The dowel pins may be hardened or unhardened steel and may be straight or tapered.
Tapered pins are easier to remove than straight pins. Hardened pins are normally used
where strength, shock, or wear are factors. If the components are held together with
threaded fasteners, and the pins will see little shear stress or wear, unhardened dowel pins
When doweled components are moved during the alignment process following the
overhaul of a unit, redoweling is necessary. Depending on the location and size of
the dowels and the amount of available room, it will be necessary to either relocate
the dowels or bore the e2dsting holes to accept a larger dowel. If the dowels are
relocated on components subject to varying temperatures, such as the generator
stator, care should be taken to install the dowel so that they do not impede
differential thermal expansion.
A magnetic-based drill should be used to bore the hole for the dowel. To ensure a
tight fit, the hole is always drilled undersized and reamed to match the dowel pin.
The reaming can be accomplished using the magnetic-based drill and a chucking
reamer, or it can be done with a hand reamer and a wrench. If the drill is used, care
should be taken to properly align the reamer and to choose the right cutting speed to
prevent oversized or bellmouth holes and to achieve a good finish. If the
components were only moved a few thousands of an inch, expansion reamers can
sometimes be used to ream the e2dsting hole to use a slightly oversized dowel.
5.5 Coupling boltholes
When new turbine runners are installed as part of the overhaul, it is usually necessary to
rebore the turbine shaft to turbine runner coupling boltholes. On rare occasions, the
coupling bolts on the generator-turbine shaft coupling may gall upon removal to the point
reboring is required. The normal procedure is to temporarily bolt the shaft to the runner or
the shafts together using four bolts. The bolts should be tightened sufficiently to ensure the
mating surfaces are making full contact. 'right wires or laser alignment equipment is then
used to check for dogleg and offset. Offset can be corrected by shifting the shaft, but dogleg
requires remachining the shaft or the runner mating face. When it is determined that the
shaft is correctly positioned, the holes are bored slightly oversized from the original diameter
to accept new coupling bolts.
The boring equipment used will depend on the size of the hole being bored. In some
cases, a magnetic-based drill can be used with a boring bar. If a magnetic-based drill
is used, a bearing for the boring bar should be mounted at the bottom of the hole to
add additional support. Cylinder boring machines for engines can also be used in
some instances. These machines are designed to bore the cylinders of large diesel or
gasoline engines but can be used for boring coupling boltholes with some
modifications. On large holes, a boring bar similar to that used on wicket