PumpsDOE-HDBK-1018/1-93POSITIVE DISPLACEMENT PUMPSWith the large number of teeth usually employed on the gears, the discharge is relativelysmooth and continuous, with small quantities of liquid being delivered to the discharge linein rapid succession. If designed with fewer teeth, the space between the teeth is greater andthe capacity increases for a given speed; however, the tendency toward a pulsatingdischarge increases. In all simple gear pumps, power is applied to the shaft of one of thegears, which transmits power to the driven gear through their meshing teeth. There are no valves in the gear pump to cause friction losses as in the reciprocating pump.The high impeller velocities, with resultant friction losses, are not required as in thecentrifugal pump. Therefore, the gear pump is well suited for handling viscous fluids suchas fuel and lubricating oils.OtherGearPumpsThere are two types of gears used in gear pumpsFigure 15 Types of Gears Used In Pumpsin addition to the simple spur gear. One type isthe helical gear. A helix is the curve producedwhen a straight line moves up or down thesurface of a cylinder. The other type is theherringbone gear. A herringbone gear iscomposed of two helixes spiraling in differentdirections from the center of the gear. Spur,helical, and herringbone gears are shown inFigure 15.The helical gear pump has advantages over thesimple spur gear. In a spur gear, the entirelength of the gear tooth engages at the sametime. In a helical gear, the point of engagementmoves along the length of the gear tooth as thegear rotates. This makes the helical gear operatewith a steadier discharge pressure and fewerpulsations than a spur gear pump.The herringbone gear pump is also amodification of the simple gear pump. Itsprincipal difference in operation from the simplespur gear pump is that the pointed center sectionof the space between two teeth beginsdischarging before the divergent outer ends ofthe preceding space complete discharging. Thisoverlapping tends to provide a steadier dischargepressure. The power transmission from thedriving to the driven gear is also smoother andquieter.Rev. 0ME-03Page 23
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