9.3 Laser Shaft Alignment Systems
Key Takeaways
- A laser alignment system replaces mechanical indicators with a laser emitter and a position-sensitive detector, so there is no bracket sag to measure.
- Prealignment checks are still required: a laser system will not detect soft foot, pipe strain, or a bent shaft unless the mechanic checks for them.
- Laser systems support live move, showing the machine position updating in real time as it is shimmed and jacked.
- Measurement dimensions must be entered accurately, because the system projects the shaft centerlines out to the feet using those numbers.
- Thermal growth targets are entered as deliberate cold offsets so the system aligns the machine to be correct at operating temperature, not at rest.
How a laser system measures
A laser shaft alignment system mounts a laser emitter on one shaft and a position-sensitive detector on the other — or an emitter and detector on each shaft in a dual-beam system. As the shafts are rotated together, the beam strikes the detector at different points, and the system computes where each shaft centerline lies in space.
The output is what the mechanic actually needs: offset and angularity in both planes, plus the exact shim change required at the front and back feet of the movable machine.
| Dial indicator methods | Laser system | |
|---|---|---|
| Bracket sag | Must be measured and applied | None — nothing mechanical spans the gap |
| Reading error | Operator reads a dial | Electronic, repeatable |
| Calculation | Manual, graphical, or calculator | Automatic |
| Live move | Not possible; measure, move, re-measure | Yes, position updates in real time |
| Documentation | Hand recorded | Stored reports |
| Sensitivity to setup dimensions | Moderate | High — the entered dimensions drive the result |
| Cost and training | Low | Higher |
Measurement modes
- Three-point (clock) mode. Take readings at three positions, commonly 9, 12, and 3 o'clock, or at any three positions spanning at least about 70 degrees. Useful where a shaft cannot be rotated a full turn.
- Continuous sweep mode. Rotate the shafts steadily through as much of a revolution as access allows while the system samples continuously. More data points give a better fit and a more reliable result.
As with dial methods, rotate both shafts together. A laser system computes the relationship between two centerlines, and turning only one shaft measures the coupling hub instead.
Entering the dimensions
This is where laser alignments most often go wrong. The system knows only what you tell it:
- Distance between the two measurement planes (the two mounting brackets).
- Distance from the detector or reference plane to the coupling center.
- Distance from that plane to the movable machine's front foot.
- Distance from that plane to the movable machine's back foot.
- RPM, for tolerance evaluation.
Every one of these is projected geometrically out to the feet. An error of an inch in the foot distance produces a proportional error in the recommended shim. Measure these with a tape and record them; do not estimate them.
Thermal growth targets
A laser system aligns to whatever target you give it. Enter 0.000 everywhere and it will align the machine perfectly at the temperature it is sitting at, which is the wrong answer for any machine that runs hot.
Enter the cold offset targets from the equipment manufacturer's data, or calculate them as in Chapter 8 from the coefficient of expansion, the height from the feet to the shaft centerline, and the temperature change. The system will then guide the machine to a cold position that becomes correct at operating temperature.
Live move
Live move is the feature that most changes the work. With the system in live move, the display shows the current vertical and horizontal position of the movable machine, updating as the mechanic jacks it sideways or changes shims. Instead of measure-move-measure-move, the machine is walked directly into tolerance.
Practical rules:
- Vertical first, then horizontal, the same as with dial indicators. Changing shims changes the horizontal position slightly.
- Loosen only the bolts you need to and keep the machine controlled. A free-floating machine during a live move can drop or shift.
- Re-measure with all bolts torqued. Live move shows the position with bolts loose; the final verification reading must be taken with the machine bolted down.
What the laser does not do
A laser system is a measuring instrument, not a diagnostician. It will happily align a machine that has:
- Soft foot — most systems include a soft foot check routine, but the mechanic must run it and act on it.
- Pipe strain — invisible to the laser; the machine simply will not stay where it was put.
- A bent shaft or eccentric hub — turning both shafts together reduces the effect, but a bent shaft still vibrates at speed.
- A loose or cracked baseplate or a hollow grout bed — the alignment will be perfect and will not last a week.
The prealignment sequence from Chapter 8 applies identically whether the measurement is taken with dial indicators or with a laser.
Care of the equipment
- Keep the optical surfaces clean with the supplied lens tissue; a fingerprint on a lens scatters the beam.
- Protect the units from impact — they are precision optical instruments in a plant environment.
- Mind the environment: heavy steam, heat shimmer, and airborne dust between the units degrade the beam. A large temperature gradient across the beam path can bend it enough to matter.
- Check battery state before starting a job in an awkward location.
- Follow the laser safety class instructions. Industrial alignment lasers are low power, but the beam should never be directed at anyone's eyes.
- Store the system in its case with the brackets and chains, and verify against a known-good setup periodically.
Which measurement error is eliminated by a laser alignment system but must be corrected for with dial indicators?
A mechanic aligns a hot process pump with a laser system, entering zero thermal targets, and achieves a reading well within tolerance. Three weeks later the pump has failed a mechanical seal. What was wrong?
Why must both shafts be rotated together when taking laser alignment readings?