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.
Another important factor is the coupling type. Industrial users generally agree that non-segmented elastomer boot couplings produce less damaging vibration than jaw or gear couplings, given equal amounts of misalignment. Other kinds of couplings fall between these extremes.
Figure 2. Offset and angular tolerances in relation to operating speed.
Rough Alignment: When installing the pump and driver, experienced technicians will perform a "rough alignment" based on visual indicators. They also will ensure that all machine feet are in good foot-plane and have 0.025 in to 0.050 in shims under them. Good foot-plane must be established and maintained throughout the alignment procedure to avoid stressing the machine cases.
Target Values: Pumps and drivers are moving targets due to torque strains and thermal effects, so evaluation of these factors is an important step in pump shaft alignment. Just as a marksman anticipates the location of a moving target, proper alignment procedures must predict the relative running position of the machine cases (i.e., differences between cold and running alignment positions). These target values may be determined for the relative positions at the coupling or at the feet.
Figure 3. Sample alignment target values.
Measurement: Alignment tolerances and misalignment are measured at the coupling, where the power is transmitted. The simplest way to measure these parameters is with a straightedge and feeler gauges. A taper gauge or caliper can also be used to measure angularity between the coupling faces. These methods can achieve accuracy of about 0.005 in, which is acceptable for many pumps that operate at 1,200 rpm or less.
If precision alignment is required, careful use of dial indicator methods (e.g., rim-and-face and reverse-dial), including compensation for bracket sag, can achieve accuracy of 0.0005 in. This will suffice for most pumps that run at 5,000 rpm or slower.
Laser alignment systems accomplish the task quicker and with more accuracy, eliminating math errors and other common mistakes. Most of these systems also provide graphics that show the direction of the "correction move."
Regardless of the alignment method, correction moves must be determined from measured misalignment data-whether the calculations are done manually, or automatically with a calculator, computer program or laser alignment system. Attempts to "guesstimate" correction moves often waste time and cause frustration.
Foot-plane: To maintain good foot-plane during the alignment procedure, both front feet should be raised or lowered the same amount, and similarly, both rear feet should be moved in equal amounts. Shim adjustments that would tilt the motor side to side should be avoided.
If a machine is bolt-bound or base-bound and cannot be adjusted sufficiently, it will be necessary to move the opposite (fixed) machine case. Most calculator and laser systems provide the means to recalculate for base- and bolt-bound conditions.
Documentation: It is essential to record pre-alignment data, target values, tolerances and final aligned condition. This information can help maintenance personnel determine when to perform maintenance tasks and spot developing problems that may otherwise result in unexpected, costly failures and downtime. The documentation features included with many alignment calculators and laser systems may not be adequate for recording data about foundations, machine feet, shims, coupling components and observations. If so, a paper or an electronic work order system should be used to capture this information (see an example data form below).
Assessing Alignment Needs
Pump and alignment tool manufacturers offer "suggested" alignment tolerances, most of which do not consider the coupling type. But application-specific variables make a single tolerance for pumps unrealistic. The best approach is to evaluate each installation based on operating speed, thermal movement, spool-piece length and coupling type.
For a large 1,200 rpm pump with an elastomer-in-shear coupling, alignment with a straight edge and feeler gauge to precision of 0.005 in would suffice. A medium-size 3,600 rpm hot water circulation pump with a jaw coupling, however, would require precision alignment with dial indicators or a laser system. A high-temperature (400 deg F) refinery pump may have a spool-piece coupling to accommodate thermal movement, and would require an operational assessment of target values.
While target value assessment and precision alignment techniques could be used for any of these pumps, the required time and expense would outweigh the benefits for the 1,200 rpm pump. Thermal growth analysis also would be unwarranted on the hot water circulation pump, because the driver and pump will have similar thermal growth. Common sense dictates that evaluation of the alignment needs of individual pumps-including coupling type, thermal changes, rotating speed and spool piece length-will ensure use of the most cost-effective shaft alignment procedure.
Successful pump alignment requires careful planning and execution, beginning with evaluation of tolerances and target values based on pump speed, thermal characteristics, coupling type and spacing. The technician must also be adequately trained and systematically document the entire procedure, including the original misalignment, the final alignment condition, and any observations related to machine reliability. The expense or sophistication of the alignment tools-whether laser, dial indicator or manual-will not produce the desired results if these essentials are ignored.