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allowing the blades to interlock at the platforms and
assume a different radial position during each
balancing run. This only occurs at the relatively low
r.p.m. used for balancing, because, during engine
running, the blades will assume a consistent radial
position as they are centrifuged outwards.
45. To obtain authentic balance results when blade
scatter is present, it is necessary to record readings
from several balance runs, e.g. 8 runs, thereafter
determining a vector mean.
46. A typical dynamic balancing machine for
indicating the magnitude and angular position of
unbalance in each plane is shown in fig. 25-6.
Correction of unbalance may be achieved by one or
a combination of the following basic methods; redistribution
of weight, addition of weight and removal of
weight.
47. Redistribution of weight is possible for such
assemblies as turbine and compressor discs, when
blades of different weight can be interchanged and,
on some engines, clamped weights are provided for
positioning around the disc.
48. The addition of weight is probably the most
common method used, certain parts of the assembly
having provision for the fitting of screwed or riveted
plugs, heavy wire, balancing plates or nuts.
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269
Fig. 25-5 Unbalance couples due to
centrifugal force.
49. Removing weight by machining metal from
balancing lands is the third basic method, but
normally it is only employed on initial manufacture
when balancing a component, e.g. a turbine shaft or
a compressor shaft, that is part of a larger assembly.
50. Modular assembled engines demand different
balancing methods which include the use of
simulated engine rotors. The dummy rotors must
reproduce the bearing span, weight, centre of gravity
and dynamic characteristics of the sub-assembly it
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270
Fig. 25-6 Dynamic balancing machine.
Fig. 25-7 Simulated engine rotor assemblies.
replaces and must be produced and maintained so
that their own contribution to the measured
unbalance is minimal. In order to obtain the correct
dynamic reactions when balancing a compressor
and/or turbine rotor assembly on its own, with the
intention of making it an independent module, a
simulated engine rotor must be used to replace the
mating assembly, ref. fig. 25-7. The compressor
and/or turbine rotor assembly having then been independently
balanced with the appropriate dummy
rotor is thus corrected both for its own unbalance and
influence due to geometric errors on any other
mating assembly.
Moment weighing of blades
51. With the introduction of the large fan blade,
moment weighing of blades has assumed a greater
significance, ref. fig. 25-8. This operation takes into
account the mass of each blade and also the position
of its centre of gravity relative to the centre line of the
disc into which the blade is assembled. The
mechanical system of blade moment weighing may
be integrated with a computer, ref. fig, 25-9, which
will automatically optimise the blade distribution. The
moment weight of a blade in units i.e. g.mm. or oz.in.,
is identical to the unbalance effect of the blade when
installed into a disc. The recorded measurement of
blade moment weights enables each blade to be
distributed around the disc in order that these
unbalances are cancelled.
Assembling
52. The engine can be built in the vertical or
horizontal position, using the ram or stand illustrated
in fig. 25-TO and 25-11 respectively. Assembling of
the engine sub-assemblies or modules is done in
separate areas, thus minimizing the build time on the
build rams or stands.
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271
Fig. 25-8 Principle of blade moment weighing.
Fig. 25-9 Integrated blade moment
weighing.
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272
Fig. 25-10 Engine assembly
--- vertical.
Fig. 25-11 Engine assembly
--- horizontal.
53. During assembling, inspection checks are
made. These checks can establish dimensions to
enable axial and radial clearances to be calculated
and adjustments to be made, or they can ascertain
that vital fitting operations have been correctly
effected. Dimensional checks are effected during
disassembly to establish datums which must be
repeated on subsequent re-assernbly.
54. To ensure that the nuts, bolts and setscrews
throughout the engine and its accessories are
uniformly tight, controlled torque tightening is
applied, fig. 25-12, the torque loading figure is
determined by the thread diameter and the differing
coefficients of friction allied with thread finish i.e.,
　
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