最新泵外文翻譯.doc

1、The pump in pumping station selection problemRotary pumpsThese are built in many different designs and are extremely popular in modern fluid-power system. The most common rotary-pump designs used today are spur-gear, generated-rotary , sliding-vane ,and screw pump ,each type has advantages that make

2、 it the most suitable for a given application .Spur-gear pumps. these pumps have two mating gears are turned in a closely fitted casing. Rotation of one gear ,the driver causes the second ,or follower gear, to turn . the driving shaft is usually connected to the upper gear of the pump .When the pump

3、 is first started ,rotation of gears forces air out the casing and into the discharge pipe. this removal of air from the pump casing produces a partial vacuum on the pump inlet ,here the fluid is trapped between the teeth of the upper and lower gears and the pump casing .continued rotation of the ge

4、ars forces the fluid out of the pump discharge .Pressure rise in a spur-gear pump is produced by the squeezing action on the fluid ad it is expelled from between the meshing gear teeth and casing ,.a vacuum is formed in the cavity between the teeth ad unmesh, causing more fluid to be drawn into the

5、pump ,a spur-gear pump is a constant-displacementunit ,its discharge is constant at a given shaft speed. the only way the quantity of fluid discharge by a spur-gear pump of type in figure can be regulated is by varying the shaft speed .modern gear pumpsused in fluid-power systems develop pressures u

6、p to about 3000psi.Figure shows the typical characteristic curves of a spur-gear rotary pump. These curves show the capacity and power input for a spur-gear pump at various speeds. At any given speed the capacitycharacteristic is nearly a flat line the slight decrease in capacity with rise in discha

7、rge pressure is caused by increased leakage across the gears from the discharge to the suction side of the pump. leakage in gearpumps is sometimes termed slip. Slip also increase with arise pump discharge pressure .the curve showing the relation between pump discharge pressure and pump capacity is o

8、ften termed thehead-capacity or HQ curve .the relation between power input and pump capacity is the power-capacity or PQ curve .Power input to a squr-gear pump increases with both the operating speed and discharge pressure .asthe speed of a gear pump is increased. Its discharge rate in gallons per m

9、inute also rise . thus the horsepower input at a discharge pressure of 120psi is 5hp at 200rpm and about 13hp at 600rpm.thecorresponding capacities at these speed and pressure are 40 and 95gpm respectively, read on the 120psi ordinate where it crosses the 200-and 600-rpm HQ curves .Figure is based o

10、n spur-gear handing a fluid of constant viscosity , as the viscosity of the fluidhandle increases (i.e. ,the fluid becomes thicker and has more resistance to flow ),the capacity of a gear pump decreases , thick ,viscous fluids may limit pump capacity t higher speeds because the fluid cannot into the

11、 casing rapidly enough fill it completely .figure shows the effect lf increased fluid biscosity onthe performance of rotary pump in fluid-powersystem .at 80-psi discharge pressure the pp has acapacity lf 220gpm when handling fluid of 100SSU viscosity lf 500SSU . the power input to the pump also rise

12、s ,as shown by the power characteristics.Capacity lf rotary pump is often expressed in gallons per revolution of the gear or other internal element .if the outlet of a positive-displacement rotary pump is completely closed, the discharge pressure will increase to the point where the pump driving mot

13、or stalls or some part of the pump casingor discharge pipe ruptures .because this danger of rupture exists systems are filledwith a pressurerelief valve. This relief valve may be built as of the pump or it may be mounted in the dischargepiping.Sliding-Vane Pumps ,These pumps have a number of vanes w

14、hich are free to slide into or out of slots in the pup rotor . when the rotor is turned by the pump driver , centrifugal force , springs , or pressurized fluid causes the vanes to move outward in their slots and bear against the inner bore of thepump casing or against a cam ring . as the rotor revol

15、ves , fluid flows in between the vanes when they pass the suction port. This fluid is carried around the pump casing until the discharge port is reached. Here the fluid is forced out of the casing and into the discharge pipe.In the sliding-vane pump in Figure the vanes in an oval-shaped bore. Centri

16、fugal force starts the vanes out of their slots when the rotor begins turning. The vanes are held out by pressure which is bledinto the cavities behind the vanes from a distributing ring at the end of the vane slots. Suction is through two ports A and AI, placed diametrically opposite each other. Tw

17、o discharge ports are similarly placed.This arrangement of ports keeps the rotor in hydraulic balance, reliving the bearing of heavy loads. When the rotor turns counterclockwise, fluid from the suction pipe comes into ports A and AI is trappedbetween the vanes, and is carried around and discharged t

18、hrough ports B and BI. Pumps of this design are built for pressures up to 2500 psi. earlier models required staging to attain pressures approximatingthose currently available in one stage. Valving , uses to equalize flow and pressure loads as rotor sets areoperated in series to attain high pressures

19、. Speed of rotation is usually limited to less than 2500rpm because of centrifugal forces and subsequent wear at the contact point of vanes against the cam-ringsurface.Two vanes may be used in each slot to control the force against the interior of the casing or the cam ring. Dual vanes also provide

