servo and proportional valves are particularly sensitive to this form of component wear.
Adhesive wear is caused when the lubrication layer between two moving surfaces breaks down; this can allow the two surfaces to come together under extreme loads and friction. The two surfaces are then “cold welded” together. When the two surfaces are forced apart, particles from both surfaces are released. These particles can be very damaging to the systems as they are small and are can be much harder than the surfaces that created them.
Lastly, fatigue wear happens over time, because of loads and pressures. Here, you begin to get fissions or little ripples in surfaces. Over time, you will get cracks. Contaminants get forced down into those cracks and act as wedges that chip away at the surface to make it rough — and pull away particles and send them downstream.
Why is wear so important? It is important because it causes dimensional changes or clearance changes within the components, which increases the leakage and lowers the efficiency of the system. Even worse, the small particles that are generated accelerate the wear process. It’s like a snowball at the top of the hill. It starts small and as it rolls down, it gets bigger and bigger and bigger.
The objective of the filtration process is to reduce the level of contaminants present in a hydraulic system, and to maintain an acceptable level of cleanliness — no matter what contaminants are being generated and ingressed into the system. By maintaining this balance, the following benefits are achieved:
- Extended component life
- Enhanced system reliability
- Reduced downtime and servicing costs
- Safety of operation
- Extended fluid life
The filter performance is essential to reducing contaminants in the systems. There are standardized tests that give us common ground when comparing different filter options. Filter performance can be measured by efficiency (beta ration), dirt holding capacity (DHC), and pressure drop (delta P). The performance criteria as well as fluid and temperature compatibility should be considered when choosing a filter.
The location of the filter will ultimately depend upon the reason for fitting it. If it is for direct component protection, then the filter must be installed immediately upstream of the component concerned. If it is fitted to control potential sources of dirt, then it must be installed downstream of that source. If it is for general control, then it can be positioned in any of the flow lines which see the majority of the flow.
There are many locations for fitting filters in hydraulic systems. The potential positions are can be in the pressure line, the return line, or offline. Wherever the filter is located, it is essential that it is easily seen, so the blockage indicator can be observed, and it is easily accessible, so that the element can be readily changed.
About the Expert
Cameron MacNeil has been a Filtration Product Manager at Stauff for over fifteen years.
MacNeil presented the education session How to Control and Limit Contamination in a Hydraulic System at IFPE 2020. IFPE, the International Fluid Power Exposition is the leading North American exhibition bringing together the fluid power, power transmission and motion control industries.
For more information on how to control and limit contamination in a hydraulic system, listen to Cameron MacNeil’s interview on NFPA's Fluid Power Forum podcast.
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