Figure 4-7.—Lobe pump.previously. The lobes are considerably larger thangear teeth, but there are only two or three lobeson each rotor. A three-lobe pump is illustratedin figure 4-7. The two elements are rotated, onedirectly driven by the source of power, and theother through timing gears. As the elementsrotate, liquid is trapped between two lobes of eachrotor and the walls of the pump chamber andcarried around from the suction side to thedischarge side of the pump. As liquid leaves thesuction chamber, the pressure in the suctionchamber is lowered, and additional liquid is forcedinto the chamber from the reservoir.The lobes are constructed so there is acontinuous seal at the points where they meet atthe center of the pump. The lobes of the pumpillustrated in figure 4-7 are fitted with small vanesat the outer edge to improve the seal of the pump.Although these vanes are mechanically held intheir slots, they are, to some extent, free to moveoutward. Centrifugal force keeps the vanes snugagainst the chamber and the other rotatingmembers.SCREW PUMPScrew pumps for power transmission systemsare generally used only on submarines. Althoughlow in efficiency and expensive, the screw pumpis suitable for high pressures (3000 psi), anddelivers fluid with little noise or pressurepulsation.Screw pumps are available in several differentdesigns; however, they all operate in a similarmanner. In a fixed-displacement rotary-type screwpump (fig. 4-8, view A), fluid is propelled axiallyFigure 4-8.—Screw pumps.4-7
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