An air-gap survey is a geometry record, not a single feeler-gauge number. The result changes with axial position, rotor angle, bearing clearance, support condition and access. Before interpreting a difference, make sure every reading describes the same mechanical state.
Map the readings
| Pattern | Check before diagnosis | Possible source |
|---|---|---|
| One side consistently smaller | Repeat method and frame level | Stator offset, support distortion or bearing position |
| Small point moves with rotor | Re-measure at several safe rotor positions | Rotor geometry or shaft condition |
| Difference changes axially | Confirm probe depth and end positions | Rotor/stator taper or assembly alignment |
| Readings change after bolt work | Record foot and frame condition | Frame distortion or soft-foot influence |
| Bottom gap differs with bearing state | Confirm shaft lift and bearing clearance | Rotor sag or bearing position |
These patterns guide inspection; they are not stand-alone acceptance rules. Use the motor drawing and manufacturer limits for the specific machine.
Control the measurement state
Isolate the machine and follow the approved turning and access procedure. Mark angular positions and axial planes. Record whether the rotor is resting, lifted or coupled, and whether end shields or bearings have been disturbed.
Use the same instrument and contact method for the comparison set. Where access is restricted, document the limitation rather than filling missing positions with estimates.
Link correction to evidence
Do not move the stator or bearings from one reading alone. Review shaft runout, bearing condition, frame level, hold-down bolts and alignment. After any correction, repeat the same map and retain both sets.
For adjacent checks, see soft-foot inspection and motor shaft alignment.