Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGFigure147.11515-18.— Lobe pump (heliquad type).than gear teeth; as a rule, there are only two orthree lobes on each rotor.There are several different types of screwpumps. The mainpointsof differencebetweenthevarious types are the number of intermeshingscrews and the pitch of the screws. A doublescrewlow pitch pump is shown in figure 15-20,and a triple-screw high-pitch pump in figure15-21. Both of these pumps are widely usedaboard ship to pump fuel oil and lubricating oil.In the double-screw pump, one rotor is driven bythe drive shaft and the other by a set of timinggears. In the triple-screwpump, a central powerrotor meshes with two idler rotors.The rotating element in a rotating plungerpump (fig. 15-22) is a plunger which is set offcenteron a drive shaft that is rotated by thesource of power. The plunger is driven up anddown and around the chamber by the rotation ofthe shaft, in such away as to make a sliding sealwith the walls of the chamber. In moving,the plunger alternately opens and closes a passageto the discharge.Because of its valveless construction, the rotatingplunger pump is suitable for pumping oilthat may contain sand or other sediment and forpumping high viscosity liquids. The pump canproduce a very high suction lift.A moving vane pump (fig. 15-23) consists ofa cylindrically bored housing with a suction inleton one side and a discharge outlet on the otherside; a cylindrically shaped rotor of smallerdiameter than the cylinder is driven about an axisplaced above the centerline of the cylinder in sucha way that the clearance between the rotor and thecylinder is small at the top and at a maximumvalue at the bottom.The rotor carries vanes which move in and outas the rotor rotates, thus maintaining sealedspaces between the rotor and the cylinder wall.The vanes trap liquid on the suction side andcarry it to the discharge side; contraction of thespace expels the liquid into the discharge line.The vanes may swing on pivots, as shown in theillustration, or they may slide in slots in therotor.The moving vane type of pump is used for lubricatingoil service and transfer and, in general,for handling light liquids of medium viscosity.An internal gear pump is shown in figure 15-24. In the gear pumps previously described, theteeth project radially outward from the center ofthegears. In an internal gear system, the teethof one gear project outward but the teeth of theother project inward toward the center. In an internalgear pump, one gear stands inside theother.A gear directly attached to the drive shaft ofthe pump is set offcenter in a circular chamberfitted around its circumference with the spursof an internal gear. The two gears mesh on oneside of the pump chamber, between the suctionand the discharge. On the opposite side of thechamber a crescent-shaped form stands in thespace between the two gears in such a way thata close clearance exists between each gear andthe crescent.The rotation of the central gear by the shaftcauses the outside gear to rotate, since the twoFigure147.11615-19.— Two-lobe pump.406