30. When the requirement for a detailed inspection on a component such as a turbine disc is necessary, etching of the disc surfaces would be followed by binocular inspection of the blade retention areas. The whole disc would then be subjected to magnetic crack test, followed by re-inspection of the disc including a further binocular inspection of the blade retention areas.
Repair
31.
To ensure that costs are maintained at the lowest possible level, a wide variety of techniques are used to repair engine parts to make them suitable for further service. Welding, the fitting of interference sleeves or liners, machining and electro-plating are some of the techniques employed during repair.

32.
The welding techniques detailed in Part 22 are extensively used and range from welding of cracks by inert gas welding to the renewing of sections of flame tubes and jet pipes by electric resistance welding.

33.
On some materials now being used for gas turbine engine parts, different techniques may have to be employed. An example of this is the high strength titanium alloys which suffer from brittle welds if they are allowed to become contaminated by oxygen during the cooling period. Parts made in

Overhaul

these alloys, which have to withstand high stress loadings in service, are often welded in a bag or plastic dome that is purged by an inert gas before 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 X-ray 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

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 dis-tribution 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.
　
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