Machinery Lubrication

Machinery Lubrication Jan Feb 2015

Machinery Lubrication magazine published by Noria Corporation

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2 | January - February 2015 | www.machinerylubrication.com AS I SEE IT JIM FI T CH | NORI A CORPOR AT ION H y d r a u l i c s D RECOGNIZING the Causes and Hazards of SILT LOCK o you know about silt lock? This common condition causes motion impediment failure in certain machines. While this type of failure usually doesn't involve wear or permanent internal damage to the machine, it is sudden and abrupt. Silt lock is the result of solid contamination and is amplifi ed by other conditions that must be controlled where possible. It is most often associated with hydraulic systems and produces seizure or jamming of components. Because of its lack of warning or predict- ability, silt lock is responsible for some of the most devastating and destructive catastro- phes in mechanical machinery. These include mechanical casualties, prolonged production losses and even loss of human life. Silt lock has been found to be the root cause of countless failures related to aircraft, spacecraft, passenger cars, elevators, turbine generators, tower cranes, etc. Obliteration-Prone Electrohydraulic Valves Electrohydraulic valves are used exten- sively in modern hydraulic systems. They couple the immediate response of electrical control with the high power of hydraulics. In other words, they rapidly convert electrical energy to powerful and responsive mechan- ical energy. Because the structure of these valves is generally more complicated and intricate than ordinary control valves, they are less tolerant to solid contamination. Typical examples of such valves include solenoid, pulse-width modulated (PWM), proportional control and servo valves. For example, electrohydraulic valves are critical to steam turbine performance and include steam throttle valves, trip solenoid valves and intercept valves. In gas turbine service, you might have an inlet guide vane valve (on peaking units) and/or a gas control valve (in base load units). As shown in Figure 1, silt particles less than 10 microns can enter the clearances between the spool and bore in the leakage path. This obliteration of the clearance space can result in increasing static friction of the spool when the valve is actuated. This can cause a stick-slip movement, which is also known as a hard-over condition. Servo valves are particularly prone to this form of contaminant failure due to their high performance demands. For instance, a stick-slip of valve movement in commercial aircraft hydraulics might result in the loss of critical control during a landing sequence. Critical Risk Factors Particles generally don't act alone in causing silt-induced motion impediment failures. Most often it is a team effort that increases both the magnitude and frequency of the risk. These risk factors are described below: Particle Size and Clearance As with contaminant wear, particle size matters. Very large particles can't enter the working clearance and are swept aside. Very small particles pass through the clearance without restriction. However, clear- ance-sized particles cause the highest risk. This is seen in Figure 2 where 10-micron particles exhibited greater static friction in spool valves compared to particles that were 30 microns and 0-5 microns. In valves, silt lock generally occurs as a result of an eccentric position of the spool as it rests in the bore. Due to the weight of the spool, the annular clearance varies from submicron in the bottom position to 10-50 microns in the space above. A single tramp particle may be all that's needed to seed the obliteration process, as shown in Figure 1. This primary particle narrows the clear- ance, allowing the smaller and more highly populated secondary particles to load into the clearance space. These secondary parti- cles are the "silt" that causes the high static friction (stick-slip or complete seizure), resulting in motion impediment failure. Dwell Time Many valves are in constant motion, while others remain at rest until needed. High Pressure Low Pressure Spool Bore Seed Particle Silt Particles Figure 1. Silt particles migrate into the clearances between a valve's spool and bore, increasing friction when the valve is actuated. Silt Lands

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