Machinery Lubrication magazine published by Noria Corporation
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These may be governor valves in a turbine elec- trohydraulic control (EHC) system or landing gear hydraulic valves in commercial aircraft. When a valve remains stationary, the fl uid leakage path across the silt land (the zone where silt particles pile up) is exposed to more and more particles. Given enough dwell time, the particle buildup (obliteration) is suffi cient to arrest movement of the valve and cause motion impediment failure. The infl uence of dwell time can be observed in Figure 2. Most aircraft valves mitigate the dwell time/ obliteration risk by keeping the valve spool fl ut- tering, a design condition called "dither." Nonetheless, as most seasoned airline pilots will tell you, these valves are still known to stick on occasion. Television audiences were able to see an example of contaminant-induced valve stiction a few years ago on "America's Funniest Home Videos." A life-sized hydraulic Abraham Lincoln was shown moving erratically during a showing of Disneyworld's Hall of Presidents. Oil Pressure Differential Oil is always trying to move from high to low pressure. In typical electrohydraulic valves, the annular space between the spool and the bore is all that separates high pressure from low pressure. Oil leaks through this pathway at a higher velocity with increasing operating pres- sures. This results in more particles being exposed to the silt lands and more packing force on the particles, causing static friction. Water Contamination Free and emulsifi ed water in oil will prefer- entially occlude to most solid particles such as dirt. The hydrogen bonding in the water mole- cules causes strong attractive forces similar to the clumping of wet sand. Laboratory studies have shown that moisture sharply aggravates bird's-nest obliteration in valves, orifi ces, glances and other tight-clearance fl ow paths. Varnish and Sludge Varnish is a soft, gummy deposit that collects on internal machine surfaces. It tends to be more acute on metal surfaces that are cooler than the oil. Varnish condenses on these cool surfaces, producing a sticky residue. Electrohy- draulic valve surfaces are a common destination for varnish insolubles in oil. Nearby particles can get stuck on these adherent surfaces. This condition is known as the "fl y paper" effect (see Figure 3). The synergistic result greatly increases the risk of silt lock and stiction. Controlling the Silt Lock Risk Maintenance and reliability professionals don't have the convenient ability to redesign machiner y or even change operating demands to reduce the risk of silt lock. However, they can infl uence the conditions to which the machine and its components are exposed. This would include controlling particle contamination (exclusion and removal), water contamination and varnish potential. With suitable diligence, not only is the risk of silt lock signifi cantly reduced, so too is the risk of wear and corrosion mode failures. About the Author Jim Fitch has a wealth of "in the trenches" expe- rience in lubrication, oil analysis, tribology and machinery failure investigations. Over the past two decades, he has presented hundreds of courses on these subjects. Jim has published more than 200 technical articles, papers and publications. He ser ves as a U.S. delegate to the ISO tribology and oil analysis working group. Since 2002, he has been director and board member of the International Council for Machinery Lubrication. He is the CEO and a co-founder of Noria Corporation. Contact Jim at jfi tch@noria.com. PUBLISHER Mike Ramsey - mramsey@noria.com GROUP PUBLISHER Brett O'Kelley - bokelley@noria.com EDITOR-IN-CHIEF Jason Sowards - jsowards@noria.com SENIOR EDITOR Jim Fitch - jfi tch@noria.com TECHNICAL WRITERS Jeremy Wright - jwright@noria.com Wes Cash - wcash@noria.com Alejandro Meza - ameza@noria.com Bennett Fitch - bfi tch@noria.com Loren Green - lgreen@noria.com CREATIVE DIRECTOR Ryan Kiker - rkiker@noria.com GRAPHIC ARTISTS Julia Backus - jbackus@noria.com Terry Kellam - tkellam@noria.com Josh Couch - jcouch@noria.com Patrick Clark - pclark@noria.com ADVERTISING SALES Tim Davidson - tdavidson@noria.com 800-597-5460, ext. 224 MEDIA PRODUCTION MANAGER Ally Katz - akatz@noria.com CORRESPONDENCE You may address articles, case studies, special requests and other correspondence to: Editor-in-chief MACHINERY LUBRICATION Noria Corporation 1328 E. 43rd Court • Tulsa, Oklahoma 74105 Phone: 918-749-1400 Fax: 918-746-0925 Email address: jsowards@noria.com MACHINERY LUBRICATION Volume 15 - Issue 1 January-February 2015 ( USPS 021-695) is published bimonthly by Noria Corporation, 1328 E. 43rd Court, Tulsa, OK 74105-4124. 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Machinery Lubrication is an independently produced publication of Noria Corporation. Noria Corporation reserves the right, with respect to submissions, to revise, republish and authorize its readers to use the tips and articles submitted for personal and commercial use. The opinions of those interviewed and those who write articles for this magazine are not necessarily shared by Noria Corporation. CONTENT NOTICE: The recommendations and information provided in Machinery Lubrication and its related information properties do not purport to address all of the safety concerns that may exist. It is the respon- sibility of the user to follow appropriate safety and health practices. Further, Noria does not make any representations, warranties, express or implied, regarding the accuracy, completeness or suitability of the information or recommendations provided herewith. Noria shall not be liable for any inju- ries, loss of profi ts, business, goodwill, data, interruption of business, nor for incidental or consequential merchantability or fi tness of purpose, or damages related to the use of information or recommendations provided. Machinery Lubrication January - February 2015 | 3 0 10 20 30 40 50 60 60 50 40 30 20 10 0 Particle Size >10 μm >30 μm 0-5 μm Silt Lock Risk Increases with "Dwell" Time Force required to free valve (pounds) Spool dwell time (minutes) (Clearance-sized particles) Figure 2. Clearance-sized particles produce the highest risk of silt lock. Figure 3. "Fly paper" particles