The centralized hydraulic system you needed was finally approved, installed, plumbed, and tested. The new system should provide a big boost to your plant’s productivity. It has state-of-the-art load-sensing pumps which will save energy and provide more than enough power to run the production line. Proportional valves with on-board electronics are mounted on the cylinders to control speed, acceleration, and positioning, and radial-piston motors with resolver feedback will provide powerful and precise control of conveyors.
The design of the centralized hydraulic system also includes a pressure filter to protect proportional valves, return line filters to protect any contamination coming back from components, an offline filtration system for continuous filtration and cooling of the oil, and last, a suction strainer with a 3-psi bypass to keep out large contaminants. The boss is excited because of the promised energy savings and production uptime resulting in more profit.
This is where most plant personnel wash their hands, relax, and set the cruise control, because they got this centralized hydraulic system running which will solve all their problems for the next 20 years. That sounds amazing…but the reality is this is only the beginning.
Another step that’s just as critical, if not more so, is to develop a maintenance strategy/program. This step is the backbone of energy savings and reliability. You cannot afford any downtime; otherwise, you’ll have to suffer the wrath of the plant manager for agreeing to spend the capital on this new hydraulic system. The question is, what does a maintenance program look like?
Mapping Out a Plan
Many plant personnel believe a maintenance strategy is to fix something when it breaks down and change the oil at specified intervals. This plan is reactive and preventive, which are lagging indicators of failures and provide us with no information about how the hydraulic system is running. If we move into the predictive and proactive realm, understanding how failures occur becomes relevant, making the 20-year goal now attainable.
The how component is the first of three components of a holistic approach to extending equipment life. The second component builds on how failures occur and leads to a strategy around contamination control. The third component builds even more to develop a contamination control strategy, which allows us to evaluate the progress and identify opportunities for improvement.
Machines fail due to machine wear, which can be directly affected by a lack of contamination control. Contamination is a primary source for machine wear, with particle contamination accounting for up to 82% of surface degradation. Water ingression promotes detrimental conditions such as oxidation, corrosion, and hydrolysis. The asset’s life substantially decreases when significant levels of particles and water are present in the hydraulic fluid. The loss of opportunity at hand depends on the levels of contamination: the higher the contamination level, the shorter the life expected.
Cleanliness levels according to ISO 4406 clearly indicate that the cleaner the fluid, the longer the life of hydraulic components and systems. The first number in each rating indicates the number of particles 4 µm and larger in a 1-cc sample of fluid. The second number represents the number of particles 6 µm larger than 6 µm, and the third number indicates the number of particles 14 µm and larger.
Many would agree that hydraulic systems can be temperamental. However, no matter how intricate a hydraulic system is, common methods can evaluate how a machine is running and, ultimately, how close the hydraulic system is to failure from contamination. Oil sampling, oil analysis, and membrane patch colorimetry (MPC, the “Patch Test”) are methods commonly used to detect fluid contamination and lubricant degradation.
A contamination control strategy is the heart of precision lubrication within the maintenance of the centralized hydraulic system. It involves:
- Protecting against particle and water contamination by filtering new oil
- Storing oil in a climate-controlled environment
- Keeping dirt and moisture out of the hydraulic reservoir with desiccant breathers
- Delivering clean oil to the centralized hydraulic system through return filters
- Installing an offline filtration circuit (kidney looping) in the reservoir
- Installing a heat exchanger in the kidney loop, when necessary, to keep the fluid within a specific temperature range because excessive heat can also degrade oil
- Monitoring the condition of the oil through oil sampling, and
- Educating and training the workforce
Desiccant Breather as a Troubleshooting Aid
Protecting the centralized hydraulic system is simple and cost-effective through desiccant breathers and oil filters. Desiccant breathers filter particles and adsorb moisture from incoming air to prevent both from coming entering the reservoir. But desiccant breathers don’t just remove moisture only from air entering the reservoir. If moisture is present in the reservoir’s headspace (the air above the hydraulic fluid), the desiccant breather will adsorb the moisture and keep the reservoir dry.
The desiccant changes color as it adsorbs more moisture. This feature actually serves as a troubleshooting tool, because if the desiccant breather repeatedly becomes saturated, either the ambient air is extremely humid, or water is getting into the hydraulic system somewhere. This means a desiccant breather can be used as a troubleshooting tool.
For example, consider two identical hydraulic systems—one in Florida and one in Arizona, representing humid and arid environments, respectively. If the breather’s desiccant changes color from bottom to top, the more-humid environment exists outside the reservoir (Florida). This is because air moisture laden air entering the desiccant breather (from the bottom) becomes drier as it travels upward through the desiccant bed.
Likewise, when the desiccant changes color from top to bottom, the more-humid environment exists inside the reservoir (Arizona). Desiccant at the top of the breather (where air from the reservoir enters) adsorbs more moisture than desiccant at the bottom (closest to the dry atmosphere). This condition indicates that the more-humid air is inside the reservoir—evidence of water in the hydraulic fluid. If the source of the water is not identified and corrected, problems with water contamination will occur.
Determining Filtration Needs
Solid particles can do the most damage when they are near size to the clearances between the moving parts of hydraulic components. Extreme operating conditions—such as high operating pressures and temperatures—make the problem worse.
When determining which type of filter to use in a hydraulic system, note that pore size and beta rating (ß) hold the greatest influence in a filter’s effectiveness. The beta rating measures the efficiency of the filter at a specified pore size. The higher the beta rating, the more efficient the filter is at capturing the specified pore size particles. For example, for the hydraulic system with proportional valves, the filter should have a pore size of 3-µm and a beta rating of 200 (β3 ≥ 200). At this pore size and beta rating, each pass through the filter will remove 99.5% of all the particles sized 3-µm and larger. This value is calculated by:
E = (β–1)/ β = 200 – 1/200 = 99.5%
Where E is efficiency, %
Although both breathers and filters work collectively for a good contamination control strategy, breather filters may be more effective because it costs more to remove contaminants than to exclude them—according to some studies, 10 times more. A successful contamination control strategy should incorporate both methods.
Steve Musil is fluid power manager for Motion Industries' Central Group. Ed Duda is technical consultant for Des-Case Corp., Goodlettsville, Tenn. Click here to watch a video describing service and repair capabilities at Motion Industries.