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1.20. COMPRESSOR CONSTRUCTION

Centrifugal - flow compressors are usually made of titanium. The diffuser is generally manufactured of a stainless steel alloy. A close fit is important between the compressor and its case to obtain maximum compressor efficiency. Correct rotor assembly balancing is essential for safe operation because of the high rpm. Balancing the rotor can be accomplished by removing metal from specified areas of the compressor or by using balancing weights installed in holes in the hub of the compressor. On some engines where the compressor and turbine wheel are balanced as a unit, special bolts and nuts having slight variations in weight are used.

Axial-flow compressors are constructed of many different materials, depending upon the load and temperature under which the unit must operate. The rotor blades are generally cast of stainless-steel alloy. Some manufacturers use mdybdenum coated titanium blades to dampen vibrations on some stages of rotor blades. The clearance between the rotor blades and the outer case is most important. Some companies coat the inner surface of the compressor case with a soft material that can be worn away by the blades as they expand because of the heat generated from compressing the air. This type of compressor uses the "wear-fit" method to form its own clearance between the compressor case and the rotor blade tip.

Methods of attaching the blade to the disk or hub vary between manufacturers, with the majority using some variation of the dove-tail method to hold the rotor blades to the disk. Various other methods are used to anchor the blades in place. Some blades do not have a tight fit in the disk, but rather are seated by centrifugal force during engine operation. By allowing the blades to move, vibrational stress is reduced during start and shutdown. Stator vanes, shown in figure 1.16, can be either solid or hollow construction, and are connected together at their tips by a shroud. This shrouding serves two purposes. First, it provides support, and second, it provides the necessary air seal between rotating and stationary parts. Most manufacturers use the split compressor cases, while some others favor a weldment, forming a continuous case. The advantages of the split case lie in the fact that the compressor and stator blades are readily available to inspection. The one-piece case offers simplicity and strength because it is one piece; in most instances, it is a principal structural part of the engine and is usually made of cast aluminum, magnesium, or steel. Figures 1.16 and 1.17 show shrouded compressor stators in both the split case and the one-piece case.

Shrouded Compressor Stators
Figure 1.16. Shrouded Compressor Stators.

One-Piece Compressor Case
Figure 1.17. One-Piece Compressor Case.

 

 

David L. Heiserman, Editor

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Revised: June 06, 2015