44.
When balancing assemblies such as L.P. compressor rotors, the readings obtained are incon-sistent due to blade scatter. Blade scatter is caused by the platform and root or retaining pin clearances 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; redis-tribution 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.
Overhaul
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
Overhaul
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 inde-pendently 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.
Overhaul
Overhaul
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., silver or cadmium plating and the lubricant used.
Testing
55.
On completion of assembly, every production and/or overhauled engine must be tested in a 'sea-level' test cell (fig. 25-14), i.e. a test cell in which the engine is run at ambient temperature and pressure conditions, the resultant performance figures being corrected to International Standard Atmosphere (I.S.A.) sea-level conditions (Part 21).
56.
To ensure that the engine performance meets that guaranteed to the customer and the require-ments of the Government licensing and purchasing authorities, each engine is tested to an acceptance test schedule.
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