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1、Fluid Power PumpsPurpose of pumpsEvery fluidpower system uses one or more pumps to pressurize the hydraulic fluid. The fluid under pressure, in turn, performs work in the output section of the fluid-power system. Thus, the pressurized fluid may be used to move a piston in a cylinder or to turn the s
2、haft of a hydraulic motor.The purpose of a pump in a fluid-power system is to pressurize the fluid so that work may be performed. Some fluid-power systems use low ressures-100psi or less-to do work. Where a large work output is required, high pressures-1000pse or more-may be used. So we find that ev
3、ery modern fluid-power system uses at least one pimp to pressurize the fluid.Types of PumpsThere types of pumps find use in fluid-power systems：(1)rotary, (2)reciprocating, and(3)centrifugal pumps.Simple hydraulic systems may use but one type of pump. The trend is to use pumps with the most satisfac
4、tory characteristics for the specific tasks involvedIn matching the characteristics of the pump to the requirements of the hydraulic system, it is not unusual to find two types of pumps in series. For example, a centrifugal pump may be used to supercharge a reciprocating pump, or a rotary pump may b
5、e used to supply pressurized oil for the controls associated with a reversing variable-displacement reciprocating pumpRotary PumpsThese are built in many different designs and extremely popular in modern fluid-power system. The most common rotary-pump designs used today are spur-gear, internal-gear,
6、 generated-rotary, sliding-vane, and screw pump. Each type has advantages that make it 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 d
7、riving shaft is usually connected to the upper gear of the pump.When the pump 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 suction side of the pump inlet. Here the fluid is trapped
8、 between the teeth of the upper and lower gears and the pump casing. Continued rotation of the gears forces the fluid out of pump discharge.Pressure rise in a spur-gear pump is produced by the squeezing action on the fluid as it is expelled from between the meshing gear teeth and casing. A vacuum is
9、 formed in the cavity between the teeth as unmeshed, causing more fluid to be drawn into the pump. A spur-gear pump is a constant-displacement unit; its discharge is constant at a given shaft speed. The only way the quantity of fluid discharge by a spur-gear pump of the type in Figure can be regulat
10、ed is by caring the shaft speed. Modern gear pumps used in fluid-power systems develop pressures up 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 carious speeds. At any given speed th
11、e capacity characteristic is nearly a flat line. The slight decrease in capacity with rise in discharge pressure is caused by increased leakage across the gears, from the discharge to the suction side of the pump .Leakage in gear pumps is sometimes termed slip. Slip also increases with arise in pump
12、 discharge pressure. The curve showing the relation between pump discharge pressure and pup capacity is often termed the head-capacity or HQ curve. The relation between power input and pump capacity is the power-capacity or PQ curve.Power input to a spur-gear pump increase with both the operating sp
13、eed and discharge pressure. As the speed of a gear pump is increased, its discharge rate in gallons per minute also rises. Thus the horsepower input at a discharge pressure of 120pse is 5hp at 200rpm and about 13hp at 600rpm. The corresponding capacities at these speeds and this pressure are 40 and
14、95gpm, respectively, read on the 120psi ordinate where it crosses the 200-and 600-rpm HQ curves.Figure is based on spur-gear handing a fluid of constant viscosity, As the viscosity of the fluid handled increased, the capacity of a gear pump decreases. Thick, viscous fluids may limit pump capacity at
15、 higher speeds because the fluid cannot into the casing rapidly enough fill it completely. Figure shows the effect of increased fluid viscosity on the performance of rotary pump in a fluid-power system. At 80-psi discharge pressure the pump has a capacity of 220gpm when handing fluid having a viscos
16、ity of 100SSU viscosity. Capacity of this pump decreases to 150gpm when handing fluid having a viscosity of 500SSU. The power input to the pump also rises, as shown the power characteristics.Capacity of rotary pump is often expressed in gallons per revolution of the gear or other internal element. I
17、f the outlet of a positive-displacement rotary pump is completely closed, the discharge pressure will increase to the point where the pump driving motor stalls or some part of the pump casing or discharge pipe ruptures, because this danger of rupture exists systems are fitted with a pressure-relief
18、valve. This relief valve may be built as of the pump or it may be mounted in the discharge piping.These pumps have a number of vanes which are free to slide or out of slots in the pump rotor. When the rotor is turned by the pump driver, centrifugal force, springs, or pressurized fluid causes the van
19、es to move outward in their slots and bear against the inner bore of the pump casing or against a cam ring. As the rotor revolves, 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 for
20、ced out of the casing and into the discharge pipe.In the sliding-vane pump in Figure the vanes in an oval-shaped bore. Centrifugal force starts the vanes out of their slots when the rotor begins turning. The vanes are held out by pressure which is bled into the cavities behind the vanes from a distr
21、ibuting ring at the end of the vane slots. Suction is through two ports A and A1, placed diametrically opposite each other. Two 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 counterclock
22、wise, fluid from the suction pipe comes into ports A and A1 is trapped between the vanes, and is carried around and discharged through ports Band B1Pumps of this design are built for pressures up to 2500psi. Earlier models required staging to attain pressures approximating those currently available
23、in one stage. Valving, used to equalize flow and pressure loads as roter sets are operated in series to attain high pressures. 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-ring surface. Figur
24、e shown that the characteristic curves of the pump when operating at 1200rpm and handing oil having a viscosity of 150SSU at 100F.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 a tighter seal, reducing the leakage f
25、rom the discharge side to the suction side 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 gallons per minute cannot be changed wit
26、hout changing the speed of rotation unless a special design id used. Figure shoes a variable-capacity sliding-vane pump. It does not use dual suction and discharge ports. The rotor runs in the pressure-chamber ring, which can be adjusted so that it is off-center to the rotor. As the degree of off-ce
27、nter or eccentricity is changed, a variable volume of fluid is discharged. Figure shows that the vanes 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 beginning of the fluid intake function an
28、d the beginning of the fluid discharge.Figure shows how maximum flow is available at minimum working pressure. As the flow decreases to aminimum valve, the pressure increases to the maximum. The pump delivers only that fluid neended to replace clearance floes resulting from the usual slide fit in ci
29、rcuit components.A relief valve is not essential with a variable-displacement-type pump of this design to protect pumping mechanism. Other conditions within the circuit may dictate the use of a safety or relief valve to prevent localized pressure build up beyond the usual working levels.For automati
30、c control of the discharge, an adjustable spring-loaded governor is design to protect pumping mechanism. Other conditions within the circuit may dictate the use of a safety or relief valve to prevent localized pressure build up beyond the usual working levels.For automatic control of the discharge,
31、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 by the spring. If the pump discharge pressure rises above that for which the by governor spring is set, the spring is compressed.
