Helical Gear PumpThe helical gear pump (fig. 4-4) is stillanother modification of the spur gear pump.Because of the helical gear design, theoverlapping of successive discharges fromspaces between the teeth is even greater than itis in the herringbone gear pump; therefore, thedischarge flow is smoother. Since the dischargeflow is smooth in the helical pump, the gearscan be designed with a small number of largeteeth—thus allowing increased capacity withoutsacrificing smoothness of flow.The pumping gears of this type of pump aredriven by a set of timing and driving gears thathelp maintain the required close clearanceswithout actual metallic contact of the pumpinggears. (Metallic contact between the teeth of thepumping gears would provide a tighter sealagainst slippage; however, it would cause rapidwear of the teeth, because foreign matter in theliquid would be present on the contactsurfaces.)Roller bearings at both ends of the gear shaftsmaintain proper alignment and minimize thefriction loss in the transmission of power. Suitablepackings are used to prevent leakage around theshaft.Off-centered Internal Gear PumpThis pump is illustrated in figure 4-2, view B.The drive gear is attached directly to the drive shaftof the pump and is placed off-center in relation tothe internal gear. The two gears mesh on one sideof the pump, between the suction (inlet) anddischarge ports. On the opposite side of thechamber, a crescent-shaped form fitted to a closetolerance fills the space between the two gears.The rotation of the center gear by the driveshaft causes the outside gear to rotate, since thetwo are meshed. Everything in the chamber rotatesexcept the crescent. This causes liquid to betrapped in the gear spaces as they pass thecrescent. The liquid is carried from the suction portto the discharge port where it is forced out of thepump by the meshing of the gears. The size of thecrescent that separates the internal and externalgears determines the volume delivery of the pump.A small crescent allows more volume of liquid perrevolution than a larger crescent.Figure 4-4.—Helical gear pump.4-5
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