By Alan Luedeking, LUDECA Inc.
Proper shaft alignment of directly coupled machines has proven to be one of the greatest money savers for machinery-intensive industries over the last 25 years. When poorly done, usually for lack of time, accurate measurement tools, or because of improper procedures, a bad alignment on a single motor-pump unit can cost a plant thousands of dollars in unnecessary downtime, failed seals, premature bearing wear, increased vibration, reduced efficiency and higher power consumption. Often it is a hidden ailment, only noticed with surprise when a coupling, bearing or seal fails unexpectedly.
When two centerlines of rotation are not aligned correctly and are forced to spin together, the machines’ shafts undergo a destructive stress reversal of double the magnitude of the misalignment at twice the speed of rotation. At 1,800 rpm this will happen 5,184,000 times a day. The flexing of the shafts and of the coupling’s flexible elements produce an excitation force that travels through the shafts and into the bearings, rotors and seals. This vibration can destroy them in far less time than they are designed to last. It costs the machine power to do thisthe improvement of a precision alignment over a straightedge rough alignment on a larger compressor train in continuous service could cut power consumption enough to shave thousands of dollars off the electric bill per year. (For instance, a 30 KWh reduction at $0.07 per KWh saves over $18,000 in one year.)
What Is Misalignment?
Offset is the radial separation of the actual or projected centerlines of rotation of two coupled shafts, at any location of interest along their lengths. The magnitude of this offset should be kept within tolerance at the location of the coupling flex planes. (See Figure 1.)
Angular misalignment (angularity) is the slope of one shaft centerline relative to the other. It is usually described as a rise over a run, in mils per inch. It is the amount by which the offset changes for every inch you travel along their lengths. Often the angularity is described as the gap difference between coupling faces (in mils) occurring across the diameter of the coupling (in inches.)
Usually, offset at the coupling should be kept to within 2 mils and angularity to within 0.3 mils per inch at 1,800 RPM for “short couplings”. At 3.600 RPM, 1 mil of offset and 0.2 mils per inch of angularity is standard. For spacer couplings, or where the flex planes are further apart axially than they are big radially, allow 0.6 mils per inch of span length at 1,800 RPM, or 0.3 mils per inch at 3,600 RPM. These values are commensurately tighter at higher RPM’s and looser at lower speeds. For critical machines these tolerance values should be applied as absolute vector values rather than as independent values for the horizontal and vertical planes of alignment. (See Tables 1 and 2 for recommended alignment tolerances.)
Also contributing to alignment problems are “soft foot” conditions (machine frame distortions) which occur when tightening (or loosening) the anchor bolts over uneven mounting surfaces, flimsy bases or the presence of pipe stress. An important part of the overall alignment job consists of detecting, measuring and correcting any soft foot distortions on all machines in the train prior to actually performing the final shaft alignment.
How to Measure Alignment
The traditional method of laying a straightedge across the coupling hubs and feeler-gauging the coupling faces increasingly is inadequate at today’s speeds of rotation. At best, it is a useful procedure only to get you in the ballpark (rough-aligned) or for very slow speed machines (less than 600 RPM.) The problem with the straightedge method is that any out-of-roundness of the hubs or shafts, a misbored coupling or an imperfectly seated coupling makes these reference locations different from the positions of the centerlines of rotation you are trying to measure. Therefore, it is important that both shafts be rotated with accurate measuring tools (dial indicators or laser alignment system) affixed to the shafts or the solid coupling hubs. Then, even if you have shaft or hub out-of-roundness, or surface imperfections, the fixtures mounted upon them will describe a circle as you rotate the shafts, allowing true measurement of the misalignment of the centerlines of rotation.
One of the best methodologies today involves using a good laser alignment system. The best such systems shoot a single visible laser beam across the coupling directly into a five-axis position detecting sensor or into a roof prism which reflects the beam back into a position detector. As the shafts are rotated, the components change their position relative to each other, causing the beam to move across a laser position sensing target. The transducer then feeds this information to a dedicated hand-held computer which precisely calculates the offset and angularity between the shafts. It also calculates the vertical shimming and horizontal move correction values for either machine.
How to Buy a Laser Alignment System
If you intend to buy a laser alignment system and you are unsure of which system to buy, get a demonstration from the vendor. Let the salesman point out his system’s features and make sure you understand them. If the salesman makes any negative comments about a competitor’s product, make sure you ascertain the veracity of these comments by getting a demonstration of that product as well.
Figure 2: Rotalign® Ultra
Ideally, the laser system you choose should have, at a minimum, the following features:
- Ability to measure with a resolution of 1 micron (40 millionths of an inch).
- Ability to take accurate readings with only 70 degrees of shaft rotation, starting anywhere and stopping anywhere.
- Ability to measure and display soft foot for all feet of both machines, or all machines in the machine train. Look for a system that offers a soft foot “wizard” functionality to assist in diagnosing and correcting soft foot problems.
- Ability to allow for expansion of your measuring range with a readjustment feature while taking readings or monitoring a moveextremely important for longer spans or when bad misalignment is present. What do you do when your dial indicator runs out of stem or bottoms out? You reposition it and reset it to zero for more range. It is essential that your laser system have this capability also, and automatically incorporate these readjustments into the calculation of all results.
- Ability to offer a “spacer shaft function”, so that you can see alignment results in terms of projected offset-offset or angle-angle at both flex planes of the coupling. This capability aids in achieving the alignment quickly and prevents you from wasting time by aligning machines better than you have to by attempting to meet short coupling tolerances at the midpoint between the flex planes.
- Ability to monitor your horizontal and vertical moves live as you perform them. This eliminates the need for attaching indicators against the feet, which usually get in the way.
- Abiliy to allow input target specifications for thermal growth at the coupling. This feature then automatically recalculates shim and move values to allow deliberate misalignment to compensate for thermal and dynamic position changes in the machine centerlines after startup.
- Abiliy to allow input of thermal growth values for the feet of the machines, if target specs at the coupling are unknown. As in the above point, the system then compensates for the growth that will occur.
- Ability to take readings in any order, coupled or uncoupled, and even with the shafts in continuous slow rotation (clockwise or counter-clockwise) essential for heavy machines whose momentum makes it difficult to stop the shafts exactly at the required positions. The uncoupled alignment feature is especially important: Not always will your coupling already be made up when you are aligning the machines, and the shafts may have to be rotated separately, or worse, you may have one shaft that can’t be turned while the other can.
- Ability to offer a “machine train function” that lets you see the alignment of three or more machines in a train graphically.
Figure 3: Optalign® Smart
There are several systems on the market, some more expensive than others, but only few offer the accuracy and all the combination of features which make laser alignment a worthwhile tool for the maintenance department.
Considering the costs of not doing proper shaft alignment, a good alignment system will save a company tens or hundreds of thousands of dollars a year. As alignment improves, so does machine performance, repair expense and unscheduled downtime, all of which in turn frees up resources and manpower for productive utilization in other areas.
Alan Luedeking is the manager of Alignment Tech Support and Training for LUDECA Inc.