measured after every few hits to determine the stretch. With experience, the person
swinging the sledge hammer can usually get a feel for when the stretch on the bolt is
The use of hydraulically operated wrenches is becoming more common. This
system uses a high pressure, hydraulic cylinder to drive the wrench. By observing
the system pressure, the torque on the bolt can be determined and used as a guide in
stretching the bolt. The preload of the bolt should still be checked by measuring the
stretch with a micrometer. Hydraulic wrenches allow much closer control of the
bolt stretch as well as being much less strenuous.
Another method of stretching bolts is with the use of bolt heaters. The bolt is
measured while still cold and then the heater is inserted in an axial hole through the
center of the bolt. As the bolt is heated, it expands, and while it is still hot, the nuts
are tightened to retain the expansion. The final stretch is measured with a
micrometer after the bolt has cooled. This method will usually take some trial and
error to achieve the desired stretch in the bolt.
6.2 Unit alignment
It is critical that the generator and turbine shaft be properly plumbed and the guide bearings
be made plumb and concentric. The exact procedure will vary depending on the design of
the thrust and the guide bearings
describes in detail the alignment procedures for the various type of bearings as well
as tolerances for the alignment.
Once the shaft alignment is determined to be acceptable, preparation for installing
the guide bearings can begin. Since the alignment procedures call for plumbing the
center of runout and not the shaft, the shaft will not be perfectly plumb and will
probably not be in the desired running position with reference to the bearing centers.
This will not be a problem if the guide bearings are all nonadjustable and the bearing
housings have already been made concentric and plumb as part of the alignment
process. In this case, the shaft is moved to center in the upper generator guide and
turbine guide bearing housings and the bearings installed.
If the upper and lower guide bearings are the adjustable segmented shoe type, the
shaft will have to be positioned so that it can be used as a reference for setting the
clearances on the bearing segments. To make the bearing centers all concentric, the
shaft must be perfectly plumb. This can be accomplished by installing the turbine
bearing and shimming the shaft to center in the turbine bearing. Plumb readings, are
then taken to determine the position of the shaft. From these plumb readings, the
top of the shaft can be moved to make it plumb. The upper and lower guide bearing
segments can then be installed and set using the shaft as a reference.