Fundamentals of Rotating Machinery Diagnostics
6 Average Shaft Centerline Plots
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The orbit, timebase, and orbit/timebase plots present dynamic (rapidly changing) shaft position data, but they do not show changes in the average shaft position, which is also an important characteristic of system response. These changes are caused by changes in the static radial load or changes in the stiffness characteristics of the rotor system. They routinely occur during startups or shutdowns and during steady state operation of the rotor system, over relatively short or long time spans. When a rotor system with fluid-film bearings changes speed, there are changes in the stiffness characteristics of the bearing, which cause a change in the average position of the shaft. Thus, changes in shaft position can provide very important diagnostic information.