How does Order Analysis measure phase lag, particularly super-sync amplitudes?

Question

In recent testing of a high speed gear box, an 8X amplitude was observed (on the radial displacement probes) as the gear passed through a certain speed range. An NI4472 pci card and vi's written in 7 Express, w/ Order Analysis tool kit were configured with radial displacement probes and keyphasor as input signals. The behavior of the phase lag measurement is significant. If the phase lag of the 8X amplitude rolled as the gear passed through a particular speed range then this could suggest one particular problem, but if the phase lag remained constant then this could indicate a different problem. So, someone asked: How does the system measure phase lag of supersynchronous amplitudes?

Answer 1

See attached file for figure 1,2,3
The Phase measurement in the LabVIEW Order Analysis Toolkit uses the phase lag convention. Phase is defined as the angle difference measured from the peak of a vibration signal backward in time to the reference trigger point. This means the directions of numerically increasing angles are always set against the shaft rotation. The trigger point here is the tachometer pulses.
 Figure 1 shows the relationship of the vibration signal and reference signal to zero degree phase. The shaft has a heavy spot and a keyway slot. When the keyway slot passes the tachometer, the tachometer detects a trigger pulse. The heavy spot causes the shaft to vibrate as the shaft rotates. When the heavy spot passes the proximity probe, the vibration reaches a peak. When the heavy spot passes the proximity probe and the keyway slot passes the tachometer simultaneously, the peak of the vibration does not lag or lead the reference trigger point. At this point the phase is zero degrees.
 The other part of the convention dictates that 90 degrees means that the peak of vibration lags 90 degrees behind the trigger point. Figure 2 1illustrates the 90 degree phase. When the vibration signal reaches the peak, rotate the shaft backward (counter the rotation direction) until the keyway slot passes the tachometer. The number of degrees you rotate is the phase lag, or the phase value in machinery vibration measurement. Figure 2 shows the relationship of the vibration signal and reference signal to the 90 degrees phase convention.
For the 8x phase, the measurement is conducted in the similar way. The difference is that the 8x order component go 8 times faster than the trigger pulses. Figure 3 shows the 90 degree phases ( tacho pulse in rising edge) of the 8x order components. The phase measurement is still conducted through calculating the delay between the peak in the 8x vibration components and the trigger pulses and comparing the delay to one cycle of the 8x components. In figure 3, the signals between the two green grid lines are used to output one phase results.