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.
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.
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.
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).
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