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Tribology and Lubrication Technology May 2016 : Page 37

MEET THE PRESENTER This article is based on a Webinar originally presented by STLE University on Feb. 5, 2015. “Efficient Hydraulic Systems and Fluids” is available at www.stle.org : $39 to STLE members, $59 for all others. Paul Michael, a research chemist at the Milwaukee School of Engineering’s Fluid Power Institute, has more than 35 years of experience in formulating and testing lubricants. His research interests include energy-effi-cient fluids, fluid compatibility, oil aeration, filtration, particle characterization and machine failure diagnosis. He is active in standards committees for ASTM, the International Organization for Standardization and the National Fire Protection Association. He also serves as a lubrication subject matter expert for the U.S. military. Paul is the principal investigator for energy-efficient hydraulic fluids in the Center for Compact and Efficient Fluid Power, a National Science Foundation engineering research center in Minneapolis that brings together researchers, educators, students and industry representatives to develop more compact, efficient and effec-tive hydraulic and pneumatic technology. You can reach Michael at michael@msoe.edu . Paul P l Mi Michael h l Unlike centrifugal pumping systems, where the flow de-pends on pressure, hydraulic systems use positive displace-ment pumps and motors—where flow is independent of pres-sure. In reality no pump is 100% efficient, so pressure always influences flow to some extent. Hydraulic systems produce kinetic energy in the form of flow and potential energy in the form of pressure. Thus, it is imperative to maintain separation between high-pressure and low-pressure zones in a hydraulic system. This requirement drives many of the design concepts in fluid power technol-ogy; moving machine components must seal at tribological interfaces to minimize leakage through gaps. Internal leakage, the migration of fluids from high-pressure zones to low-pressure zones inside hydraulic components, reduces the amount of power that a system can deliver. As system pressures and temperatures increase, pressure-driven flow losses through internal gaps also increase. This effect is more significant in mobile applications—the smaller oil reservoirs and heat exchangers required for mobile hydraulic systems means that they operate at higher temperatures than industrial systems. FLUID REQUIREMENTS Reliability and efficiency demand different properties of the hydraulic fluid. Reliability standards are well defined and re-quired for all hydraulic fluids. These standards include viscos-ity, wear protection, thermal stability, corrosion inhibition, foam resistance, demulsibility, oxidation life and cleanliness. Pressure-dependent fluid properties, which include bulk modulus, density and traction, can have a large impact on hydraulic system efficiency, but these properties rarely appear in hydraulic fluid specifications. Bulk modulus represents the ratio of volume change to pres-sure change in a liquid. As a rule of thumb, the volume of a hydraulic fluid decreases by about 0.5% for every 1,000 psi increase in pressure. A fluid’s bulk modulus depends on pres-sure, temperature, chemistry and structural rigidity. Bulk mod-Volumetric, Mechanical and Overall Efficiency Curves HOW DO WE MEASURE EFFICIENCY? The overall efficiency of a hydraulic pump or motor is its volu-metric efficiency multiplied by its mechanical efficiency. Volu-metric efficiency relates to the output flow per revolution of the input shaft of a pump. It measures the pump’s ability to mini-mize leakage between high-pressure and low-pressure regions. Mechanical efficiency relates to the output torque of a motor and is an indicator of the ability of the fluid to prevent friction. At high pump pressures and low motor speeds, where most practical operations take place, volumetric efficiency increases rapidly with increasing pump speed (or fluid viscos-ity), and then levels off. Meanwhile, mechanical efficiency de-creases nearly linearly as the pump speed (or the viscosity of the fluid) increases. This relationship is commonly illustrated using a Stribeck curve, which plots efficiency as a function of speed, viscosity and pressure (load) ( see Figure 1 ). WWW .S TLE. OR G Figure 1 | Stribeck curves plot efficiency in a hydraulic system as a function of Z (speed), N (viscosity) and p (load or pressure). Multiply-ing volumetric efficiency by mechanical efficiency yields the overall efficiency. In this plot, 16 gear pumps produced 1,789 data points. MA Y 2 016 • 37 TRIBOL OG Y & L UBRIC A TION TE CHNOL OG Y

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