Proximity Probe
A proximity probe (eddy current transducer) is a non-contact displacement sensor that measures relative motion between a rotating shaft and its bearing housing by detecting changes in a magnetic field caused by eddy currents in the conductive target. It outputs DC voltage for shaft position/gap and AC voltage for vibration/displacement.
In shop floor maintenance, proximity probes are installed in pairs (X & Y) 90° apart at each bearing to monitor critical rotating equipment like turbomachinery, compressors, and generators per API 670 standards. Integrated into CMMS/Asset Reliability systems (e.g., ServiceGrid), they provide real-time data on shaft orbit, misalignment, rubs, oil whirl, and thrust position. This enables predictive maintenance and machinery protection by detecting issues early, reducing unplanned downtime, and optimizing repair schedules in industrial environments.
- Cabling Errors: Improper grounding, damaged extension cables, or interference causing signal noise and false trips.
- Incorrect Gap Setting: Installing the probe outside the linear range (e.g., wrong tip-to-shaft distance), leading to non-linear output or loss of signal.
- Target Material/Finish Issues: Using non-ferrous targets or poor surface finish (e.g., rough shaft, coating) that distorts eddy current formation and measurement accuracy.
How does a proximity probe differ from an accelerometer?
Probes measure absolute shaft displacement relative to the bearing (low-frequency to DC), while accelerometers measure casing vibration (high-frequency); probes detect inner race defects and rubs more effectively.
What frequency range does it cover?
DC to ~10 kHz, capturing slow roll and static position.
Why is it the industry standard for turbomachinery?
It provides direct rotor dynamics data (shaft orbit, critical speeds) unattainable by casing-mounted sensors, preventing catastrophic failures in high-speed equipment.