20、a tighter seal , reducing the leakage from the discharge side to the suctionside of the pump . the opposed inlet and discharge port in this design provide hydraulic balance in the same way as the pump, both these pumps are constant-displacement units.The delivery or capacity of a vane-type pump in g

21、allons per minute cannot be changed without changing the speed of rotation unless a special design is used. Figure shows a variable-capacitysliding-vane pump. It dose not use dual suction and discharge ports. The rotor rums in the pressure-chamber ring, which can be adjusted so that it is off-center

22、 to the rotor. As the degree of off-center or eccentricity is changed, a variable volume of fluid is discharged. Figure shows that thevanes create a vacuum so that oil enters through 180 of shaft rotation. Discharge also takes place through 180 of rotation. There is a slight overlapping of the begin

23、ning of the fluid intake function and the beginning of the fluid discharge.Figure shows how maximum flow is available at minimum working pressure. As the pressure rises, flow diminishes in a predetermined pattern. As the flow decreases to a minimum valve, the pressure increases to the maximum. The p

24、ump delivers only that fluid needed to replace clearance floes resulting from the usual slide fit in circuit components.A relief valve is not essential with a variable-displacement-typepump of this design to protectpumping mechanism. Other conditions within the circuit may dictate the use of a safet

25、y or relief valveto prevent localized pressure buildup beyond the usual working levels.For automatic control of the discharge , an adjustable spring-loaded governor is used . this governor is arranged so that the pump discharge acts on a piston or inner surface of the ring whose movement is opposed

26、by the spring . if the pump discharge pressure rises above that for which the bygovernor spring is set , the spring is compressed. This allows the pressure-chamber ring to move and take a position that is less off center with respect to the rotor. The pump theb delivers less fluid, and thepressure i

27、s established at the desired level. The discharge pressure for units of this design varies between 100 and 2500psi.The characteristics of a variable-displacement-pump compensator are shown in figure. Horsepowerinputvaluesalsoshownso thatthe powerinputrequirementscan be accuratelycomputed.Variable-vo

28、lumevane pumps are capacity ofmultiple-pressurelevels in a predetermined pattern.Two-pressure pump controls can provide an efficientmethod of unloading a circuitand still holdsufficient pressure available for pilot circuits.The black area of the graph of figure shows a variable-volume pump maintaini

29、ng a pressure of100psi against a closed circuit.Wasted power is the result of pumpingoilat 100psi through anunloading or relief valve to maintain a source of positive pilot pressure. Two-pressuretype controlsinclude hydraulic, pilot-operated types and solenoid-controlled, pilot-operated types. The p

30、ilot oil obtained from the pump discharge cannot assist the governor spring. Minimum pressure will result. Theplus figure shows the solenoid energized so that pilot oil assists compensator spring. The amount of assistance is determined by the small ball and spring, acting as a simple relief valve. T

31、his provides the predetermined maximum operating pressure.Another type of two-pressure system employs what is termed a differential unloading governor. Itis applied in a high-low or two-pumpcircuit. The governor automatically, Through pressure sensing,unloads the large volume pump to a minimum deadh

32、ead pressure setting. Deadhead pressure refers to aspecificpressure levelestablished as resultingaction ofthe variable-displacement-pumpcontrolmechanism. The pumping action and the resulting flow at deadhead condition are equal to the leakage inthe system and pilot-control flow requirements. No majo

33、r power movement occursat this time, eventhough the hydraulic system may be providinga clamping or holding action while the pump is indeadhead positionThe governor is basically a hydraulically operated, two-pressure control with a differential pistonthat allows complete unloading when sufficient ext

34、ernal pilot pressure is applied to pilot unload port.The minimumdeadhead pressure setting is controlledby the main governor springA.themaximumpressure is controlledby the relief-valveadjustment B. the operating pressure forthegovernor is generated by the large-volume pump and enters through orifice

35、C.To use this device let us assume that the circuit require a maximum pressure of 1000psi, which will be supplied by a 5-gpm pump. It also needs a large flow (40gpm) at pressure up to 500psi; it continuesto 1000pso at the reduced flow rate. A two-pump system with an unloading governor on the 40-gpm

36、pump at 500psi to a minimum pressure setting of 200psi (or another desired value) , which the 5-gpm pump takes the circuit up to1000psi or more.Note in figure that two sources of pilot pressure are required. One ,the 40-gpm pump, provides pressure within the housing so that maximum pressure setting

37、can be obtained. The setting of the spring,plus the pressure withinthe governor housing, determines the maximum pressure capacity of the40-gpm pump. The second pilot source is the circuit proper, which will go to 1000psi. this pilot line enters the governor through orifice D and acts on the unloadin

38、g piston E . the area of piston E is 15percent greater than the effective area of the relief poppet F. the governor will unload at 500psi and be activated at 15percent below 500psi, or 425psi. By unloading, we mean zero flow output of the 40-gpm pump.As pressure in the circuit increases from zero to

