Importance of Proper Alignment
Proper shaft alignment is critical when setting up a reliable power transmission drive train. Not only does proper alignment promote lower vibration and decreased reaction forces on seals and bearings, but it also plays a significant role in the service life of the coupling. One of the primary purposes of a flexible coupling is to accommodate misalignment between shaft ends, but there is a limit to that capacity. Operating a coupling at a greater angle increases the stresses on the flexible elements thus reducing the usable life of the coupling. When a coupling is operated within its designed misalignment capacity, the full potential service life of the equipment can be realized.
The Modified Goodman Diagram (see below) or similar fatigue diagram, is used to determine if the coupling can achieve infinite life when selecting a coupling for an application. The theoretical constant (torque, centrifugal, and axial misalignment) and alternating (angular misalignment and torsional oscillations) stresses, are quantified and plotted against each other. If the coupling is operated below the safe working stress line after the required Factor of Safety has been applied (dashed red line), the coupling can theoretically achieve infinite life.
While torque and speed are typically well understood quantities during the design phase, the alignment range of a coupling during operation can be more difficult to determine. The graphic below highlights how a specific load case is effected by an increase in alignment.
Typical Misalignment Capacity
Recommended Angle - 0.010°
Rated Angle - 0.020° (per API 671)
Effect of increased misalignment on disc pack stress and infinite life - FEA result of "high angle" illustrated above
Following a criticality analysis, a Failure Mode and Effects Analysis (FMEA) is conducted to determine all the potential failure types, what the failure effect is, and what can be used to mitigate or eliminate the failure mode. The FMEA is then used as the foundation in the decision-making process to determine the correct maintenance strategy for new equipment. The most common maintenance strategies are listed below:
Disc and diaphragm couplings are designed for infinite life when operated within their application design limits; however, a change in operating conditions may a couplings service life. The following sections highlight potential failure modes which can be mitigated or eliminated through the Kop-Flex Factory Recertification service. Used couplings are reconditioned to like “Like New” condition and returned with the original factory warranty, for a significant value over buying new.
While recertifying high performance couplings during every turnaround would be the most conservative maintenance strategy, it may not be the most cost effective or feasible due to scheduling logistics. Guidelines which combine Factory Recertification and condition monitoring, with the understanding and prevention of past issues, yields an effective Proactive Maintenance strategy for your critical rotating equipment. The following sections provide a foundational understanding of the Factors which may result in the reduction of the couplings effective service life.