Valve's design reduces hysteresis
Minimizes friction during movement of solenoid plunger
By Charles J. Murray, Senior Regional Editor -- Design News, March 5, 1995
Broadview, IL-By dramatically reducing internal friction, a new linear proportional solenoid valve can precisely meter the flow of fluids. Initially employed in commercial refrigeration systems, the device addresses one of the chief problems of proportional valves: hysteresis caused by friction.
To alleviate the problem, engineers from Parker Hannifin Corp.'s Automotive and Refrigeration Group developed a new valve called the Ana-Loid. The Ana-Loid enhances performance by employing a proprietary magnetic circuit and a patented plunger design.
The plunger design addresses the friction issue by employing tiny ball bearings in four longitudinal slots in its outer diameter. Each slot captures two ball bearings-one at each end.
During operation, the ball bearings reduce friction between the plunger and plunger tube. That's critically important when the solenoid coil creates a flux path that attracts the plunger. Typically, such flux paths tend to pull the plunger toward one side of plunger tube, as well as providing the desired axial pull force. As a result, friction develops between the plunger and the contacting wall. In conventional valves, that friction detracts from the axial force and precludes accurate positioning of the plunger.
The new design, however, alleviates those problems through the use of the magnetic circuit and the ball-bearing-equipped plunger. The proprietary circuit provides an output force that is truly proportional to the level of dc current. And the ball bearings reduce friction associated with the radial forces.
Engineers further enhanced the valve's precision by adding a post-and-sleeve flow mechanism. Pressure-balanced, it therefore also keeps hysteresis at low levels.
Until now, no solenoid designs used ball bearings on the plunger. Parker's design not only facilitates robotic assembly and eliminates the space requirements of conventional ball races, it also provides pure rolling contact, something not available in traditional ball bearing designs.
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The resulting expansion device accurately controls refrigerant flow in response to either an analog dc signal or a high-frequency PWM signal. Most importantly, it exhibits hysteresis levels of only 5% or less, says Lee Banks, national sales manager for Parker Hannifin's Automotive and Refrigeration Group. Says Banks: "The key aspect of this valve is that you get a consistent and predictable response to current input."
Additional details...Contact Roger Riefler, Parker Hannifin Corp., Automotive and Refrigeration Group, 777 Bennett Dr., Longwood, FL 32750, (407) 767-2922.
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