Should I Install Single Plane or XY Proximity Probes?

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Metrix SETPOINT Contition Monitoring Group; Minden, NV


Vibration displacement is equal to force / dynamic stiffness.

The vibration can change if:

                     the forces change,

                     the dynamic stiffness changes, or


                     if both happen simultaneously.

Let’s focus on the second possibility for just a moment, i.e. a change in the system dynamic stiffness.  In the presence of a constant force due to unbalance[1] and an asymmetrical support stiffness[2] which exists with most turbomachinery, we realize that the resulting vibration displacement will vary as the constant unbalance force rotates and encounters a continually varying, asymmetric support stiffness.  If we were to plot this motion, we would generate an elliptical shape; i.e. the shaft relative orbit, which would accurately depict the motion of the centerline of the shaft in the radial or xy plane.  Maximum vibration will occur along the major axis of the ellipse (plane of lowest support stiffness) while along the minor axis of the ellipse, the vibration response will be at its minimum because the support stiffness is maximum.  More than likely, the major axis of the ellipse (plane of maximum shaft vibration) will not be aligned with either the x or y proximity probe.  Consequently by itself, as a single plane measurement, neither probe will see the maximum vibration.  However, it can be observed by noting the vibration amplitude along the major axis of the ellipse or by calculating Smax.  However, in order to either generate the shaft relative orbit or calculate Smax,  xy transducers must be installed at 90°.  These measurements cannot be made with a single plane measurement.  The worst case scenario occurs when the major axis of the ellipse (plane of maximum vibration) is rotated 45° with respect to the angular location of the proximity probe.  For this case, the proximity probe will only see 71% of the maximum vibration because the plane of maximum vibration is rotated 45° from the axis of the transducer.

The following vibration signal characteristics are critical when evaluating the health of  turbomachinery operating in hydrodynamic bearings:

vibration frequency

determining actual maximum vibration amplitude (requires xy proximity probes)

vibration precession; vibration either in the direction of or opposite of the direction of rotation  (requires xy proximity probes)

shaft average centerline radial position (requires xy proximity probes)

full spectrum  (requires xy proximity probes)

 At a minimum, the risk assessment should include:

•          Quantify past and present unit performance, maintenance history, and unit availability.

•          Review existing data relative to any previous failures, maintenance history and unit availability.

•          Quantify expected return on investment of expected changes or improvements.


[1] A fixed mass at a fixed radius rotating at operating speed generating a constant, uniform force in all directions

[2]Nnon-uniform support stiffness; i.e. different vertical support stiffness vs. horizontal stiffness.



  • Endres, Ned, “Machinery Health Monitoring Helps Plants Achieve Financial Goals”, Combined Cycle Journal, third quarter 2006, page OH-40  
  • Bently, D. E.  with Hatch, Charles, T.; Fundamentals of Rotating Machinery Diagnostics; Bently Pressurized Bearing Press; 2002
  • Eisenmann, Robert C. Sr. and Eisenmann, Robert C. Jr; Machinery Malfunction Diagnosis and Correction; Prentice Hall; 1998 
  • Standard 670, "Noncontacting Vibration and Axial Position Monitoring System,"; Standard 612, "Special Purpose Steam Tur­bines for Refinery Services,"; Standard 617, "Centrifugal Compressors for General Refinery Services,";  American Petroleum Institute, Washington, D.C., and Standard 426.01, "Specification for Meas­urement of Lateral Vibration on High Speed Helical and Herringbone Gear Units," American Gear Manufacturers Association, Arlington, Virginia.
  • Maxwell, A.S., "Vibration Monitoring - The Search for Optimum Protection," presented at the fourth Tur­bomechanics Seminar, Ottawa, Ontario, 23 September 1976.