Shaft alignment is a technical skill that is not common in the construction and maintenance professions, but categorized more like a specialty. It requires unique and expensive measurement instruments, some calculation capability, and relies heavily on experience for successful results on heavy, high-speed, or high-temperature machines. At present there are no universally accepted standards that define good results. The U.S. Navy has some alignment specifications, as do some industrial companies. Unfortunately, the various specifications do not appear similar, nor even cover the same subject matter.
There is also no testing or certification of alignment craft people. With no common training, no certification, and no common standards, it should come as no surprise that there is large variability in the results.
Some requirements for the measurement system are specified. The most important requirement for any shaft alignment system is repeatability of the readings. This is evaluated with a 360 deg repeatability test. It is also a good way to evaluate a fixture system when considering a purchase. Basically, measuring systems that do not return to zero (within 0.002 inch) after a 360 deg rotation should be rejected. Be suspicious of plastic straps or other flexible fixture components.
The choice of measuring systems and methods is up to the aligner. The two fundamental choices are dial indicators or lasers. Dial-indicator systems are the most useful because they can be used to measure shaft runout, bearing alignment, and soft foot directly. All of the above measurements are required by the standard, and needed to assure a good-running machine, but not attainable with lasers. Lasers require batteries, are not intrinsically safe for use in explosive environments, and cannot do face-and-rim measurements.
The aligner is required to consider other factors that affect the running condition, besides just shaft alignment. These are coupling axial position, casing distortion, bearing alignment (if the bearings are disturbed), uneven bases, thermal growth, bent shafts, pipe strain, and bar sag. It is the responsibility of the aligner to determine if any of these are factors and to make the appropriate corrections.
Vibration should not be used as rejection criteria, but it could be used as an acceptance criteria. That is, many other mechanical defects can cause excessive vibration even with an excellent alignment (such as unbalance or resonance). So vibration should not be used as a symptom to fault the alignment. However, a smooth-running machine is evidence that the alignment is satisfactory, and it should be accepted.
All alignment jobs should be on a time-and-materials basis. Since the existing condition is unknown until the first readings are taken, the aligner does not know the extent of correction required. For this reason, it is inappropriate to require a fixed-price bid before the aligner has an opportunity to examine the machine. The range of contract service rates for alignment are $45 to $145/hour per person. Most alignment jobs are one-person tasks, or one alignment specialist with some helpers. MT
The purpose of this standard is to guarantee reliability of mechanical equipment when first placed into service and after major repair. It specifies the alignment condition of components to reduce vibration and minimize wear.
Reducing dynamics forces at mechanical joints is the objective of alignment, but vibration shall not be used as a judgment criterion for acceptable alignment. Other defects can cause vibration, including the foundation and other building parts. The craftsperson who performs the alignment uses static measurements when the machine is stopped, and the same static methods shall be used to judge acceptability.
This standard defines acceptable limits for shaft-to-shaft alignment of coupled machines. The limits are defined in terms of maximum offset and angularity. It also defines axial spacing for thrust conditions. Acceptable shim materials are defined. Safety procedures and how to move machines without introducing additional damage are covered.
The following complicating factors are discussed in terms of acceptable fixes: Uneven bases, resonances, thermal growth, bent shafts, bolt-bound conditions, piping strain, casing distortion, and bar sag.
The final alignment is done when the machine is in a ready-to-run condition. Additional hot alignment checks can also be done after some running in time. However, under no circumstances should the driver machine be energized before an alignment check is made. In other words, all coupled machine systems shall have the alignment checked and verified to be acceptable, prior to start-up.
This standard places no requirements on the types of instruments or the methods to achieve alignment. Rather, the final orientation is defined as an objective. The aligner is free to use whatever equipment is most suitable for the task at hand.
The measurement system needs to be repeatable to within 0.002 inch when exercised through one complete cycle. Repeatability is the significant characteristic that guarantees adherence to the specifications. The measurement system shall be checked for repeatability at the start of each alignment task after the system is fixtured in place on the machine. The machine shafts shall be rotated (a full 360 deg if possible) and the shaft orientation returned to the starting point. The measuring system shall read to within 0.002 inch of the initial reading. If it does not, the fixture is too flexible and must be rigidized. If 0.002-inch repeatability is not achievable, then the measurement system is not useable for alignment purposes.
All sources of energy to the machine system, that pose a hazard to the aligner, shall be de-energized. The controls shall be physically locked to prevent operation during the alignment process. Typical energy sources to be locked out are electrical controls, but could also be steam valves, or gas controls.
Shaft-to-shaft alignment is part of the total task of setting up machinery. The aligner is in a position to affect long-term reliability by detecting and correcting other factors. He/she is in position with measuring instruments, tools, and a window of opportunity to make some changes prior to start-up. It will be the responsibility of the aligner to recognize when these factors are active players and to properly respond. The proper response may be to correct it immediately or to advise the owner when correction is more than a routine alignment task.
Final alignment is normally done just prior to start up after all utility connections have been made, especially piping. Preliminary alignments can be done to roughly position machines, but a final alignment check should also be done after all movement or strain causing activity is done.
Newly-assembled piping shall have flanges that mate well without excessive force. Prior to bolting the flanges, an alignment inspector shall verify that the two flanges can be brought together into intimate contact and assembled with no more than 200-pounds force (an average adult male can arm push 200 pounds).
Resonant foundations or bases are dynamic structural defects. This will cause high vibration at specific speeds. Resonances are not detectable during static alignment measurements. They are only apparent during operation of the machine. The aligner is not responsible for detecting or correcting resonances.
Any change, thermal or mechanical, from cold-alignment conditions to hot-running conditions are the responsibility of the aligner to estimate and correct for. Thermal growth calculations shall be made for any temperature changes greater than 100 F.
Misaligned bearings have vibration symptoms identical to misaligned shafts. The damage also follows a similar pattern. If installing or moving bearings is part of the alignment task, then bearing alignment shall be checked and adjusted according to Appendix A.
Prior to de-energizing the machine and beginning the alignment task, the aligner shall verify that the proper tools are on hand to safely and efficiently move the machines. This includes lifting devices, wrenches, shims, and measuring instruments.
The shaft-to-shaft residual misalignment is acceptable when the intersection point of the two shafts is within the coupling area and the included angle between the shaft centerlines is small. These two criteria must be applied in two orthogonal directions, typically horizontal and vertical for convenience, and normalized to speed. That is, slow-speed machines are allowed a larger tolerance. High-speed machines are required to be better aligned.
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