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Disc Couplings Failures


ANGULAR MISALIGNMENT

Possible Symptoms:

  • High axial vibration at 1x, 2x and 3x (2x may be higher than 1x).
  • Phase relationship across coupling typically ~180°.
  • 1x radial vibration trends up over time due to “unraveling” of disc pack.

General appearance

  • Fracture near bolt hole.
  • Fretting at fracture location.
  • Fracture may occur through bolt hole.

Typical causes

  • Thermal movement.
  • Foundation setting.
  • Pipe strain.
  • Loose anchor bolt/ studs.
  • Poor initial alignment.

Corrective actions

  • Check parallel offset AND face runout on each side as the angularity (offset + face runout) may be different between ends.
  • Review thermal growth values of equipment (if applicable) and perform hot alignment.
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AXIAL MISALIGNMENT

Possible Symptoms:

  • Elevated axial vibration and fluctuation in motor current if motor rotor off magnetic center.
  • Increased thrust bearing temperature.
  • Can see a “pulsing effect” in axial readings due to thrusting.

General appearance

  • Fracture near bolt hole.  
  • May occur on both sides of bolt hole.

Typical causes

  • Incorrect thermal growth.
  • Equipment not on mag. center.
  • Incorrect magnetic center.
  • Poor initial alignment.

Corrective actions

  • Check axial spacing, coupling axial capacity, and equipment end play.
  • Verify motor rotor is on magnetic center.
  • Review thermal growth values of equipment.
 

TORSIONAL OSCILLATION

Possible Symptoms:

  • Difficult to detect without torque measurements.
  • Correlation between vibratory torque frequency and radial vibration may be present in geared trains.

General appearance

  • Fracture in center of disk link.
  • Fretting at clamping area and in the center of the link.

Typical causes

  • Excitation from VFD drive.
  • Torsional issue.
  • "Whipping action” on start due to motor slip.

Corrective actions

  • Review system torsionals and tune coupling stiffness/ inertias, if necessary.
  • Troubleshoot with torque monitoring system.
  • Review VFD drive parameters (if applicable).
 

 

TORQUE OVERLOAD

Possible Symptoms:

  • Typically instantaneous failure, difficult to predict.
  • Imbalance created from buckled disc packs.
  • Audible noise due to yielding of disc pack.

General appearance

  • Buckling of disc pack.
  • Severe spreading.
  • Spreading in consecutive links.
  • Disc pack bolts may contact clearance holes.

Typical causes

  • Electrical fault.
  • Liquid slug in compressor.
  • High start up torque.
  • Operation/process event.

Corrective actions

  • Inspection for crack initiation, replace coupling elements at a minimum immediately.
  • Check sizing and Service Factor of Coupling – possible application for shear pin/shear spacer design.
  • Review operational history for any atypical events.