39、 500psi, the pressure within the governor housing also increases until the relief-valve setting is reached, at which time the relief valve cracks open, allowing flow to the tank.The pressure drop in the hosing is a maximum additive value, allowingthe pump to deadhead.Meanwhile, the system pressure c

40、ontinues to rise above 700psi, resulting in a greater force on the bottom of piston E than on the top. The piston then completely unseats poppet F, which results in a further pressure drop within the governor horsing to zero pressure because of the full-open position ofthe relief poppet F. flow ente

41、ring the housing through orifice is directed to the tank pass the relief poppet without increasing the pressure in housing. The deadhead pressure of the 40-gpm pump then decreases to the lower set value. Thus , at the flow rate to the unloading governor ,the 40gpm pumpgoes to deadhead. The flow rate

42、 to the circuit decreases to 5gpm as the pressure to 1000psi, the 5-gpm pump is also at its deadhead setting, thus only holding system pressure.The 4-gpm pump unloads its volume at 500psi. It requires a system pressure of 600psi to unload the 40-gpm pump to its minimumpressure of 200psi. the 600-psi

43、 pilot supply enters through orifice D and acts on the differential piston E. The pumps volume is reduced to zero circuit-flow output at 500psi. The additional 100-psi pilotpressure is required to open poppet F completely and allow the pressure within the housing to decrease to zero.As circuit press

44、ure decreases ,both pumps come back into service in a similar pattern.Axial Piston PumpsIn axial piston pumps of the in-line type, where the cylinders and the drive shaft are parallel ,the reciprocating motion is created by a cam plate, also known as a wobble plate ,tilting plate ,or swash plate .Th

45、is plate lies in a plane plate that cuts across the center line of the drive shaft and cylinder barrel and does not rotate .In a fixed-displacement pump ,the cam plate will be rigidly mounted in a positionso that it intersects the center line of the cylinder barrel at an angle approximately 25 degre

46、es from perpendicular .Variable-deliveryaxial piston pumps are designed ,so that the angle that the cam plate makes withaperpendicular to the center line of the cylinder barrel may be varied from zero to 20 or 25 degrees to one or both sides. One end of each piston rod is held in contact with the ca

47、m plate as the cylinder block andpiston assembly rotates with the drive shaft This causes the pistons to reciprocate within the cylinders .The length of the piston stroke is proportional to the angle that the cam plate is set from perpendicular to the center line of the cylinder barrel .A variation

48、of axial piston pump is the bent-axis type is shown in 1-1 .This type does not have a tilting cam plate as the in-line pump does .Instead ,the cylinder block axial is varied from the drive shaft axis .The ends of the connecting rods are retained in sockets on a disc that turns with the drive shaft .

49、The cylinder block is turned with the drive shaft by a universal joint assembly at the intersectionof the drive shaft and the cylinder block shaft ,In order to vary the pump displacement ,the cylinder block shaft and valve plate are mounted in a yoke and the entire assembly is swung in an are around

50、 a pair of mounting pintles attacked to the pump housing .Figure 1-1 Bent axis axial piston pumpThe pumping action of the axial piston pump is made possible by a universal joint or link .Figure1-2is a series of drawings that illustrates how the universal joint is used in the operation of this pump .

51、 First ,a rocker arm is installed on a horizontal shaft .(See fig 1-1view A )The arm is joined to theshaft by a pin so that it can be swung back and forth ,as indicated in view B。 Next, a ring is placedaround the shaft and secured to the rocker arm so the ring can turn from left to right as shown in

52、 view C, This provides two rotary motions in different planes at the same time and in varying proportions as maybe desired .The rocker arm can swing back and forth in one arc ,and the ring can simultaneously move from left to right in another arc ,in a plane at right angles to the plane in which the

53、 rocker arc turns .Next ,a tilting plate is added to the assembly .The tilting plate is placed at a slant to the axial of the shaft, as depicted in figure 1-1,view D. The rocker arm is then slanted at the same angle as the tiltingplate ,so that it lies parallel to the tilting plate .The ring is also

54、 parallel to ,and in contact with ,the tilting plate .The position of the ring in relation to the rocker arm is unchanged from that shown in figure 1-2,view C.Figure 1-2view E shows the assembly after the shaft ,still in a horizontal position ,has been rotateda quarter turn .The rocker arm is stilli

55、n the same position as the tiltingplate and oscar nowperpendicular to the axial of the shaft The ring has turned on the rocker pinstripe ,so that it has changed its position in relation to the rocker arm ,but it remains parallel to ,and in contact with the tilting plate .View F of figure 1-2 shows t

56、he assembly after the shaft has been rotated another quarter turn. The parts are now in the same position as shown in view D, but with the ends of the rocker arm reversed ,The ring still bears against the tilting plate .As the shaft continues to rotate ,the rocker arm and the ring turn about their pivots ,with eachchanging its relation to the other and with the ring always bearing on the plate .Figure 1-2, view G wheel added to the assembly .The wheel is placed upright and fixed to the shaft ,so that rotates with the shaft .In addition ,two rods A and B, are loosely connected to the tiltingri