Lobe pump

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Helical gear pump

Engineman 3 - Basic Engine Mechanics

Operating Troubles

LOBE PUMP.-The lobe pump is still another variation of the simple gear pump. A lobe pump (heliquad type) is illustrated in figure 13-4. The lobes are considerably larger than gear teeth, but there are only two or three lobes on each rotor. The rotors are driven by external spur gears on the rotor shafts. Some lobe pumps are made with replaceable inserts (gibs) at the extremities of the lobes. These inserts take up the wear that would otherwise be sustained by the ends of the lobes. In addition, they maintain a tight seal be-tween the lobe ends and the casing. The inserts are usually seated on a spring. In this way, they automatically compensate for considerable wear of both the gibs and the casing. Replaceable cover plates (liner plates) are fitted at each end of the casing where the lobe faces cause heavy wear.

SCREW PUMP.-There are several types of screw pumps. The main differences are the number of intermeshing screws and the pitch

Figure 13-4.-Lobe pump (heliquad type).

of the screws. Figure 13-5 shows a positive-displacement, double-screw, low-pitch pump. (In a low-pitch pump, the threads form a small angle with respect to the center line of the pumping ele-ment.) Screw pumps are used primarily for pump-ing viscous fluids, such as F-76 and F-44.

Hydraulic systems on some ships use the screw pump to supply pressure for the system.

In the screw pump, liquid is trapped and forced through the pump by the action of rotating screws. As the rotor turns, the liquid is trapped between the threads at the outer end of each pair of screws. The threads carry the liquid along within the housing to the center of the pump casing where it is discharged.