Achieving Precise Alignment
EP Editorial Staff | February 1, 2004
Without proper preparation, standards and tolerances, and precision procedures, machines may have significant residual misalignment even though the alignment computer is reading zeros.
When asked which alignment methods are employed at a plant, the staff will often proudly display the latest laser alignment technology tool. When used properly, laser tools can provide accurate alignments for a variety of machine configurations.
Based on observations in the field, however, it is apparent that many companies are actually misaligning their machinery precisely with these tools. The consistent and precision approach outlined below will maximize the effectiveness of any alignment tool—whether dial or laser—and help achieve the best possible alignment.
Before the first wrench is turned, gather all of the information required for the alignment job.
- Identify the alignment specifications for the shaft-to-shaft alignment and the base. Refer to the machine manufacturer’s documentation as well as historical records. In the absence of specifications, 0.001 in. (1 mil) offset at the coupling with no more than 0.002 in. (2 mils) offset at any foot plane will provide a precision alignment. Be aware that some high-speed or high-precision machinery may actually require a tighter specification. Do not rely on the coupling tolerances or rules of thumb based on shaft speed and shaft diameter, as these will often provide insufficient precision.
- Determine the thermal growth offsets for the machine. If well-documented thermal growth values are not available, a thermal growth study using any of a variety of methods may be necessary. Most thermal growth values provided by manufacturers are recommended starting points, and the actual growth must be determined from the installed condition. Offsets based on horsepower, for example, are not accurate and may actually worsen the condition.
- Verify the as-left readings from the previous alignment records. A significant change from the as-left readings and the current or as-found readings may indicate a base problem such as corrosion, distortion, cracking, or loose or damaged anchor bolts.
- Check bolt torque requirements.
- Review the work order for completeness and reconcile any questions or problems before continuing with the job.
Assemble tools and supplies
No matter how well calibrated the technician’s forearm is, it is no replacement for a proper torque wrench. When bolts are torqued unevenly, the machine will move unpredictably. Use the right tools for the job, which include properly sized and safe wrenches, crow’s feet, sockets, torque wrenches, etc.
If the shafts are difficult to turn manually, use an approved and appropriate turning tool, such as a strap wrench, to ensure that pipe wrenches or other tools that could damage the shaft or coupling components are not used. Check out a shim kit with the properly sized shims for the feet and the hold-down bolts.
Select the alignment system to be used for the job. Reverse dial and laser systems provide accuracy to within the thinnest shim that can be used, which is typically 0.001 in. (1 mil). Some machines may require rim-and-face fixtures if one of the shafts cannot be turned depending on the capabilities and available fixtures for the laser system the company may have.
Inspect the machine
Whether realigning an existing machine or setting up new equipment, perform a thorough inspection of the machine and base before starting the job. Look for any degradation of the base due to corrosion or concrete damage. Check for cracks in bases or frames and check any anchor bolts.
If the machine is still operating, hand-feel all joints and interfaces to determine the presence of relative motion, which may indicate looseness or other problems. Check the condition of all shims on the driver and driven machines. If any of the machine components are bolted directly to the frame or base without shims, suspect soft foot. If spacer plates are used, ensure that they are installed correctly and that they are not damaged, bent, or corroded.
Verify that the hold-down bolts are correct for the application and that they are not damaged. Hardened washers should be present under all bolts. If the washers are cupped or damaged, they should be replaced. Check for horizontal and vertical jacking screws and verify that they are free to move and that they are not providing any binding force. If jacking screws are not present, fabricate new ones, as this will speed the alignment process and ease machine movement.
On existing machinery, collect a set of as-found readings when the machine has reached ambient conditions, and compare with previous results. If there is a significant discrepancy, re-evaluate the base, foundation, piping, etc., to determine the cause of the movement.
A difference may be due to errors with the previous alignment or because offsets were used during the alignment that were not recorded. Tactfully review the previous alignment with the lead technician if available. Ask about the alignment method that was used and if there were any difficulties encountered while aligning the machine the last time.
Perform a step-by-step alignment
Every plant has slightly different work procedure rules based on safety requirements, machine types, and the labor structure. The fundamental alignment procedures necessary for consistent precision are easily adapted to any work environment. There are many benefits when using a procedure, but the most important are that it assures that every machine will be aligned with the same attention to detail and it helps to overcome many of the bad habits that may have become engrained with the workforce.
The largest portion of an alignment typically involves preparing the machine for the actual alignment moves. When the machine is on solid footing with minimal strain from attachments and the shafts and bearings are within specifications, it is much easier to move a machine consistently and quickly into the desired position.
The following provides an overview of the typical steps required prior to actual alignment. Although these steps appear to be quite basic, they are often skipped in the interest of time or due to lack of precision training and focus.
• Properly secure the machine using your company’s lockout/tagout procedures. Divert flow to fans or pumps that could cause a shaft to rotate unexpectedly.
• Eliminate soft foot in the driver and driven machines. Soft foot will have an adverse affect on the machine components due to strain and distortion. During the final alignment, soft foot often creates inconsistent readings from move to move. Generally, the maximum gap under any foot of the machine should be no more than 0.001 in. (1 mil).
Be aware that a laser system cannot determine the actual gap at a foot—it only measures the effect of the soft foot at the shaft. Always measure the gap with feeler gauges and make the appropriate parallel and angle corrections.
