Rotor construction tends to vary from vendor to vendor. The blades are attached to the outer surface of the rotor. The rotor may be of basic disc or drum type construction, with the disc type having some variations. The two most common disc construction modes are shrunk discs on a shaft and stacked discs, normally through-bolted together. A final method is the solid rotor construction.

Figure 6-10, A 14-stage axial-flow rotor. (Courtesy of Elliott Company}

When the disc construction method is used, the blades are attached by a single-iobe dovetail root design. The slots are broached into the rotor and the blade roots fitted into the slots and keyed in place. Figure 6-10 shows a bladed disc type rotor.

When the discs are of the shrunk-on design, they are made up individually and stacked onto the shaft by first heating the disc to dilate the bore. They are then allowed to cool and thus attach themselves to the shaft. Keys are normally not used. When the discs are of the stacked design, the discs are equipped with rabbet fit to radially lock the discs to maintain concentricity of assembly. The through-bolts are usually ten-sioned by stretching hydraulically to a precise value to ensure the mechanical integrity of the assembly.

An alternate method not used too much at this time is the drum design. The drum construction is somewhat different from the disc, in that the rotor body is of cylindrical construction. By using the hollow drum, conical roots, of bolted construction, can be used for the rotor, again, allowing for stagger adjustments to fine tune the axial compressor to the application if the need arises. The setting is done to a gauge at the factory, as with the stators.

Figure 6-10, A 14-stage axial-flow rotor. (Courtesy of Elliott Company}

For smaller compressors, where the speed is relatively high and space is limited, a solid rotor construction is used. This is similar to the disc type of construction, except that the discs are an integral part of the rotor. Blade attachment slots are cut into the rotor, similar to the slots cut into the discs. Rotor blades are rarely, if ever, shrouded.

Rotor material in all cases is low alloy steel with an appropriate heat treatment to match the stresses imposed by the blades and rotor weight. The rotor is generally manufactured from a forging with the material being a chrome-molybdenum alloy such as AISI 4140 or AISI4340.


Shafting takes on several forms to match the various rotor construction methods. Obviously, for the solid rotor, the shaft is a part of the overall rotor. For the shrunk-on discs, the shaft is a continuous member, carrying the discs in the center section. Concentricity of all turns and good control on the roundness of the shaft are critical, if a balanced, smooth running compressor is to result.

The more unique form of shafting is used in the bolted disc and drum designs. Here, the stub-shaft design is used. A stub-shaft is fitted to each end of the center body, whether disc or drum. The design must anticipate all possible sources of stress, so the proper shrink can be applied to the interface. An interference fit is used at the interface to ensure concentricity for all operating conditions. It should include a reasonable allowance for momentary overspeed, particularly if a turbine driver is used. The design should consider the potential temperature transients that may be encountered at startup, shutdown, and hot restarts. Some arbitrary allowance should be made for torsional transients, even when not ordinarily anticipated. The shaft material, for the separate shafting, can be made of a different material than that used for the rotor body, although there is not much reason to do it that way. The heat treatment used could be different without compromising the overall rotor.

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