centers (fig, 4-1). External pumps may use spurgears, herringbone gears, or helical gears to movethe fluid. In an internal gear pump, the teeth ofone gear project outward, but the teeth of theother gear project inward toward the center of thepump (fig. 4-2, view A). Internal gear pumps maybe either centered or off-centered.Spur Gear PumpThe spur gear pump (fig. 4-1) consists of twomeshed gears which revolve in a housing. Thedrive gear in the illustration is turned by a driveshaft which is attached to the power source. Theclearances between the gear teeth as they mesh andbetween the teeth and the pump housing are verysmall.The inlet port is connected to the fluid supplyline, and the outlet port is connected to thepressure line. In figure 4-1 the drive gear is turningin a counterclockwise direction, and the driven(idle) gear is turning in a clockwise direction. AsFigure 4-2.—Off-centered internal gear pump.the teeth pass the inlet port, liquid is trappedbetween the teeth and the housing. This liquid iscarried around the housing to the outlet port. Asthe teeth mesh again, the liquid between the teethis pushed into the outlet port. This actionproduces a positive flow of liquid into the system.A shearpin or shear section is incorporated in thedrive shaft. This is to protect the power sourceFigure 4-1.—Gear-type rotary pump.4-3
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