The three-screw, high-pitch, screw pump has many of the same elements as the two-screw, low-pitch, screw pump, and their operations are similar as shown in Figure 9.29. Three screws, oppositely threaded on each end, are employed. They rotate in a triple cylinder, the two outer bores of which overlap the centre bore. The pitch of the screws is much higher than in the low pitch screw pump; therefore, the centre screw, or power rotor, is used to drive the two outer idler rotors directly without external timing gears. Pedestal bearings at the base support the weight of the rotors and maintain their axial position. The liquid being pumped enters the suction opening, flows through passages around the rotor housing, and through the screws from each end, in opposed streams, toward the centre discharge. This eliminates unbalanced hydraulic thrust. The screw pump is used for pumping viscous fluids, usually lubricating, hydraulic, fuel oil, etc.
Figure 9.29 Three-screw, High-pitch, Screw Pump
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