Laser Alignment for Vertical Shaft Plumbness: Faster, More Accurate Measurements

For hydro‑turbine and generator shafts, plumbness—how closely the shaft’s centerline aligns with gravity—is critical. Traditional wire‑and‑micrometer techniques, while inexpensive, demand extensive shaft access, labor, and time, and their accuracy hinges on meticulous manual operation.

Ludeca Inc. introduces the PERMAPLUMB laser system, which replaces the labor‑intensive wire method with a compact, magnetically‑mounted device that delivers sub‑0.00002 inch per foot resolution in a fraction of the time. The system’s self‑leveling mirror, always plumb to the horizon, reflects a Class 1 laser beam into a 1‑micron‑resolution detector. A 14‑inch axial bracket and magnetic feet secure the unit to the shaft, allowing a single 270° rotation to capture four 90° measurements and compute angularity, correction values, and a statistical quality metric.

Plumbness is expressed as a tiny angular deviation from perfect verticality. In practice, the parameter is usually reported as thousandths of an inch per foot (or per inch), and measurements are taken in two orthogonal planes: 0°–180° and 90°–270°.

Accurate plumbness reduces bearing temperatures, limits shaft movement, curbs vibration, and improves overall turbine efficiency—making the time required to achieve it a key cost factor.

The conventional tight‑wire method uses four 0.020–0.030 inch stainless‑steel wires spaced 90° apart, each weighted 20–30 lb and submerged in an oil bath. An electronic micrometer reads the distance from the wire to the shaft, with an audible tone indicating contact. Although inexpensive and intuitive, the method demands minimal vibration, high‑quality shaft surfaces, and careful, consistent measurement—factors that increase human error and prolong the process.

Accuracy improves with longer axial measurement lengths, which necessitates more operator travel or additional personnel. The overall time and space requirements make the wire technique a bottleneck, especially when adjacent turbines must remain operational.

The PERMAPLUMB system overcomes these limitations by automating the measurement process. Its laser transducer and self‑adjusting mirror are mounted on a 14‑inch magnetic bracket that attaches directly to the shaft. The mirror’s two‑axis pivot keeps the surface plumb, reflecting the laser into a high‑resolution detector. By rotating the shaft to four 90° positions, the system captures four readings and presents angularity, corrective moves, and a live “move” function that visualizes real‑time adjustments.

With a resolution better than 0.00002 inch per foot—an order of magnitude finer than the tight‑wire method—PERMAPLUMB delivers precision while remaining insensitive to operational vibration.

To integrate PERMAPLUMB into standard procedures, ensure the shaft can rotate freely: back off adjustable bearing shoes and clear any obstructions. Perform preliminary checks for dog‑leg or run‑out; dial indicators or proximity probes can quickly confirm shaft integrity. Equal load distribution across thrust bearing pads is essential, and various load‑cell methods can assist in achieving balance.

Mount the PERMAPLUMB on an accessible shaft section—typically the deck above the turbine. Connect the unit to a laptop and power supply, then launch the WinPLUMB software. Input thrust bearing data, guide bearing positions, and tolerance targets (e.g., NEMA’s 0.25 mils/ft, or 0.0208 mils/inch). The software then guides the measurement sequence: activate high‑pressure lubrication, rotate to the 0° position, deactivate lubrication to let the shaft settle, record the first point, and repeat for the remaining 270°.

Results appear in mils per inch, with the software flagging whether the NEMA tolerance has been met. If corrections are needed, PERMAPLUMB calculates pad adjustments—both up/down and “Pad Up & Down” optimized solutions—displaying them in real time. The “live move” mode updates shaft plumbness, predicted pad corrections, and guide bearing positions as adjustments are made, alerting operators to potential obstructions.

For vibration‑prone environments, PERMAPLUMB samples 32 readings per second per point, allowing the operator to select up to 204 seconds of data per point. The standard deviation display helps determine the optimal measurement duration, ensuring stable data without unnecessary delay.

The system’s circular completion check verifies that the sum of 0° and 180° readings equals the sum of 90° and 270° readings, with deviations below 0.2 considered acceptable. Repeatability functions compare new measurements against previous ones, confirming shaft rigidity; a discrepancy over 0.004 mils/inch indicates a lack of rigidity.

PERMAPLUMB’s laser‑based approach offers rapid, highly accurate vertical shaft plumbness measurements while minimizing downtime, labor, and vibration sensitivity. The combination of quick setup, reliable data, and real‑time correction guidance makes it the preferred choice for turbine maintenance and commissioning.

Daus Studenberg is an applications engineer at Ludeca Inc., based in Miami, Fla. He earned a B.S. in Mechanical Engineering from the University of Florida and specializes in field service, technical support, training, and product development. For questions, contact Daus.Studenberg@ludeca.com.

PERMAPLUMB Product Page

WinPLUMB Software Page