Proper Pump to Motor Alignment Techniques: Part 1 of 6
Correct shaft alignment of the motor to the centrifugal pump is one of the most critical steps in the installation process. Performing this step well will set the foundation for many years of smooth pump and system operation. Failure to properly align a motor to the pump within the required tolerances can prove disastrous, resulting in high vibrations, increased shaft stress, high bearing temperatures, and possibly premature pump failure.
This multi-part guide aims to define the process as well as highlight some of the crucial steps that must be taken in order to achieve a successful alignment.
In essence, a shaft alignment can be defined as positioning two rotating machines at the point of power transfer, in this case a pump and a motor, such that their axes of rotation are co-linear when the machine system is operating under normal conditions.
Several key distinctions made in the statement above are:
- Point of Power Transfer: No shaft is perfectly straight, in fact, most pump rotating elements are designed as “non-rigid rotors”. This means that, when stationary, the rotating element will exhibit significant deflection at areas not directly supported by bearing surfaces. Therefore, for shaft alignment purposes, we consider the point of power transmission, the interface between couplings, only.
- Axes of Rotation: This is one of the most common stumbling blocks, or sources of error when performing a shaft alignment between a motor and a pump. Industry experts refer to the alignment process as a “shaft alignment” for a reason. That reason is that the alignment being performed should be between the pump shaft and the motor shaft. Coupling geometry/surfaces should never be used to define critical alignment conditions, as this geometry does not accurately represent the rotation of axis of the shafts.
- Under Normal Operating Conditions: It is important to understand that the alignment condition between two rotating machines can change significantly once the machines are in operation. A variety of factors can affect the alignment condition, including, but not limited to thermal growth, pipe strain, machine torque, foundation movement and bearing play. Setting the static, cold alignment targets to account for hot, dynamic conditions that will be present during operation is therefore critical.
Is shaft alignment necessary with flexible couplings?
Throughout industry, confusion exists on why it is necessary to align a pump to a motor if a flexible coupling is being used. After all, many high-end flexible couplings are designed to accommodate misalignments in excess of 250 mils. With that being the case, why bother aligning the two rotating machines at all?
The answer to that question lies not with the coupling itself, but with the components used in the pump and driver. Experience across a variety of industries tells us that as misalignment increases, loading on shafting increases dramatically. The increase in shaft loading causes increased reactionary forces within bearings, bearing isolators and mechanical seals, ultimately leading to premature failures.
To reiterate, a pump should always be aligned to its driver, even if a modern, high-end flexible coupling is being used. It is commonly stated that as much as 50% of rotating machine failures throughout industry can be attributed to incorrect shaft alignment.