GE Bently Nevada 7200 Series Installation Manual page 13

14 mm Proximity Transducer System 7200 Series Installation Manual
c. The probe can also be gapped electrically by connecting the probe cable to the Proximitor (see
CAUTION below). Apply the proper input voltage (-18 Vdc or -24 Vdc) to the Proximitor.
Connect a digital multimeter to the Proximitor output. Thread the probe into the mounting, and
observe the transducer output voltage. As the probe is threaded, the transducer output voltage
remains low or gives a false reading because it is sensing the surrounding mounting material.
See Figure 2-3. As the probe tip extends through the mounting, the transducer output voltage
will increase to its maximum output and then decrease as it approaches the observed surface.
By cross-referencing this voltage with the probe calibration curve in Appendix C, the proper
gap can be set.
d. When the probe is properly gapped, secure it by tightening the jam nut (locknut). Care should
be taken so the jam nut is not secured so tightly the threads are stripped.
Remember, to prevent twisting, disconnect the probe from the
Proximitor when threading the probe. Reconnect it to take readings.
Axial position probes should be gapped when the machine is not
running so that the rotor can be placed at a known position with
respect to the clearances in the thrust bearing.
After the probe is secured in position, the probe cable must be securely fastened to prevent fatigue
failure caused by oil flow, moving air, or singular stress conditions. A clip or cable holding device
will work as long as it does not exert enough force on the cable to cause the FEP insulation to cold
flow. Cold flow is change in dimension or distortion caused by the sustained application of force.
Sometimes it is necessary to seal around the probe cable. Sealing creates a special problem, because
the cable is insulated with FEP which will cold flow and cannot be subjected to high pressure
exerted by some gland seals. If the pressure differential across the seal area is one atmosphere or
less, a simple seal (such as duct seal putty) can be used. If the differential pressure is greater than
one atmosphere, a special probe must be used. Contact your nearest Bently Nevada office for more
information.
Before the probe is installed, make sure its electrical length is equal to the electrical length required
by the Proximitor.
Before routing the cable, insert an identification label (provided by the user) under the clear FEP
sleeve at the end of the cable, if desired. Secure the label by applying heat to shrink the tubing. The
heat source should not exceed 300° F (149° C).
8
CAUTION