Shaft Alignment

Pump arrangements, or most systems that include a motor, have two main components: the driver (motor) and the driven equipment (pump). The driving power of the motor is transmitted to the pump through the shaft. In some cases, the motor and pump are close-coupled but in other situations a coupling is preferred to accommodate a small degree of shaft misalignment and end play. The figures below depict a standard pump skid that would utilize a coupling device between the pump and motor shafts. Shaft misalignment may be considered the most common cause of equipment malfunction leading to increased energy usage, costly repairs, and extended production down time. It puts stress on both pieces of equipment as well as any intermediate devices such as couplings, bearings, and mechanical seals.

There are two types of shaft misalignment: parallel/offset and angular (or a combination of both). Parallel misalignment occurs when the shafts are parallel but have a slight offset between centerlines. Angular misalignment occurs when the shaft centerlines sit at an angle relative to one another. The figures illustrate examples of proper alignment as well as both types of misalignment in vertical and horizontal directions. These cases are clearly exaggerated for ease of viewing but may not be so easily detectable in the field.

Shaft alignment requires precise measurements and adjustments and starts with inspecting and repairing/replacing parts including, but not limited to, the shaft, coupling, bearings, baseplate, and foundation. Trained technicians and plant mechanics typically use dial indicators or, more conveniently, laser alignment devices when performing a shaft alignment. The pump should remain stationary during this procedure while the motor seating is adjusted. It is recommended to correct the vertical misalignment first which may be as simple as adding/removing varying sized shims as necessary below the motor feet or resurfacing the mounting surfaces. Subsequently, the horizontal misalignment can be corrected by turning jack bolts until the motor is properly aligned or lightly tapping the motor body with a rubber mallet, though not the preferred method. It is important to remember to recheck the alignment after each adjustment is made.

Industry standard recommends the shaft alignment be checked 5 times before being approved for operation. If a skid is fully aligned, then lifted and installed on site, you should not assume the shaft preserved alignment. Simple visual alignment checks should be included in routine equipment inspections and more in-depth analysis performed if possible or when necessary. With proper shaft alignment, equipment life and mean time between repairs increases bringing along steady production and reduced repair costs.