The generator rotor is usually the single heaviest
component to be removed during an overhaul. In essentially all cases,
special lifting devices and a pedestal for storage are provided. The lifting
devices should be thoroughly inspected long before the rotor is removed.
Large generators may require two cranes to be hooked to the lifting device.
The crane operators should familiarize themselves with the use of the lifting
device and signals from the designated signaler.
To uncouple the generator shaft from the turbine shaft, all of the coupling
bolts except four should be removed. Before removing the final bolts,
machined steel blocks should be set under the turbine runner on the support
ledge to limit the amount the turbine runner and shaft have to be lowered.
The shaft and runner can then be lowered using the last four bolts or by
clamping the coupling with hydraulic jacks, removing the last bolts, and
lowering the shaft with the jacks. The turbine runner is designed to
support only the weight of the turbine shaft and the runner itself, so at no
time should the weight of the generator rotor be allowed to rest on the
Plywood strips, 6 to 10 inches wide, 1/4 to 3/8 inch thick, and 8 feet long
should be cut to fit in the generator air gap to prevent the rotor from
contacting the stator while the rotor is being removed. Six or more
maintenance personnel should be stationed around the stator with the strips
inserted in the air gap moving them up and down. If the rotor gets too
close to the stator, the strips will become tight, alerting the crane operators
to stop and recenter the rotor.
The bearing shoes of segmented shoe guide bearings can
usually be lifted out of the bearing housing by one or two people once the
bearing cover is removed.
The turbine bearing, which is almost always a split shell journal bearing,
must be lifted out of its housing using come-a-longs or chain falls. The
bearing is then set on blocks on top of the bearing housing and unbolted to
split it in two so that it can be removed from the turbine shaft.
On some units, the upper and lower generator guide bearings are also split
shell journal bearings. In the case of the upper guide bearing, it is usually
one piece and can be simply lifted off with the crane once the excitor and
bearing cover are removed. Special attention should be given to the upper
guide bearing insulating gaskets to prevent breaking or misplacing any of
the segments. The lower guide bearing usually must be lowered down into
the turbine pit using threaded rods and nuts or come-a-longs after the oil
pot is removed. The bearing is lowered to rest on top of the shaft
coupling, split, and the bearing halves lowered to the scaffolding.