Shaft Orbit Plots (I)

The Orbit

•          The orbit represents the path of the shaft centerline within the bearing clearance.

•          Two orthogonal probes are required to observe the complete motion of the shaft within.

•          The dynamic motion of the shaft can be observed in real time by feeding the output of the two orthogonal probes to the X and Y of a dual channel oscilloscope

•          If the Keyphasor output is fed to the Z axis, a phase reference mark can be created on the orbit itself

•          The orbit, with the Keyphasor mark, is probably the most powerful plot for machinery diagnosis

Precession

Once a gyroscope starts to spin, it will resist changes in the orientation of its spin axis. For example, a spinning top resists toppling over, thus keeping its spin axis vertical. If a torque, or twisting force, is applied to the spin axis, the axis will not turn in the direction of the torque, but will instead move in a direction perpendicular to it. This motion is called precession. The wobbling motion of a spinning top is a simple example of precession. The torque that causes the wobbling is the weight of the top acting about its tapering point. The modern gyroscope was developed in the first half of the 19th cent. by the

 

Construction of an Orbit

•          XY transducers observe the vibration of a rotor shaft

•          A notch in the shaft (at a different axial location) is detected by the Keyphasor transducer.

•          The vibration transducer signals produce two time base plots (middle) which combine into an orbit plot (right)

 Probe Orientation and the Orbit Plot

•          On the left side, when the probes are mounted at 0^o and 90^oR, the orbit plot and oscilloscope display show the same view.

•           On the right, when the probes are mounted at 45^oL and 45^oR, the orbit plots are automatically rotated

•           The oscilloscope, however, must be physically rotated 45^o CCW to display the correct orbit orientation 

Examples of 1X and Subsynchronous Orbits

 

•          Orbit at left shows subsynchronous fluid-induced instability. Note the multiple keyphasor dots because the frequency is not a fraction of the running speed

•          The orbit at right is predominantly 1X. The keyphasor dots appear in a small cluster indicating dominant 1X behavior 

 

Slow Roll Vector Compensation of 1X Filtered Orbit

•          Slow roll vector compensation can considerably change the amplitude and phase of the orbit

•           Slow roll vectors of X= 1.2 mil pp /324^o and Y= 1.4 mil pp /231^o

Slow roll Waveform Compensation of a Turbine Orbit

Note how compensation makes the orbit (right) much clearer