Why is shaft alignment necessary for rotating machinery?
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Why is shaft alignment necessary for rotating machinery?
Prevent vibration in rotating machinery caused by unbalanced centrifugal forces during operation
1. Why alignment is necessary: The rotation centers of the two shafts connected by the coupling between the pump and the motor must be strictly concentric. The coupling must be accurately aligned during installation; otherwise, it will cause significant stress on the coupling, which in turn will severely affect the proper functioning of the shafts, bearings, and other components on the shafts. This can even lead to vibrations or damage to the entire machine and its foundation. Therefore, aligning the coupling between the pump and the motor is one of the crucial steps in the installation and maintenance process.
2. Analysis of coupling misalignment: When installing a new pump, it may not be necessary to check the perpendicularity between the coupling’s end faces and the axis. However, when installing an old pump, this check is essential. If there is any deviation from perpendicularity, it must be corrected before alignment can take place. Generally, there are four possible scenarios:
1) S1 = S2, a1 = a2: The end faces of the two coupling halves are both parallel and concentric. In this case, the two axes must lie on the same straight line.
2) S1 = S2, a1 ≠ a2: The end faces are parallel, but the axes are not concentric. There is a parallel radial displacement of e = (a2 – a1)/2 between the axes.
3) S1 ≠ S2, a1 = a2: The end faces are concentric but not parallel. There is an angular displacement of α between the axes.
4) S1 ≠ S2, a1 ≠ a2: The end faces are neither concentric nor parallel. There is both a radial displacement of e and an angular displacement of α between the axes. The first scenario is the one we strive to achieve during alignment. The second, third, and fourth scenarios are incorrect and require adjustments to bring them back to the first scenario. When installing equipment, first install the driven unit (the pump) with its axis in a horizontal position, and then install the driving unit (the motor). Thus, alignment only requires adjusting the driving unit, by using shims under its feet.
3. Alignment methods: Traditional methods:
1) Using a knife-edge gauge and feeler gauge to measure the misalignment of the coupling, as well as wedge-shaped gaps or feeler gauges to measure the non-parallelism of the coupling’s end faces. This method is suitable for low-speed equipment with elastic connections and low precision requirements. However, its drawback is that it can only be used for simple equipment in situations where the working environment isn’t complex. Its accuracy is also low. Due to the increasing demands for equipment management and maintenance, this method is increasingly being abandoned by equipment managers.
2) Using wire to align the coupling. This method is also suitable for low-speed equipment with elastic connections and low precision requirements. As shown in the diagram, wrap two pieces of wire around the two coupling halves, making sure they are tightly wrapped so that there is no relative movement between the wire and the shafts. Adjust the distance between the wires to a certain value—neither too close nor too far—and remember this initial distance. Rotate both shafts in the same direction (either clockwise or counterclockwise) for one full turn, trying to ensure that the angles rotated by both shafts remain consistent at all times. Stop every 90 degrees to check changes in the distance S. This method can detect the consistency of the end faces’ perpendicularity, i.e., the levelness of the shafts, as well as any vertical misalignment between them. It’s a relatively intuitive method, but its drawbacks are the same as those of traditional methods. It’s not commonly used these days.
3) Using a dial indicator or specialized alignment tools to measure the misalignment and non-parallelism of the couplings. This method is suitable for high-speed equipment with rigid connections and high precision requirements. However, it has three drawbacks: 1) Low accuracy; 2) Time-consuming and labor-intensive, with repeated adjustments often required. It’s difficult to complete quickly, especially when production needs to resume immediately; 3) It’s difficult to use this method for large-scale equipment or complex working environments. Advanced precision laser alignment methods: Currently, the most widely used method for shaft alignment in China is precision laser alignment
2. Analysis of coupling misalignment: When installing a new pump, it may not be necessary to check the perpendicularity between the coupling’s end faces and the axis. However, when installing an old pump, this check is essential. If there is any deviation from perpendicularity, it must be corrected before alignment can take place. Generally, there are four possible scenarios:
1) S1 = S2, a1 = a2: The end faces of the two coupling halves are both parallel and concentric. In this case, the two axes must lie on the same straight line.
2) S1 = S2, a1 ≠ a2: The end faces are parallel, but the axes are not concentric. There is a parallel radial displacement of e = (a2 – a1)/2 between the axes.
3) S1 ≠ S2, a1 = a2: The end faces are concentric but not parallel. There is an angular displacement of α between the axes.
4) S1 ≠ S2, a1 ≠ a2: The end faces are neither concentric nor parallel. There is both a radial displacement of e and an angular displacement of α between the axes. The first scenario is the one we strive to achieve during alignment. The second, third, and fourth scenarios are incorrect and require adjustments to bring them back to the first scenario. When installing equipment, first install the driven unit (the pump) with its axis in a horizontal position, and then install the driving unit (the motor). Thus, alignment only requires adjusting the driving unit, by using shims under its feet.
3. Alignment methods: Traditional methods:
1) Using a knife-edge gauge and feeler gauge to measure the misalignment of the coupling, as well as wedge-shaped gaps or feeler gauges to measure the non-parallelism of the coupling’s end faces. This method is suitable for low-speed equipment with elastic connections and low precision requirements. However, its drawback is that it can only be used for simple equipment in situations where the working environment isn’t complex. Its accuracy is also low. Due to the increasing demands for equipment management and maintenance, this method is increasingly being abandoned by equipment managers.
2) Using wire to align the coupling. This method is also suitable for low-speed equipment with elastic connections and low precision requirements. As shown in the diagram, wrap two pieces of wire around the two coupling halves, making sure they are tightly wrapped so that there is no relative movement between the wire and the shafts. Adjust the distance between the wires to a certain value—neither too close nor too far—and remember this initial distance. Rotate both shafts in the same direction (either clockwise or counterclockwise) for one full turn, trying to ensure that the angles rotated by both shafts remain consistent at all times. Stop every 90 degrees to check changes in the distance S. This method can detect the consistency of the end faces’ perpendicularity, i.e., the levelness of the shafts, as well as any vertical misalignment between them. It’s a relatively intuitive method, but its drawbacks are the same as those of traditional methods. It’s not commonly used these days.
3) Using a dial indicator or specialized alignment tools to measure the misalignment and non-parallelism of the couplings. This method is suitable for high-speed equipment with rigid connections and high precision requirements. However, it has three drawbacks: 1) Low accuracy; 2) Time-consuming and labor-intensive, with repeated adjustments often required. It’s difficult to complete quickly, especially when production needs to resume immediately; 3) It’s difficult to use this method for large-scale equipment or complex working environments. Advanced precision laser alignment methods: Currently, the most widely used method for shaft alignment in China is precision laser alignment
The main purpose is to reduce vibration, particularly the vibration at twice the frequency
If this is a short-answer question in an exam, you can approach answering such a question from a different perspective. It can be answered as follows: If rotating machinery is not aligned, it will affect the proper operation of the shafts, bearings, and other rotating components on the shaft, and in severe cases, it may lead to equipment failures. I’m sure no one would dare say you’re wrong.