How to Extend the Life of Your Coupling
Couplings are designed to be the weakest link and lowest cost component on rotating equipment. Because of this, they are often overlooked. But there are steps you can take to extend the life of your coupling (and maybe even learn something about your equipment operating conditions too). By understanding and identifying coupling failures if and when they do happen, they can be prevented in the future. This will result in increased uptime of your equipment.
The primary function of a coupling is to transmit torque between equipment and accommodate for different types of misalignment while reducing the reactionary forces on the equipment. Couplings can also be designed to dampen damaging peak torques and torsional vibrations, provide overload protection to prevent the transmission of damaging load and prevent shorting between equipment by adding electrical insulation.
Disc couplings are typically passive components compared to the other drive-train 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.
It Starts with Selecting the Right Coupling
While couplings are designed for infinite life, they 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. These issues can typically be identified through instrumentation or visual inspection, but not all coupling failures can be prevented. Selecting the right coupling for your application is the first step in ensuring it’s long performance life.
Most high-performance couplings are designed to be maintenance free, and if they are operated within their rated capacities, can outlast the equipment to which they are connected.
Gear couplings are the preferred style when an extremely high torque capacity is required. As a mechanically flexible design, they transmit load through teeth on the hub and sleeve which are designed to accommodate misalignment. The metal-on-metal contact is subject to wear and, while the rate can be mitigated through proper coupling sizing, case hardening and lubrication, they are designed with a finite service life in mind.
Disc couplings are the most common flexible element coupling due to their excellent performance, high misalignment capacity, compact design, and cost. While there are many different disc couplings designs available on the market, they all operate under the same principals. Disc couplings are designed for infinite life when operated within their recommended limits. The primary failure modes of disc couplings are due to excessive misalignment and torque overload. The failure mode can be identified through inspection of the disc pack. The potential for future failures may be reduced by performing regular inspections.
Why Do Couplings Fail?
Couplings fail for several reasons, but the primary causes are improper selection for the particular application; excessive misalignment; improper, inadequate, or insufficient lubrication; harsh environmental or operating conditions; and excessive speeds or loads.
A coupling will achieve its maximum service life when operated within its designed alignment and torque ratings. Detrimental operating conditions as a result of inaccurate thermal growth offsets, shifting equipment foundations or torsional oscillations, will shorten a coupling’s effective service life if the load case exceeds the as-designed values.
Top Reason for Failure #1: Misalignment
Angular misalignment can be caused by thermal movement, foundation settling, pipe strain, loose anchor bolts, or poor initial alignment. You will generally notice a fracture near or through the bolt hole or fretting at the fracture location. Other symptoms include high axial vibration at 1x, 2x and 3x and 1x radial vibrations trending up over time as the disc pack deteriorates. Corrective actions for angular misalignment include checking parallel offset and face runout on each side as the angularity might be different between ends. You should also review thermal growth values of equipment and perform hot alignment.
Axial misalignment is usually caused by incorrect thermal growth, incorrect magnetic center or poor initial alignment. You may notice a fracture near or on both sides of the bolt hole. Symptoms also include an elevated axial vibration and fluctuation in the motor current, increased thrust bearing temperature and a “pulsing effect” in axial readings because of thrusting. In the case of axial misalignment, check axial spacing, coupling axial capacity and equipment end play. You should also verify the motor rotor is on the magnetic center and review the thermal growth values of the equipment. High performance couplings are typically installed in a pre-stretched condition, which accommodates for the axial thermal expansion of the equipment.
Top Reason for Failure #2: Torque Overload
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.
Steps for Proper Maintenance
Once you know why a coupling fails, you can work to prevent it from occurring again in the future. Regularly scheduled visual inspections (performed during scheduled turnarounds), preventative maintenance and condition monitoring can assist in the early identification of potential problems, preventing damage to the coupling and connected equipment.
For critical equipment, scheduled turnarounds provide the best opportunity to perform a visual inspection of a coupling in service. Because these inspections are inherently invasive and cracks in the disc pack may not be visible without the removal and disassembly of the coupling, condition monitoring is widely used for inspecting critical equipment.
For non-critical equipment, periodic visual inspections of the disc packs may be a sufficient strategy to keep your equipment running. It is strongly recommended that the equipment be stopped and locked out to ensure safety.
Recertification as an Option
A coupling recertification can be performed during a plant turnaround, resulting in a “like new” coupling for 50-60% the cost of new equipment. Coupling re-certification resets the damage accumulation the coupling has endured while operating under real-world loading conditions.
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Monitoring and Diagnostics
Torque Monitoring is a valuable tool for preventative maintenance and problem solving. It provides real-time data from couplings to operators to:
- Understand the real loads under working conditions versus design loads
- Specify overload protection devices if needed
- Verify the most appropriate coupling solution
- Determine the effect of transient loads on coupling
By selecting the right coupling for the job and following these steps for proper maintenance, you can extend the life of your coupling and reduce downtime.