Couplings are typically passive components compared to the other drivetrain equipment. They do not input any power or provide a process output, but simply connect the driving and driven equipment. The coupling may act as a “litmus test” and provide an early warning to a more significant problem if the equipment experiences an issue.
While designed for infinite life, couplings must be operated within their intended design limits in order to achieve optimal performance. Due to installation issues and unforeseen events, a coupling may be subjected to loading greater than its rated capacity while in service. The modified Goodman diagram is created by modeling the theoretical mean and alternating stresses the coupling is subjected to at its maximum allowable ratings. When a coupling is subjected to a torque and/or misalignment exceeding its rating, the stresses typically cannot be quantified accurately.
The difficulty in quantifying stresses, either due to shifting equipment or transient torque spikes make it difficult to predict a coupling’s remaining service life. Although technological advances in condition monitoring have decreased unanticipated failures, they may occur without warning or so rapidly that the equipment cannot be shut down in time. Knowing why a coupling fails is the first step to preventing it from occurring again in the future.
There are several reasons why a coupling will operate in a misaligned condition. A few of the most common include:
- Installation / Human Errors
- Thermal Growth
- Worn Bearings
- Dynamic Twist of Skid
- Settling Bases / Foundations
- Pipe strain
The axial alignment, or correct spacing between the flanges, ensures the coupling is being operated in a neutral position rather than under tension or compression. High performance couplings are typically provided with axial thermal growth values, which consider the thermal expansion of the equipment, allowing the coupling to be installed in a prestretched condition. Disc couplings are designed to accommodate this axial misalignment, but incorrect axial alignment or thermal growth values may impart an additional mean stress on the coupling or the equipment, adversely affecting performance.
The maximum continuous axial misalignment rating of the coupling is determined from the geometry of the disc pack and listed on the coupling drawing. For optimal service life, it is recommended that the coupling be shimmed and installed to operate within 10% of the maximum axial alignment rating of the coupling.
When a coupling is subjected to angular misalignment, the highest stresses will be found in the outermost discs near the disc pack bolt hole. This is the location of the highest bending stresses and where disc couplings typically fail from cyclic fatigue due to high misalignment. Fretting, which can be mitigated using a low friction coating on the discs, may also be present at the failure location due to movement between the discs. The failure of a disc coupling due to axial misalignment will show similar signs as angular misalignment. The discs may crack on both sides of the disc pack bolt hole, since the coupling is in compression or tension.
Coupling Issue #1: Angular Misalignment
- 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
- Fracture near bolt hole
- Fretting at fracture location
- Fracture may occur through bolt hole
- Thermal movement
- Foundation setting
- Pipe strain
- Loose anchor bolt/ studs
- Poor initial alignment
- 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.
Coupling Issue #2: Axial Misalignment
- 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
- Fracture near bolt hole
- May occur on both sides of bolt hole
- Incorrect thermal growth
- Equipment not on mag. center
- Incorrect magnetic center
- Poor initial alignment
- Check axial spacing, coupling axial capacity, and equipment end play
- Verify motor rotor is on magnetic center
- Review thermal growth values of equipment
The torque capacity of the coupling is typically determined during the design and selection phase. Since this is generally a well understood quantity, torque related failures frequently coincide with an atypical event, such as the ingestion of a liquid slug in a compressor or a hot shut down following an equipment trip. Torque related failures typically exhibit severe spreading or buckling of the disc pack and may result in the deformation of the flanges due to contact from the disc pack hardware.
Failures due to torsional fatigue are becoming more common due to the increased use of variable frequency-controlled drives on motors which can excite damaging resonant frequencies. Additionally, synchronous motor driven trains may experience high torsional oscillations during startups, so equipment that is subject to frequent startups is at higher risk.
Coupling Issue #3:Torque Overload
- Typically instantaneous failure, difficult to predict
- Imbalance created from buckled disc packs
- Audible noise due to yielding of disc pack
- Buckling of disc pack
- Severe spreading
- Spreading in consecutive links
- Disc pack bolts may contact clearance holes
- Electrical fault
- Liquid slug in compressor
- High start up torque
- Operation/process event
- 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.
Coupling Issue #4: Torsional Oscillation
- 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
- Excitation from VFD drive
- Torsional issue
- "Whipping action” on start due to motor slip
- Review system torsionals and tune coupling stiffness/ inertias, if necessary.
- Troubleshoot with torque monitoring system.
- Review VFD drive parameters (if applicable).