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.
The amount of dial indicator sag also should be determined and considered before starting the alignment procedure. Indicator sag is the term used to describe the bending of the dial indicator’s mounting hardware. Bending can cause some errors in the indicator readings that are used to determine different misalignment elements, especially in rim-and-face readings. The degree to which the mounting hardware bends depends on the length and material strength of the hardware.
Misalignment and limits
There are a number of ways to describe misalignment at the connection or coupling and to define permissible misalignment tolerances. There also are different limits for shaft-to-shaft alignment of coupled shafts using couplings. Offset (parallel) and angularity misalignment tolerances are usually specified for this purpose, although in some simplified cases just one combined value of misalignment is mentioned as an approximation. In other words, misalignment limits are usually defined in terms of two measures of misalignment: angularity and offset. Sometimes, a very rough estimation of combined offset and gap difference (related to angular) is used.
Offset (parallel) misalignment is the distance between the shaft centers of rotation measured at the plane of shaft or coupling center. The offset tolerance describes the maximum separation that can exist between two shafts. The angular misalignment is the difference in the slope of one shaft as compared with the slope of the other shaft. The angularity may be described either directly, as an angle, or as a gap difference at a particular coupling size. The latter method is popular because it relates directly to site measurements.
Modern alignment tools have also used the mentioned offset and angularity misalignment system. For instance, a modern, laser-based shaft alignment system measures the angle between shaft centerlines; such a system can also be set to describe this angle as a gap difference set at any desired size. Proper shaft alignment is especially critical when shafts are running at high speeds; the allowable limits of misalignment decrease as shaft speeds increase.
It is not easy to detect misalignment when machineries are running. In such a situation, the vibration signature (in terms of specific elements of radial and axial vibration) might be used for the detection of misalignment, but this is often difficult. Over long times, secondary effects of misalignment can be observed: high temperatures in bearings, seals, or lubricant; damaged bearings; loose, broken, or missing coupling bolts or foundation bolts; cracks in shafts; and excessive amounts of lubricant leakage.
Periodic alignment checks
Periodic alignment checks on all coupled machinery are considered one of the best tools in a maintenance program. Such checks are important because the vibration and reliability effects of misalignment can seriously damage a piece of machinery. As a rough indication, misalignment of more than 40 microns can cause vibration and will significantly reduce equipment life.
Indications of misalignment in rotating machinery are shaft wobbling, excessive vibration (in radial and axial directions), excessive bearing temperature (even if adequate lubrication is present), noise, bearing wear, and coupling wear. Misalignment can be detrimental in particular to rolling-element bearings. The misalignment can introduce considerable changes into the contact load and the pressure between rolling elements and raceways of such bearings, and thus it can greatly reduce rolling-element bearing life.
Vibrational signature of misalignment
It is not easy to detect or trace vibrational signature characteristics of misalignment. Different experts and authors cite different signatures as important ones for misalignment. The second harmonic (known as “2× harmonics”) and axial vibration (vibration in the direction of shaft axis) are main indicators of misalignment. The vibration at axial direction is an effective tool to detect misalignment. Baseline vibration data, measured during a verified alignment situation, play a major role in the detection of the misalignment. In addition to the above-mentioned indications, as a general rule, increased misalignment has yielded increased vibration peaks and stronger harmonics.
In many cases and for many machineries, the observed changes in the vibration that occurred with shifts of misalignment do not show a typical vibration signature (vibration spectra) as predicted by simplified formulations or theories. Misalignment has complex, nonlinear dynamic effects that need great care for modeling, observations and detections. The rules provided in theoretical textbooks or courses are not usually effective, and sometimes these formulations and theories are far from reality of machineries. Different experiences and site observations have shown that vibration signature of misalignment is a strong function of machinery speed and coupling stiffness. A single-point vibration spectrum does not provide good and reliable indication of machinery misalignment. Observations of spectra in axial and radial directions at various speeds and several points are needed to diagnose misalignment effects. Orbital plots of measurements in two planes (such as vertical and horizontal or X-Y planes) in the time domain might also be needed.
This is usually difficult to define a general misalignment limit, because there are vast differences in machinery operation, speed, type of coupling, etc. Also, different manufacturers, codes and specifications have offered different limits and criteria. However, a thorough vibration measurement exercise can be very helpful. An alignment is not considered good enough until it is well within all relevant tolerances and a vibration analysis of the machinery in operation shows the vibration effects caused by misalignment are acceptable.
Flexible couplings and alignment
Most couplings can provide certain amount of flexibility to absorb a tiny amount of misalignment. A coupling should be capable of transmitting the torques under all conditions of angular and offset (parallel) misalignments, speed and temperature, simultaneously, to which it is subjected in service.
Maximum misalignments, containing offset and angular misalignments, for each service should be properly specified and requested. There is a set of misalignment values that a coupling in each service is expected to experience; this should allow for all the known effects on the machinery from thermal pressure, dynamic forces and others. As a very rough indication, angular misalignment capability across each typical flexible coupling is around 0.2 degrees.
On the other hand, it is important to understand that flexible couplings do not cure misalignment problems. Although they may somewhat absorb and dampen the effects of misalignment, flexible couplings are not a total solution. Misalignments induce stresses in a coupling. The fatigue safety factor at different combinations of speed and misalignments should be determined and used for each coupling, given that higher values of misalignment, even if within allowable range, would reduce the life of the coupling.
If all couplings were perfectly bored through their exact center and perfectly machined about their rim and face, it might be possible to align a piece of machinery simply by aligning the two coupling halves. However, coupling eccentricity often results in coupling misalignment. This does not mean, however, that dial indicators should not be placed on the coupling halves to obtain alignment measurements. It does mean that the two shafts should be rotated simultaneously when obtaining readings, which makes the couplings an extension of the shaft centerlines, whose irregularities will not affect the readings.
Although alignment operations are performed on coupling surfaces because they are convenient to use, if there is any runout (for instance, axial or radial looseness) of the shaft or the coupling, a proportionate error in alignment will result. Therefore, before making alignment measurements, the shaft and coupling should be checked and corrected for runout.