Downloaded from https://www.everyspec.com ADS-43A-HDBK where the bores or support pads for bearing supports must be accurately located. Verification of bore centers can be determined manually or with an automated coordinate measuring machine. Reference pads are sometimes machined onto a frame assembly to assist in accurate measurements. e . Engine main shafts must be carefully measured to determine proper location and size of bearing journals, radii of transition points, and location and proper configuration of splines. Hollow main shafts must be carefully measured and inspected internally as well as externally.
Finish specifications are equally critical both inside and outside of shafts to reduce the possibility of fatigue crack initiation. f. Carbon radial or face seals are common in turbine engine dynamic sealing applications. Both the carbon seal and seal contact surfaces require very fine finishes usually expressed in terms of within two to three helium light bands. Measurement of such fine surface finishes requires specialized equipment capable of generating helium light and polarization to show interference fringing on the polished surface. Labyrinth seals usually are air seals with multiple knife edges running on some type of relatively soft coating.
These seals are allowed to interfere with each other to “wear in.” Concentricity is important for good sealing. The labyrinth knife edges are sloped toward the pressure side to aid in sealing, and have a small radius (sharp) edge. Labyrinth seals will be marked or otherwise designed to prevent reverse installation. Under some circumstances, such markings could be a critical characteristic because excessive air leakage due to improper installation could adversely effect engine performance. Accessory drive seals are usually radial lip seals fabricated from a molded elastomer. Carbon seals are also used in accessory drives where the shaft speeds are very high.
Elastomeric seal wear surfaces, although not as critical as carbon seals, must also have good surface finishes with outer diameters controlled for proper seal lip pressure. g. Bearing raceways and rolling elements are polished to very fine finishes, usually less than 6 microinches arithmetic average (AA). Bearing ring and raceway dimensions are closely controlled such that mounted internal bearing clearances are in accordance with design specifications. Because of high rotational speeds, balance of the cage and rotating elements must be closely controlled. This requires careful selection of a set of elements and close control of cage dimensions.
Internal unbalance can lead to bearing instability and failure. Bearings or bearing sets are usually designed to be loaded in one direction only and therefore must be marked for proper installation. Markings such as this could be construed as a critical characteristic because reverse installation could result in a short bearing life or engine failure. h. Compressor and turbine blades are aerodynamically shaped and controlled dimensionally in the engineering drawings augmented by Master Comparator Charts (MCC) or electronic data generated by Computer Aided Design (CAD). The critical 1 4