An Overview of Shaft Alignment

What is Shaft Alignment

Shaft alignment is the process of aligning the shafts of two or more pieces of rotating equipment, typically a motor and a pump, that are connected for the purpose of transmitting power from one to the other. Shaft alignment is essential to ensure proper functioning of the equipment and reduce stress on the bearings, shafts, couplings, and the equipment itself. Shaft misalignment can lead to premature wear on bearings, seals, and other parts, excessive vibration, excessive energy consumption, coupling failure, and even equipment failure. Damage to machinery caused by shaft misalignment results in down time and repair costs as well as production losses. Proper shaft alignment can not only avoid these problems, but also extend the time intervals between regular machinery servicing. Shaft alignment is done using various tools. Two of the most popular tools are a dial indicator and a laser alignment system. Regardless of what tool is used, the principles involved in shaft alignment are the same. The tools are used to calculate how much the equipment (the motor, pump, etc.) needs to be moved vertically and horizontally in each direction to achieve the proper alignment, in which the shaft centers form a straight line (are collinear) during operation.

Calculating shaft alignment

Misalignment is described in terms of offset and angular. Offset misalignment, also called parallel misalignment, refers to the distance between the rotating axes, either on the vertical or horizontal plane, or both. Angular misalignment, also called “gap” or “face,” refers to the angle of one shaft compared to the other, and is also expressed in the vertical and horizontal planes. Misalignment is often a combination of both offset and angular types.
There are many types of shafts and couplings, and shaft alignment is not the same for all of them. While in most cases shaft alignment requires getting the shafts perfectly lined up, there are exceptions. Gear couplings and Cardan shafts are two examples. These actually require a defined misalignment to ensure correct lubrication during machine operation. In these cases, shaft alignment calculations need to take this defined misalignment into consideration. Regardless of the type of shaft alignment, precision is required. Alignment tolerance is typically in the range of 3/100–5/100 mm.

Shaft alignment requires various skills, including: 1) the ability to take measurements accurately and precisely, 2) the ability to determine whether the measured values are valid, and 3) the ability to correctly adjust the equipment based on the measured values.

Shaft alignment steps

That said, as long as correct measurements are taken and the proper order of steps followed, anyone should be able to perform shaft alignment. Shaft alignment involves the following three steps. The steps are the same regardless of how big or small the rotating equipment is.

  • Step 1. Investigate the current situation
  • Step 2. Measure the amount the equipment needs to be moved (the amount of correction).
  • Step 3. Repeat steps 1 and 2 until the alignment is within the decided allowable range (tolerance).

Calculating the required corrections involves measuring the offset and angle, the coupling diameter, and the distance from the coupling to the front and rear feet of the equipment. These values are then input into mathematical formulas.
Corrections are first made on the vertical plane, followed by the horizontal plane. Corrections to the vertical and horizontal planes are never done at the same time.

In addition, all shafts have some degree of curvature caused by their own weight. No shaft is actually perfectly straight. Shaft alignment must take into consideration shaft curvature in order to align the center of rotation of the two pieces of equipment. The curvature of the equipment shaft is determined by several factors. These include shaft rigidity, the distance and weight in between shaft supports, and the shaft shape.
Shaft alignment should also not be confused with coupling alignment. Coupling surfaces may, but do not always, represent the rotation axis of the shafts. Couplings may have rough surfaces, and the coupling mount may not be precise, resulting in differences with shaft alignment.
Other factors also need to be taken into consideration. Calculations are typically made with equipment in the cold condition, but thermal growth may be a factor when the equipment is in operation. Machine torque is another factor that may change shaft shape during operation. Some shafts are also flexible and not perfectly straight by design; their curvature varies depending on the speed of rotation when in operation.

Shaft alignment mistakes

Shaft alignment is done after pumps and fans have been installed and readied for operation. After shaft alignment, the shafts are connected to the couplings and the equipment is put into operation. While there may be pressure to get equipment operating by a preset deadline, shaft alignment should never be done in a rush. This can lead to taking shortcuts in the alignment steps and errors that result in ultimately needing more time to achieve correct shaft alignment.

Following are some typical mistakes made when aligning shafts.

  1. When the pipes of a pump are connected at the inlet and outlet openings, the pipes exert stress on the pump casing, causing it to deform and alter the shaft alignment. The same problem can occur when the pump baseplate is leveled.
  2. If there are any gaps between the equipment and the baseplate, this will exert strain on the casing, causing deformation and shaft misalignment. This problem is called soft foot, or distortion caused improper contact between the machine casing and baseplate.
  3. Shims are typically inserted under each foot of the rotating equipment to eliminate gaps. If old shims are reused, their shape may have changed, leading to misalignment and soft foot. Using too many shims may also cause soft foot, as several thin shims stacked on top of each other do not have the same strength as one thick shim.
  4. Using several people to tighten bolts may also result in excessive force being used on some of the bolts, resulting in a deformed casing.

Shaft alignment requires precise calculations of the offset and angular misalignment and an understanding of the tools and various factors that can affect alignment. That said, however, as long as the basic steps are followed, anyone should be able to perform shaft alignment.

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