Rigid and Flexible Pump Shaft Designs
The lowest critical speed is called the first critical speed, the next highest is called the second, and so forth. In centrifugal pump nomenclature, a rigid shaft means one with an operating speed lower than its first critical speed. A flexible shaft is one with an operating speed higher than its first critical speed. Once an operating speed has been selected, relative shaft dimensions must still be determined. In other words, it must be decided whether the pump will operate above or below the first critical speed.
Actually, the shaft critical speed can be reached and passed without danger because frictional forces tend to restrain the deflection. These forces are exerted by the surrounding liquid, and the various internal leakage joints acting as internal liquid-lubricated bearings. Once the critical speed is passed, the pump will run smoothly again up to the second speed corresponding to the natural rotor frequency, and so on to the third, fourth, and all higher critical speeds.
Designs rated for 1,750 rpm (or lower) are usually of the rigid-shaft type. On the other band, high-head 3,600 rpm (or higher) multistage pumps, such as those in a boiler-feed service, are frequently of the flexible-shaft type. It is possible to operate centrifugal pumps above their critical speeds for the following two reasons: (1) very little time is required to attain full speed from rest (the time required to pass through the critical speed must therefore be extremely short) and (2) the pumped liquid in the internal leakage joints acts as a restraining force on the vibration.
Experience has proved that, although it was usually assumed necessary to use shafts of such rigidity that the first critical speed is at least 20 percent above the operating speed, equally satisfactorily results can be obtained with lighter shafts with a first critical speed of about 60 to 75 percent of the operating speed. This, it is felt, is a sufficient margin to avoid any danger caused by an operation close to the critical speed.