Shaft Alignment / Predictive Maintenance

Your checklist for precision collinear alignment

These rotating shaft coupling alignment basics will help you keep everything straight.

By Richard R. Knotek, Motion Industries

You’ve heard the adage “look before you leap.” Do you also look before you align? When conducting rotating shaft centerline alignment, it’s imperative that you perform proper preliminary steps. Even though laser alignment systems are taking the place of traditional alignment practices, it is vital to adhere to best practices before and during the alignment process.

I often hear maintenance mechanics and millwrights use the phrase “coupling alignment,” but this is actually a misnomer. “Rotating equipment shaft centerline alignment” is a mouthful, but it’s a more-precise description. The centerlines of the two pieces of rotating equipment must be made collinear.

Laser systems alone do not magically ensure that machine centerlines will be collinear. The technician still has to make the moves. Think of old-school compass orienteering – someone has to do the legwork. Even with modern GPS, which tells you where you are and how to get where you want to go, you still have to take action to get there. And along the way, some hills and valleys need to be considered.

It is also important to understand that flexible shaft couplings allow for some misalignment, depending on the type, but they have limits, as well. Moment loads and forces don’t disappear; rather, they show up in the form of heat or result in distortion in the machine’s components.

Why perform precision alignment? Simply, for this reason: We want to ensure a long service life of both connected machines and reduce downtime.

What are the results of precision collinear alignment?

  • Reduced power consumption
  • Lowered bearing and operating temperatures
  • Extended bearing life
  • Lessened shaft fatigue
  • Decreased coupling wear
  • Minimized vibration
  • Prevention of seal failure
  • Assurance of proper rotor and impeller clearances
  • More-precise manufacturing

Here, then, is your collinear alignment checklist to help you achieve precision alignment:

  1. Perform lockout/tagout and verification procedures as dictated by your employer. Remove the coupling guard.
  2. Determine how precisely this machine needs to be aligned. Is it a critical A-class or D-class machine? Stay within the recommended limits of the shaft coupling. Speed, size, and machine criticality determine the alignment tolerances. Strive for perfection!
  3. Organize and have on hand all of the necessary tools before you start the alignment procedure.
  4. Inspect the machine’s feet/mounting pads along with the mounting base – no burrs, corrosion, thread eruption, dirt, etc., allowed. Use abrasive paper, fine-tooth files, and a sharpening stone to make the mounting surfaces as smooth as possible. Clean them with light machine oil.
  5. Use new fasteners and a torque wrench. Follow the proper torqueing sequence, compensating for given values if wetted. This will help prevent the twisting and distorting of the machine housing. Use hardened steel washers.
  6. Eliminate any potential piping strains that will distort machine cases when reconnected after procedure.
  7. Check bearings by rotating the shafts by hand if possible. If you can lift the shaft by hand or if it doesn’t rotate smoothly, the bearings need to be replaced.
  8. Clean and inspect the shafts for rises and swaging. Use a dial indicator to check for run-out of less than .001”/inch of shaft diameter. The same goes for the coupling hubs. Bent or damaged shafts and coupling hubs prevent precision alignment. Concentricity is important.
  9. Keep records of the process and incremental moves, even if you do so just on paper. This will help you visualize what’s going on. Most laser systems allow for this and will save under a file name.
  10. Set the coupling hubs gap between faces within 10% of the manufacturer’s recommendations. If necessary to overhang hubs, the shaft must penetrate into the hub at least the diameter of shaft.
  11. Center the machines to prevent becoming bolt-bound. Scribe or mark feet positions.
  12. Check for all types and shapes of soft foot, which is an air gap between the base and the machine mounting foot. Be it angled, parallel, squishy, or a mounting base issue, it needs to be found, measured, and corrected. Start with the worst first. Use a feeler gauge, precut shim, dial indicator, or even the laser system to map the amount of soft foot. Two-thirds foot contact is a good rule of thumb.
  13. Use stainless-steel precut shims. Stay away from plastics if at all possible. Do not have a stack of shims that looks like a head of lettuce. Stack them neatly and minimize the number of individual shims in the stack. Cut shaped shims if needed.
  14. Compensate for and calculate any potential thermal growth, both vertical and horizontal.
  15. Initially rough-align the machines using a straight edge in both the vertical and horizontal planes. Get them as close as possible visually.
  16. Determine which machine will be the movable and adjusted machine – usually the motor. Make gradual, incremental machine moves, adjusting shim packs one pad at a time. Tighten and loosen hold-down bolts consistently with a torque wrench.
  17. Establish a reference point. Belly to the back of the motor is recommended.
  18. Remember: Accurate measurements cost minimal time but are essential to the process.
  19. With laser systems, follow the procedure as dictated by the program. Enter the correct data and measurements. Mount the head brackets on the shaft if possible. Do not grab the laser heads while rotating. Make clean and multiple sweeps, checking for consistency. Make the system-recommended moves. Use the “live” mode to adjust horizontally.
  20. If using traditional alignment methods, set the recommended gap between the coupling faces with a taper gauge, feeler gauge, or calipers, at the 12:00, 3:00, 6:00, and 9:00 positions. Make sure those measurements are within recommended limits.
  21. Correct the vertical angularity first by raising or lowering the front or back feet. Then adjust the vertical offset/parallelism by raising or lowering all feet of the movable machine.
  22. When using dial indicators, make sure the heads and connections are tight. Compensate for “rod-sag” of the indicator set-up when spanning across more than four to five inches. Verify you are getting accurate readings by using the Validity Rule: Top-to-bottom must equal side-to-side. Read positive vs. negative numbers accurately.
  23. When moving horizontally, set up indicators to measure the moves. Use pusher/jacking bolts to make horizontal moves.
  24. Reinstall the guard.
  25. Start the machine and monitor its condition using temperature, vibration, and amp draw.

The race isn’t always to the swift, but to those who keep on going. In a similar way, the alignment victory belongs to those who are precise.