Orbits have historically been used to measure relative shaft movement within a journal-type bearing. The shape of the orbit told the analyst how the shaft was behaving within the bearing as well as the probable cause of the movement. This was accomplished using proximity probes usually mounted through the bearings with a 90-degree separation and a tip clearance set to around 0.050 inches. With today’s modern analyzers, it is possible to also collect an orbit using case-mounted velocity probes or accelerometers to see how the machine housing is moving. Another way of putting it would be the orbit represents the absolute path in space that the machine housing moves through (see Figure 1).
Figure 1 Figure 2
This is accomplished utilizing a two-channel instrument and collecting an orbit with the sensor of choice being a velocity probe or accelerometer. This is what’s referred to as a poor man’s operating deflection shape or ODS (see Figure 2).
The analyst can interpret the data to determine machine movement at a particular measurement location or a section of the machine if a tachometer trigger is used during orbit collection as a phase reference. Analysts must keep in mind the exact location of each sensor so that when they look at the shape of the orbit it is possible to tell the movement in relationship to the sensor’s location. The sensors should be placed 90 degrees apart or at least as close as possible to 90 degrees. Keep in mind that a properly wired sensor shows motion toward the sensor as a positive signal and motion away from the sensor as a negative signal.
An orbit is usually collected while the machine is at its normal operating state or speed, but it can also be collected while the machine is increasing or decreasing in speed, such as during a coast-down or startup. The data can be collected in a steady state, in what is known as an unfiltered orbit, requiring no tachometer (see Figures 3 and 3A), or at multiples of running speeds such as first, second or third order to look for issues relating to that or another specific frequency (see Figures 4, 5, 6 and 7).
Figure 3 - Unfiltered displacement orbit Figure 3A - Unfiltered velocity orbit
Figure 4 – Second order setup Figure 5 – Second order results
Figure 6 – Third order setup Figure 7 – Third order results
About the Author