Shaft alignment is the process of aligning two or more shafts to within a tolerated margin. It is a critical requirement for turbomachinery trains before they are put in service.
Shafts are in alignment when they are colinear at the coupling point, i.e., the rotational centerlines of two mating shafts are parallel and intersect. In other words, they join to form one line.
When this is the case, the coupled shafts theoretically operate like a solid shaft. Any deviation from the aligned or colinear condition results in abnormal wear of turbomachinery components, such as bearings, shaft seals and the shafts. Proper alignment reduces power consumption, vibration and noise, as well as helping to achieve the intended life of turbomachinery and its components.
The goal of alignment procedures is to obtain a common axis of rotation during operation for two coupled shafts. This is particularly important for high-speed, high-power units. Although some flexible couplings might allow for a tiny amount of misalignment, correct alignment remains critical for shafts with flexible coupling.
Shaft alignment can change due to factors, such as operational variation and thermal movement. These can cause movements of components, rotor assemblies and shafts, leading to misalignment.
It is important for final turbomachinery train alignment to compensate for actual operating conditions as machinery often moves after start-up as the result of wear, thermal differences, dynamic loads, and support or structural shifts. These factors should be considered and compensated for during the alignment process.
Tools used to achieve alignment may be mechanical, optical (laser shaft alignment) or gyroscope-based. Before shaft alignment, the foundation should be checked and verified.
Alignment is usually accomplished through corrections, such as shimming or moving a component. Both angular and offset alignment should be performed in two planes.
This is accomplished by moving other components to align the shaft(s) with the rotational centerline. Too often, alignment operations are performed randomly, and adjustments are made by trial and error, resulting in a time-consuming procedure.
“The use of alignment tools and procedures that take thermal movements into account is a mandatory requirement for turbomachinery operating at extreme temperatures.”
Simplistic versus 3D
Since shafts exist in three-dimensional space, misalignment can occur in any direction. It is best to break three-dimensional space into vertical and horizontal planes and to describe the specific amount of offset and angularity in each plane simultaneously at the location of the connection or coupling.
Thus, there are four specific values of misalignment needed to accurately express alignment, two offsets and two angularities. A traditional single limit of perhaps 50 microns is a simplified expression of misalignment used for ordinary machinery and basic alignment exercises. For a thorough investigation, all four components should be considered and verified.
The conditions of misalignment should be described at the location of the coupling: it is at the shaft connection that harmful machinery vibration and forces are created whenever misalignment exists. The magnitude of a misalignment tolerance or the desired alignment quality should be expressed in terms of these offsets and angularities or any other suitable 3D method.
Additionally, the size, geometry or operating temperature of turbomachinery should be considered in its alignment. Vibration and the resulting efficiency loss due to misalignment of shaft centers depends on shaft speed and the details of the coupling or type of connection.
Note that bearing load increases with misalignment. As a rough indication, bearing life decreases as the cube of the load increase, i.e., doubling the load will shorten bearing life by a factor of eight.
Do not forget to take into account thermal movement as a factor in alignment. Aligning center-to-center without paying attention to thermal movement can often lead to misalignment during operation and consequent failures.
The use of alignment tools and procedures that take thermal movements into account is a mandatory requirement for turbomachinery operating at extreme temperatures, whether cold (cryogenic machinery) or hot.
Different criteria have been used related to alignment and allowable misalignment. As a rough indication, limits between 40 and 55 microns have been specified and used. ■