7.5.1 Alignment technique
The basic intention is to have zero shear and bending moment at the couplings, so that drivng torque is the only force transmitted. This means that the shaft centrelines must be coincident at the point where the coupling faces meet, and that the axes of the two shafts must be parallel at the same point.
Using one rotor as a datum, it is possible to set the next rotor relative to the datum rotor, using coupling gap and periphery measurements, and so on down the line.
To carry out gap measurements, it is necessary to separate the couplings slightly. The gap between the coupling faces is then measured at a number of points around the rotor, for a number of different rotor positions. If the coupling is correctly aligned, the gap should be uniform all the way round. If this is not so, the bearing positions can be modified by the appropriate amount and the gap remeasured.
The concentricity of the two rotors is checked by measuring, using dial gauges, the radial difference between the periphery of one coupling and the periphery of the other coupling while rotating the separate rotors. Any errors can be corrected by bearing adjustments, as for gap errors.
Other methods of alignment, based on strain gauges and optical techniques, have been developed.
The strain gauge technique is not widely applied but can be used on the coupled shaft at a suitable point near the coupling to be checked. It measures the bending stress of the shaft at that point by the application of strain gauges. The presence of bending stress is indicated by the strain gauge giving a sine wave output as the shaft is rotated. To use this tech nique fully, it would be necessary to establish a datun at each coupling for a properly aligned machine, am to carry out a calibration exercise to see how bending stress is affected by shaft alignment changes.
Low power laser-based optical alignment technique: can also be used with the shaft fully coupled. Thes< are suitable for initially setting up the pedestals t( the required catenary. These techniques have the dis advantage that, when adjacent turbines are operating the resulting atmospheric disturbances and structure borne vibration may preclude their use.
Alignment of the shaft necessitates the outer bear ings being raised. On large turbine-generators, the outer bearings could be up to 25 mm above the leve of the central bearings.
It is often necessary to deliberately misalign par ticular bearings when cold so that an acceptable ho alignment is achieved. This makes allowance for suppor structure expansions when the turbine is at operating temperature. The required offsets are included eithei by removing/adding shims from/to the bearings U be adjusted, or by deliberately allowing a known erroi in a coupling. The amount of offset can be decidec by measurements taken from a multilimb-manomete] system, or derived from operational experience.