Alignment and misalignment are defined by visualizing the shaft centerlines of rotation of two connected machineries as two straight lines in space. Alignment aims to get them to coincide, as close as possible, so as to form one straight line. If they do not, then there exists offset misalignment, angular misalignment, or a combination of the two.
Alignment of machineries demands great care and consideration. For many facilities, alignment remains a difficult trial-and-error task that consumes considerable time and resources. Misalignment and its associated effects, including vibration and equipment damage, are critical issues for any machinery and high-speed or large machineries in particular. All parties, then – machinery manufacturers, contractors, operators, and maintenance teams – need to be aware of alignment’s criticality in ensuring the successful start-up, operation, and long-term reliability of machines. We’ll focus our attention here on alignment for machineries and rotating equipment as well as the consequences of misalignment when it comes to machinery safety and reliability.
Alignment in machineries
The ultimate goals of shaft alignment are to increase the operating life and reliability of rotating machinery and to achieve high efficiency. When two shafts require alignment, the process usually calls for one machinery shaft to be permanently mounted and the other one to be movable. The fixed machinery is usually the driven equipment. The second machinery element – for instance, the electric motor driver – is moved into approximate alignment in preparation for measurements that will determine the magnitude and direction of moves required to put it in final alignment with the fixed shaft. It is the movable machinery whose shaft will be aligned with the shaft of the fixed machinery.
The position of the movable machinery is adjusted vertically by adding or removing shims from under the machinery’s feet and horizontally by making small lateral moves as required until satisfactory final alignment is obtained. Therefore, the process depends on trial and error. Sometimes, many attempts are needed to achieve the desired result.
Dial indicators and lasers are two good choices in measuring systems. Dial indicators provide accurate and reliable measurement of shaft alignment. They are useful because they can be used to measure bearing alignment, shaft run-out, and soft foot directly. As an indication, measurement accuracy down to 0.01 mm or 0.02 mm (10 or 20 microns) may be achieved if care is taken in mounting and reading the indicators correctly and controlling or accounting for such variables as indicator sag, axial end play in the shaft, and vibration from outside sources.
Many alignments have been performed by the trial-and-error method. Although this method may eventually produce respectable results, it is extremely time-consuming. Some simple trigonometric principles will take the guesswork out of the process and allow alignment to be performed properly with the estimated required corrections measured or calculated. These accurate measurements and calculations will make it possible to align a piece of machinery on two or three attempts. The data obtained from properly installed dial indicators are converted by proper equations into the vertical and horizontal movements (or X-Y movements) required to bring the movable machinery into alignment with the fixed machinery.
Laser measurement systems are another popular choice for shaft alignment work, although the cost of such systems is much greater than that of dial indicators. Accuracy of 0.002 mm (2 microns) or even better is possible, and setup and operation is generally faster and simpler than with dial indicators. Many laser systems can perform some or all of the calculations required to obtain the horizontal and vertical movements (or X-Y movements).
Practical notes on alignment
Soft foot is the condition when all four of machinery’s feet do not support the weight of the machinery. Before starting the alignment procedure, any soft foot must be detected and corrected. Dial indicator readings taken as part of the alignment procedure can be different each time the hold-down nuts are tightened, loosened, or retightened. This can be extremely frustrating because each attempted correction can cause a soft-foot condition in another location. If the nuts securing the feet to the base loosen, machinery looseness or misalignment can result. Either of these conditions can cause high vibration and damages. Therefore, correction of soft foot must be the first priority.