7.3.3 Rotor balancing
When assembled with blades, the rotor is balanced both statically and dynamically. For built-up rotors, each bladed disc is balanced individually prior to assembly.
Static balance means that the weight is evenly disposed around the axis of the shaft. It can be checked by rolling the rotor on horizontal knife-edge supports.
Dynamic balance means that the moments of the out-of-balance weights along the axis about either bearing add up to zero. This is checked by spinning the rotor on resilient bearings, detecting the vibration, and adding or subtracting weights until the vibration is negligible.
A modern balancing machine enables balancing to be carried out with a high degree of accuracy and eliminates to a large extent the trial and error processes used in the past.
Rotors are normally balanced at low speed (400 r/min) and weight adjustment made in two convenient planes, one at each end of the rotor. This adjustment may be by varying screwed plugs in tapped holes, or by adding balance weights at specific circumferential positions. Tee-slots are machined circumferentially in the periphery of the rotor front and rear half-coupling flanges to permit the weights to be positioned and retained.
The aim of balancing is to reduce the amplitude of vibration to a tolerable level, which can be taken to be about 25 /mi at the bearing pedestals.
As rotors become larger and more flexible, it is increasingly important to understand their modal behaviour so that balancing can ensure smooth running over the speed range.
The larger LP rotors and generator rotors having critical speeds below running speed are overspeeded and, if necessary, balanced in the fully-bladed condition in a vacuum chamber, where they can be run without being overheated due to windage.
In the vacuum chamber and the high speed pit, rotors are run in bearing bushes and pedestals as nearly as possible simulating site conditions. Facilities are available for measuring journal or pedestal vibration and balancing can be carried out at closely controlled speeds anywhere in the range required. Standards of balance obtained during the factory testing must provide sufficient vibration margin to take account of site conditions. Balance on-site is likely to be affected by variations in the stiffness of the bearing, possible shaft misalignment and the coupling of the individually balanced rotors to form the complete shaft system.
Multi-plane balancing is carried out at speeds associated with each critical within the running range. Balance weights are attached to correct imbalance in any of the modes related to these criticals, as well as for imbalance in the modes of up to two criticals above running speed. Typical HP and IP turbines are provided with two balance planes. The LP rotors have three balance planes and generators have facilities to attach balance weights in almost any axial position.
Should site vibration levels exceed those specified, provision is made for site trim balancing on the LP rotors via access holes in each gland housing. Figure 1.111 shows the balance planes and site trim balance arrangements for a typical 660 MW LP rotor.