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welding commences.
34. More advanced materials and constructions
may have to be welded by electron-beam welding.
This method not only enables dissimilar metals to be
welded, but also complete sections of the more
advanced fabricated constructions, e.g. a section of
a fabricated rotor drum, to be replaced at a low
percentage cost of a new drum.
35. Some repair methods, such as welding, may
affect the properties of the materials and, to restore
the materials to a satisfactory condition, it may be
necessary to heat treat the parts to remove the
stresses, reduce the hardness of the weld area or
restore the strength of the material in the heat
affected area, Heat treatment techniques are also
used for removing distortions after welding. The parts
are heated to a temperature sufficient to remove the
stresses and, during the heat treatment process,
fixtures are often used to ensure the parts maintain
their correct configuration.
36. Electro-plating methods are also widely used for
repair purposes and these range from chromium
plating, which can be used to provide a very hard
surface, to thin coatings of copper or silver plating,
which can be applied to such areas as bearing
locations on a shaft to restore a fitting diameter that
is only slightly worn.
37. Many repairs are effected by machining
diameters and/or faces to undersize dimensions or
bores to oversize dimensions and then fitting shims,
liners or metal spraying coatings of wear resistant
material. The effected surfaces are then restored to
their original dimensions by machining or grinding.
38. The inspection of parts after they have been
repaired consists mainly of a penetrant or magnetic
inspection. However, further inspection may be
required on parts that have been extensively
repaired and this may involve pressure testing or Xray
inspection of welded areas.
39. Re-balancing of the main rotating assembly will
be necessary during overhaul, even though all the
original parts may be refitted, and this is done as
described in para 40.
Overhaul
268
Fig. 25-4 Cracks revealed by magnetic
crack detection.
Balancing
40. Because of the high rotational speeds, any
unbalance in the main rotating assembly of a gas
turbine engine is capable of producing vibration and
stresses which increase as the square of the
rotational speed. Therefore very accurate balancing
of the rotating assembly is necessary.
41. The two main methods of measuring and
correcting unbalance are single plane (static)
balancing and two plane (dynamic) balancing. With
single plane, the unbalance is only in one plane i.e.,
centrally through the component at 90 degrees to the
axis. This is appropriate for components such as
individual compressor or turbine discs.
42. For compressor and/or turbine rotor assemblies
possessing appreciable axial length, unbalance may
be present at many positions along the axis. In
general it is not possible to correct this combination
of distributed unbalance in a single plane. However,
if two correction planes are chosen, usually at axially
opposed ends of the assembly, it is always possible
to find a combination of two unbalance weights which
are equivalent for the unbalances present in the
assembled rotor, hence two plane balancing.
43. To illustrate this point refer to fig. 25-5, the distribution
of unbalance in the rotor has been reduced
to an equivalent system of two unbalances ’A’ and
’B’. The rotor is already in static balance because in
this example ’A’ and ’B’ are equal and opposed.
However, when the part is rotating, each weight
produces its own centrifugal force in opposition to the
other causing unbalance couples, with the tendency
to turn the part end-over-end. This action is restricted
by the bearings, with resultant stresses and vibration.
It will be seen, therefore, that to bring the part to a
state of dynamic balance, an equal amount of weight
must be removed at ’A’ and ’B’ or added at ’P’ and ’O’.
When the couples set up by the centrifugal forces are
equal, it is said that a part is dynamically balanced.
Unbalance is expressed in units of ounce-inches,
thus one ounce of excess weight displaced two
inches from the axis of a rotor is two ounce inches of
unbalance.
44. When balancing assemblies such as L.P.
compressor rotors, the readings obtained are inconsistent
due to blade scatter. Blade scatter is caused
by the platform and root or retaining pin clearances
　
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