The rotor is made up of the shaft, impellers, impeller spacers, thrust collar, and the balance drum. Figure 200-34 shows several rotor configurations with various impeller types.
If a rotor always operates below the lowest critical speed, it is known as a stiff-shaft rotor. In contrast, a rotor with a normal operating range above one or more of its criticals is a flexible-shaft rotor. Most multistage centrifugal compressors have flexible-shaft rotors; and therefore, must pass through at least one critical during startup or shutdown. From an operational point of view, stiff shafts would be preferable. However, it is not practical since the shafts would become prohibitively large.