The 3 Main Alignment Methods: Pros and Cons
As discussed in "Top alignment issues for centrifugal pumps", incorrect alignment can cause all sorts of issues for centrifugal pumps. Accuracy is essential to ensure the longevity of your pump. There are three methods used to align pumps that are commonly used across industrial plants. These are:
This method involves placing the straight edge on the pump and coupling, then visually checking to see if the components are aligned. Feeler gauges are used to measure the gap at the top and bottom of the coupling. Alignment is then done through a process of trial and error.
While the easiest and least expensive method of alignment, straight edge is the least accurate and least likely to achieve manufacturer's alignment specifications. It's usually used for small pump/motor combinations that don't allow enough room to use more accurate methods. In today's world, this is also the least likely method to be used, as high-performance machinery requires exact measurement.
Dial indicators form the basis of several alignment methods. Rim and face is usually used when only one shaft can be rotated, cross dial takes shaft-to-shaft readings from two indicators mounted 180 degrees apart and reverse dial does the same except indicators are in the same plane.
While they can give very accurate readings, dial indicators can be difficult to use, requiring a high level of technical competence. Unless indicator stems are positioned precisely, errors can occur. Reading errors can also occur through coupling backlash, looseness and indicator bracket sag. Dial indicators must also be removed and reinstalled after each adjustment, making the process for obtaining critical measures such as feet and coupling values very lengthy.
Laser alignment is the most accurate method available. This method employs state-of-the-art lasers to determine shaft positions. It then relates this information to the computer, allowing very precise recommendations for adjustments. While more expensive, laser alignment offers significant advantages over time, and initial costs can be offset by reduced friction and energy use, lower levels of vibration and noise and longer lasting bearings, seals, shafts and couplings.
Once familiar with this method, the operator can align the pump/motor very quickly, which allows for highly accurate alignment even when shafts are separated by more than a few inches - extending the life of the pump. Laser alignment software can also calculate the exact shim changes required. A significant advantage of laser is that it doesn't require much skill, thermal growth data can be included to allow the machine to compensate, and the operator can check the pump while it is running and at temperature. In addition, the laser system tool documents recorded values, which can be downloaded to a computer and used to benchmark future alignment inspections.
Laser alignment doesn't have gravitational hardware sag, can work with or without couplings in place, is fast and easy to mount, can detect and measure "soft foot" and can feed misalignment data to the microprocessor for correction, both horizontal and vertical. It's only real disadvantages are cost, an initially steep learning curve, and the fact that laser devices are often quite large and thus unsuitable for smaller pump/motor systems. Heat or steam can also affect the accuracy of readings.
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