Why Proper Coupling Alignment Matters
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.
Types of MisalignmentAngular Misalignment can cause fracture near the bolt hole, as well as fretting near the fracture. This type of misalignment is typically caused by thermal movement, foundation setting, pipe strain, loose anchor bolts, or poor initial alignment. With angular misalignment, you will notice higher axial vibration, as well as radial vibration trending up over time.
Axial misalignment can cause fracture near or on both sides of the bolt hole. It is typically caused by incorrect thermal growth, incorrect magnetic center, or poor initial alignment. Noticeable symptoms can include increased axial vibration, fluctuation in the motor current, increased thrust bearing temperature, and a pulsing in axial readings due to thrusting.
Benefits of Proper Alignment
The misalignment discussed above can lead to several types of equipment failure. Premature wearing can occur on a seal or bearing when it can’t run at optimal speed because of vibration problems.
Proper alignment will not only help you avoid equipment failure, but can also provide additional benefits:
- Run with lower bearing temperatures
- Reduced vibration levels
- Extended life of the equipment
- Extended maintenance intervals
Methods of AlignmentThe most common methods of coupling alignment are dial indicator and straight edge:
Dial IndicatorAttach a dial indicator bracket to one hub with dial indicator probe contacting the opposite hub’s alignment surface. Rotate the hub on which the dial indicator is attached and take readings at 4 points 900 apart. The offset (TIR) should be less than .002 inches times the coupling size. For greater hub separation (such as spacer or floating shaft couplings) use suitable fixtures to span the separation and measure the four 90° points. Shim machine until best possible alignment is obtained.
Straight EdgeShim one machine and align shafts using a straight edge until it appears to be at right angles to the shafts. Repeat at three additional points 90° apart. Recheck angular alignment and hub separation.
Alignment Isn’t ConstantIt’s important to remember that alignment isn’t a constant. Alignment can change with time and operating conditions, such as seasonally or aging and settling of equipment. You need to recheck your coupling alignment periodically so that you don’t start to dip into problems as conditions change over time. Any time there is a major rebuild, equipment is moved, or you replace a coupling, it’s important to double check the coupling alignment. Ensuring you are still working in the right parameters will help to improve overall equipment life.
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
Achieving Infinite LifeA Modified Goodman Diagram 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, 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.
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.
As always, it is important to remember to follow the manufacturer’s recommendations regarding limits.