is, all pumps are either nonpositive displacementor positive displacement.Basically, pumps that discharge liquid in acontinuous flow are referred to as nonpositivedisplacement, and those that discharge volumesseparated by a period of no discharge are referredto as positive displacement.Although the nonpositive-displacement pumpnormally produces a continuous flow, it does notprovide a positive seal against slippage; therefore,the output of the pump varies as system pressurevaries. In other words, the volume of fluiddelivered for each cycle depends on the resistanceto the flow. This type of pump produces a forceon the fluid that is constant for each particularspeed of the pump. Resistance in the dischargeline produces a force in a direction opposite thedirection of the force produced by the pump.When these forces are equal, the fluid is in a stateof equilibrium and does not flow.If the outlet of a nonpositive-displacementpump is completely closed, the discharge pressurewill increase to the maximum for that particularpump at a specific speed. Nothing more willhappen except that the pump will churn the fluidand produce heat.In contrast to the nonpositive-displacementpump, the positive-displacement pump providesa positive internal seal against slippage. Therefore,this type of pump delivers a definite volume offluid for each cycle of pump operation, regardlessof the resistance offered, provided the capacityof the power unit driving the pump is notexceeded. If the outlet of a positive-displacementpump were completely closed, the pressure wouldinstantaneously increase to the point at which theunit driving the pump would stall or somethingwould break.Positive-displacement pumps are furtherclassified as fixed displacement or variabledisplacement. The fixed-displacement pumpdelivers the same amount of fluid on each cycle.The output volume can be changed only bychanging the speed of the pump. When a pumpof this type is used in a hydraulic system, apressure regulator (unloading valve) must beincorporated in the system. A pressure regulatoror unloading valve is used in a hydraulic systemto control the amount of pressure in the systemand to unload or relieve the pump when thedesired pressure is reached. This action of apressure regulator keeps the pump from workingagainst a load when the hydraulic system is atmaximum pressure and not functioning. Duringthis time the pressure regulator bypasses the fluidfrom the pump back to the reservoir. (See chapter6 for more detailed information concerningpressure regulators.) The pump continues todeliver a fixed volume of fluid during each cycle.Such terms as fixed delivery, constant delivery,and constant volume are all used to identify thefixed-displacement pump.The variable-displacement pump is con-structed so that the displacement per cycle can bevaried. The displacement is varied through the useof an internal controlling device. Some of thesecontrolling devices are described later in thischapter.Pumps may also be classified according to thespecific design used to create the flow of fluid.Practically all hydraulic pumps fall within threedesign classifications-centrifugal, rotary, andreciprocating. The use of centrifugal pumps inhydraulics is limited and will not be discussed inthis text.ROTARY PUMPSAll rotary pumps have rotating parts whichtrap the fluid at the inlet (suction) port and forceit through the discharge port into the system.Gears, screws, lobes, and vanes are commonlyused to move the fluid. Rotary pumps are positivedisplacement of the fixed displacement type.Rotary pumps are designed with very smallclearances between rotating parts and stationaryparts to minimize slippage from the dischargeside back to the suction side. They are designedto operate at relatively moderate speeds.Operating at high speeds causes erosion andexcessive wear which results in increasedclearances.There are numerous types of rotary pumpsand various methods of classification. They maybe classified by the shaft position—eithervertically or horizontally mounted; the type ofdrive—electric motor, gasoline engine, and soforth; their manufacturer’s name; or their serviceapplication. However, classification of rotarypumps is generally made according to the type ofrotating element. A few of the most commontypes of rotary pumps are discussed in thefollowing paragraphs.GEAR PUMPSGear pumps are classified as either externalor internal gear pumps. In external gear pumpsthe teeth of both gears project outward from their4-2
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