Before making the soft foot corrections, it is advisable to place the driver and driven machines at the center or midpoint of their horizontal movement limits. This can save time later and may avoid the necessity of making bolt-bound corrections.
• Minimize pipe, duct, and conduit strain on all of the machine components in the machine train. Measure shaft movement horizontally and vertically at the coupling as each flange is attached. Movement greater than 0.002 in. (2 mils) indicates corrective action is necessary. Some mechanical seals may have requirements that are more stringent. Refer to the seal manufacturer’s specifications for the recommended values.
• Inspect all shafts and bearings to ensure that axial and radial runout and play are within tolerance. Before rotating any shaft, however, be sure that the bearings are properly lubricated. A dry rolling element bearing can be damaged simply by rotating the shaft during an alignment. Circulating oil systems may need to be energized to provide an oil film.
A machine with a severely bent shaft can be aligned with a laser or reverse dial system because the rotation occurs at the centerline of the shaft’s two bearings. The effective shaft centerlines can be aligned, but the actual offset at the coupling may exceed its capabilities, and it is likely that the machine also will exhibit evidence of unbalance.
• Inspect the coupling and all components. Missing components or incorrect key length can create unbalance. Worn parts on gears, spiders, grids, etc., can cause the coupling to lock axially and may produce thrust-related bearing problems. If the coupling is lubricated, remove the covers and hand-pack the coupling with the correct type and quantity of grease.
• Inspect all bolts and shims to ensure that the proper types are being used and that they are not damaged. Replace any suspect parts. Ensure that hardened washers are used under all hold-down bolts or nuts. Plates may have to be fabricated to provide a smooth bolting surface so that proper clamping force can be obtained and to minimize horizontal shifting of the machine when the bolts are tightened. Make sure that the hardware is not bottoming out and that there is no binding.
If possible, limit the number of shims under each foot to three. This avoids uneven shim compression and a soft foot–like condition and makes the machine movements more consistent. Make sure that all shims are of high quality precut stainless and are sized for the bolts and for the machine footprint.
• Verify soft foot condition one last time before beginning the alignment.
The order in which these steps are performed will vary from plant to plant. For example, some may prefer to inspect the shafts first. If the shafts are not within specifications, the alignment should stop until the shaft has been repaired or the machine replaced.
At this point, the final alignment can be performed. What may seem to be an inordinate amount of preparation time actually speeds the entire alignment process because it insures that the machine will move predictably in the horizontal and vertical planes. When problems are encountered, check for the following conditions:
• Base- or bolt-bound component
• Pipe strain
• Soft foot
• Improper shims
• Failure to use a torque wrench and tightening sequence
• Lack of vertical and horizontal jacking screws
• Bent shaft, excessive bearing clearance, inconsistent oiling of plain bearings
• Locked coupling or uncorrected coupling backlash, shaft end-float
• Setup and/or interpretation of alignment system results. For example, bar sag or parallax errors, laser interference, loose or slipping fixtures, incorrect dimensions, etc.
Standard practice should include a reading repeatability check. If the readings do not repeat, a setup problem must be addressed. When in doubt, go back to the preliminary steps and then take a new set of readings.
In some cases, a visual inspection will reveal a misalignment condition—even when the dials or laser read zeros when the job had been completed. Improper shimming, cupped washers, etc., all reveal an inadequate alignment and the likely presence of soft foot, case strain, and even altered dynamic characteristics.
Figure 1 shows that plates were installed under the motor feet on both sides to raise the motor’s shaft centerline up to the level of the lube oil pump. Notice that the plate does not match the footprint of the motor, which could compromise the stiffness or rigidity of the motor. It may have simply been installed backwards or upside down. Notice also that the plate is made of what appears to be mild steel. The uneven surface of the plate due to corrosion and apparent field flattening will make a precision vertical alignment of this machine difficult.
Figure 2 shows the typical condition of the hold-down bolts and washers found on the motor. Notice that the soft washer has been ground to accommodate the radius of the foot but that the washer was installed backwards. The cupped condition of the washer would make this machine very difficult to align horizontally as it would tend to locate around the washer.
The shims shown in Fig. 3 under one of the lube oil pump’s feet are grossly oversized. The slot in the shim is nearly as large as the foot. Tightening the hold-down bolt will actually pull the foot down into the shim slot, which will compromise the stiffness of the foot and make the vertical misalignment change dramatically depending on the bolt torque.
Laser alignment of this machine alone will not correct its misalignment problems. The precision-minded technician must identify the corrections necessary to bring this machine back on-line at 100 percent (or better) of its design condition.
A quality and precision alignment is possible on virtually any basic horizontally mounted machine when a step-by-step approach is followed. When the integrity of all machine components is verified and the machine is resting on a solid foundation, the alignment moves will be predictable and the final results will be confirmed with a smooth, long-running machine.
None of these steps can correct inadequate time to do the job correctly or a “close enough, let’s start it up” attitude. These are management and reliability issues that must be in place before a maintenance staff can realistically begin performing true precision alignments. MT
Gary Patrick is supervisor, proactive reliability maintenance skills and training, at SKF Reliability Maintenance Institute, Norristown, PA ZIP; (303) 979-0506
Misalignment conditions are not always corrected by laser alignment. Visual inspections can identify machine installation problems (Figs. 1 and 2) and incorrect shimming (Fig. 3) that need to be addressed before proper alignment can be achieved. (Photographs courtesy of SKF Reliability Maintenance Institute.)