Pumps and Systems, June 2009
Proper alignment of the pump shaft with the driver can reduce vibration and significantly improve reliability. For appropriate applications, the time, expertise and instruments needed to achieve precision alignment (tolerances of less than 0.005 in) will prevent seal leakage and extend bearing life.
Depending on such factors as operating speed and coupling type, not all pumps will require such precise alignment. When assessing a plant's alignment needs, it helps to understand basic shaft alignment concepts and procedures, as well as application-specific factors that dictate the required tolerances.
Effects of Misalignment
A common misconception about pump shaft/driver misalignment is that it increases bearing load, causing bearings to fail prematurely. In fact, except in cases of extreme misalignment, the resulting vibration is what damages bearings and seals. Since some vibration is normal for pumps, it is best to have an experienced vibration technician determine if the vibration is due to shaft misalignment, and whether it is severe enough to affect pump reliability.
The purpose of shaft alignment is to minimize the vibration resulting from forces transmitted across the coupling. The goal is to have both shafts rotating on a common axis, referred to as collinear. All misalignment of shaft centerlines (i.e., deviation from the collinear condition) can be described in terms of offset and angularity.
Theoretically, two perfectly aligned shafts would rotate in the same axis, and if properly balanced and coupled, would not generate abnormal vibration during operation. If instead the two shafts are misaligned in the horizontal or vertical plane (or both), or are at an angle with respect to one another, they will rotate in different axes. The amplitude of the resulting vibration will vary, depending on such factors as the severity of the misalignment, operating speed and coupling type.
In addition, the relative positions of a horizontal pump and driver can be viewed independently in the horizontal and vertical planes. Reducing alignment conditions to offset and angularity, independently in the horizontal and vertical planes, simplifies manual calculation of required "correction moves." Automated techniques for calculating corrections also use this convention. (Vertical pumps, solid couplings and hollow-shaft motors present unique concerns and require special procedures not discussed here.)
Alignment (or misalignment) is measured at the coupling-the point of power transmission-not at the feet. The amount of shims to be added or removed beneath the feet does not directly indicate the alignment condition at the coupling.
Figure 1. Alignment tolerances in relation to operating speed.
Alignment tolerances specify how close the pump and driver shaft centerlines should be to collinear at running conditions. Offset tolerances are measured in thousandths of an inch (or mils), centerline-to-centerline at the coupling. Angularity tolerances are expressed as pitch or slope (mils/inch).
Alignment tolerances for pumps range from the "rough alignment" that a conscientious technician can accomplish with visual indicators (accuracy of about 0.02 in) to precision alignment (accuracy of 0.0005 in or greater). The latter requires an experienced technician and accurate instruments (e.g., dial indicators or a laser alignment system). Accuracy of about 0.005 in can be accomplished with a simple straightedge and feeler gauge.
The degree of precision required for a specific pump/driver will depend on the pump's rotating speed, the distance between the pump and driver shafts (spool-piece length) and the application's thermal characteristics. The required precision increases exponentially with operating speed; proportionally less precision is necessary with longer coupling spool pieces. For applications where temperature changes occur during operation, evaluation of thermal effects is also needed to determine target values.