Reducing vibration with laser alignment

Alignment errors in wind turbine drive trains are among the main causes of vibration in this machinery. They are easy to identify by evaluating the vibration velocity spectra: If the amplitudes at the single and/or double frequency of the generator shaft are too high, the most recent alignment reports should be checked or the shaft alignment should be measured using laser-optical alignment equipment.

Drive trains in a wind turbine exhibit highly variable alignment between the flexibly mounted generators and gearboxes. The generator shaft shifts toward the gear output shaft, depending on load and speed.

To ensure that power can be transmitted during all alignment conditions that arise during operation, system manufacturers usually use couplings with a very large working range (Fig. 1). Here it is important that alignment conditions during operation actually remain within the working range of the coupling in use.

This can be accomplished using couplings as shown in Fig. 2 that are capable of a high degree of displacement. However, even this type of coupling has limitations when it comes to parallel offset, which manifests itself in greater restoring forces, stronger vibrations and premature wear.

Using modern laser measurement methods, a near-perfect shaft alignment can be achieved between the gearbox and the generator with the system at a standstill. In reality, however, it is not desirable to align flexibly mounted drive trains to ‘zero’. Rather, both components must be misaligned while at a standstill so that the ideal shaft alignment is obtained during operation.

To achieve this, the direction of the displacements that occur during operation must be known. The data can be entered as so-called target values in all Prftechnik alignment systems.

Where to obtain alignment target values The alignment target values can either be taken from the operating manual or obtained from the system manufacturer. However, the wide variety of drive train components and basic frame types makes it difficult to provide general quantitative figures.

The actual alignment target values can be determined using a Permalign alignment monitoring system. For this purpose, highly sensitive laser sensors are mounted on the generator and gearbox (Fig. 3) to measure the displacements of the machines in intervals of one second.

The measurements can be recorded and further evaluated with any online CMS from Prftechnik. Thus, customers who already have an online system can simply connect the Permalign components to the system via an RS-232 interface and the measurements can be transmitted to the monitoring centre by remote access.

To determine the alignment target values, measurement results are then evaluated under different wind conditions: without wind (system at a standstill), with light wind (system rotating irregularly), with moderate wind and with strong wind. Fig. 4 shows the measurement results in an XY-graph. The resulting alignment target values are then entered into the alignment system and automatically taken into account during the alignment procedure.

Typically, the required measurement campaign lasts between one and four weeks — depending on the location and wind conditions.

Dr. Edwin Becker and Ole Holstein are with Prftechnik Alignment Systems GmbH, Ismaning, Germany. More on the topic of shaft alignment can be found in the company’s manual for technicians and engineers. Request a free copy by e-mail at info@pruftechnik.com.

Reader Service Card No. 412

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DID YOU KNOW?

Here is a glossary of terms for alignment projects. Erecting: Installation of a machine on-site.

Alignment: Adjustment of the machine into the correct position when it is erected.

Alignment target value: Specification of final condition after alignment (set values).

Alignment report: Description of actual condition achieved after alignment (actual values).

Alignment condition: Condition after alignment with the machine at a standstill and cold. This will change when the machine is in operation and warm.

Displacement: Change in the position of the shaft when changing from one operating state to another, especially from a standstill with a cold machine to a state of continuous operation with a warm machine.

Influences on the alignment condition (in accordance with VDI 2726):

• Heat expansion between the installation and operating temperatures (foundation, housing, shafts)

• Flexible deformation during operation (foundation, housing, shafts)

• Displacements between the installation and operating conditions (due to play, forces and lubrication)

• Skewed position of gearbox shafts due to externally applied masses (brake discs, couplings)

• Radial clearance and wobble of flange

• Radial and axial rigidity of connection coupling.

Angular offset: Measured in degrees, millirad (mrad) or as a relative quantity in millimetres per metre (mm/m). Example: A coupling with a 100 mm diameter has a gap at the top edge of 1 mm, which causes an angle of 10 mrad (note: 1 mrad = 1 mm per metre; rad is short for radian).

Parallel offset in mm: The distance between the points of intersection of the rotating axes with a certain plane perpendicular to both axes. Usually the parallel offset refers to the distance between the rotating shaft axes in the centre of the coupling.