Principles of naval engineering - Historic Naval Ships Association

Chapter 15. -PUMPS AND FORCED DRAFT BLOWERSHELICAL GEARINTAKEFigure15-17.— Helical gear pump.147,114herringbone gear pump; and the discharge flowis, accordingly, even smoother. Since the dischargeflow is smooth in the helical gear pump,the gears can be designed with a small numberof teeth, thus allowing increased capacity withoutsacrificing smoothness of flow.The pumping gears in this type of pump aredriven by set of timing and driving gears, whichalso function to maintain the required closeclearances while preventing actual metal-tometalcontact between the pumping gears. As amatter of fact, metallic contact between the teethof the pumping gears would provide a tighter sealagainst slippage; but it would cause rapid wearof the teeth because foreign matter in the pumpedliquid would be present on the contact surfaces.Roller bearings atbothendsof the gear shaftsmaintain proper alignment and thus minimizefriction losses in the transmission of power.Stuffing boxes are used to prevent leakage at theshafts.The helical gear pump is used to pump nonviscousliquids and light oils at high speeds andto pump viscous liquids at lower speeds.Figures 15-18 and 15-19 illustrate twotypesof lobe pumps. Although these pumps look somewhatlike gear pumps, they are not true gearpumps because the rotary elements are not capableof driving each other. One rotor is poweredby the drive shaft; the other is driven by a set oftiming gears. The lobes are considerably larger405