Achieve effective measurements
The radial runout of the low speed roller can be measured by the following procedure:
The induction coil is excited by alternating current and creates an alternating magnetic field.
When a changing magnetic field interacts with a conductive material (such as a shaft), a small current called an eddy current is induced in the material.
In turn, the eddy current creates an opposite magnetic field that opposes the initial magnetic field.
The interaction between the two magnetic fields depends on the distance between the detector probe and the target material. As the distance changes, the change in interaction between the two magnetic fields translates into the output of the voltage.
The voltage output is then converted to a unit of displacement vibration in mils or micrometers.
A common mounting configuration consists of two eddy current proximity detectors mounted on the bearing housing at 90 degrees apart and at 45 degrees on either side of the vertical centerline of the shaft.
The detector can be mounted on the inside or outside of the bearing journal, depending on the design of the motor. The detector is mounted above the shaft and its position is specially machined and close to the bearing journal. This part of the shaft, called the detector track area, is machined to achieve minimal mechanical and electrical radial runout. The width of the track area depends on the size of the detector probe. It is recommended that the minimum width of the track area be 1.5 times the diameter of the detector probe. This ensures that the magnetic field induced from the detector probe can cover the entire area being machined.
API 541 requires the radial runout of the low speed roll to be measured during the coasting test phase of the rotor at a speed between 200 and 300 rpm. In this speed range, the offset recorded by the detector is almost pure radial runout without any vibration. On non-API motors, the radial runout of the low speed rolls can be recorded at approximately 10% to 15% of the operating speed. The total radial runout record must meet the limits stated on the motor specifications.