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Section 5, Precautions in Determining Pickup Direction

When measuring and recording phase, it is especially important that the pickup direction be noted and taken into consideration. For example in Fig. 1, assume that both ends of the machine are vibrating in-phase with each other. With the pickup oriented in one direction, the reference mark will show up at one angular position. But with the pickup oriented in the opposite direction, the reference mark will show up 180° opposite the first indicated angular position even though the axial vibrations at each end are vibrating in-phase.

Therefore, when taking readings on a structure, make sure that the pickup direction is properly chosen. If due to physical obstructions, you must orient a pickup in the opposite direction required for comparison with a vibration at another point, then use one pickup direction as the "standard," recording the phase as seen or indicated digitally. Then, with the pickup at the other point, pointing in the opposite direction, you will see a phase reading. Simply record that reading 180° out-of-phase from what was seen or indicated digitally. For digital phase readings of less than 180°, add 180°. For readings greater than 180°, subtract 180°. See special instructions for axial readings on the Balloon Diagram Form. This enables you to treat the readings from both pickups as if they were both pointing in the same direction. To record axial phases properly, review the left column of the analysis form "Running Speed Amplitude and Phase Data." As shown in Fig. 2, note that to show pickup direction, the arrows are to be added by the specialist.

As seen in Fig. 3, when comparing the axial vibration of the motor with the axial vibration of the pump, the pickups should both be oriented in the same direction, as shown. However, if obstructions or other difficulties prevent the proper orientation as illustrated in Fig. 4, then as with the previous example, the phase relationship of one or the other pickup should be mentally reversed by 180° before recording. When using a strobelight, always view the image from the same direction.

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