32、 This allows the pressure-chamber ring to move and take a position that is less off center with respect to the roter. The pump then delivers less fluid, and the pressure is established at the desired leval. The discharge pressure for units of this design varies between 100 and 2500psi.The characteri
33、stics of a variable-displacement-pump compensator are shown in Figure. Horsepower input values also shown so that the power input requirements can be accurately computed. Variable-volume vane pumps are capacity of multiple-pressure levels in apredetermined pattern. Two-pressure pump controls can pro
34、vide an efficient method of unloading a circuit and still hold sufficient pressure available for pilot circuits.The black area of the graph of Figure shoes a variable-volume pump maintaining a pressure of 100psi against a closed circuit. Wasted power is the result of pumping oil at 100psi through an
35、 unloading or relief valve to maintain a source of positive pilot pressure. Two-pressure-type controls include hydraulic, pilot-operated types and solenoid-controlled, pilot-operated types. The minus of Figure shoes the solenoid energized so that the pilot oil is diverted to the tank. Thus, the pilo
36、t oil obtained from the pump discharge cannot assist the governor spring. Minimum pressure will result. The plus Figure shows the solenoid energized so that oil assists compensator spring. The amount of assistance is determined by the small ball and spring, acting as a simple relief valve. This prov
37、ides the predetermined maximum operating pressure.Another type of two-pressure system employs what is termed a differential unloading governor. It is applied in a high-low or two-pump to a minimum deadhead pressure setting. Deadhead pressure refers to a specific pressure level established ad a resul
38、ting action and the resulting flow at deadhead condition are equal to the leakage in the system and pilot-control flow requirements. No major power movement occurs at this time, even though the hydraulic system may be providing a clamping or holding action while the pump is in deadhead position.The
39、governor is basically a hydraulically operated, two-pressure control with a differential piston that allows complete unloading when sufficient external pilot pressure is applied to pilot unload port.The minimum deadhead pressure setting is controlled by the main governor spring A. the maximum pressu
40、re is controlled by the relief-valve adjustment B. the operating pressure for the governor is generated by the large-volume pump and enters through orifice 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
41、 s large flow (4gpm) at pressure up to 500psi; it continues to 1000psi at the reduced flow rate. A two-pumped system with an unloading governor on the 40-gpm pump will provide the needs.We can unload the 40-gpm pump at 500psi to a minimum pressure setting of 200psi (or another desired value), which
42、the 5-gpm pump takes the circuit up to 1000psi 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 can be obtained. The setting of the spring, plus the pressure within the governor housing
43、, determines the maximum pressure capacity of the 40-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 unloading piston E. The area of piston E is 15 percent greater than the effective area of the
44、relief poppet F. The unloading differential built into this governor control is, therefore, approximately 15 percent. The governor will unload at 500psi and be activated at 15 percent below 500psi, or 425psi. By unloading, we mean zero flow output of the 40-gpm pump.As pressure in the circuit increa
45、se from zero to 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 housing is a maximum additive value, allowing the pump to deadhead. Meanwhile, the
46、 system pressure continues 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 housing to zero pressure because of the full-open position of the relief
47、poppet F. Flow entering the housing through orifice C 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 40-gpm pump goes to d
48、eadhead. The flow rate to the circuit decreases to 5gpm as the pressure to 1000psi. At 1000psi, the 5-gpm pump is also at its deadhead setting, thus only holding system pressure.The 40-gmp pump unloads its volume at 500psi. It requires a system pressure of 600psi to unload the 40-gpm pump to its min
49、imum pressure of 200psi. The 600-psi pilot supply enters through orifice D and acts on the differential piston E. The pumps volume is required to open popper F completely and allow the pressure within the housing to decrease to zero.As circuit pressure deceases, both pumps come back into service in a similar pattern.流体动力泵泵的作用每个流体动力系统都使用一个或多个泵来维持液体正常的压力。带有压力的流体在流体动力系统的高压出口部分工作。于是这部分流体可用来推动油缸的活塞或者使液压马达的轴旋转。流体动力系统中泵的作用就是维持液体的压力以便于正常工作。某些系统采用100psi的低压或更低的压力工作。当输出功率需要很大时，就用1000psi的高压或更高的压力，所以我们发现每个现代流体动力系统至少用一个泵维持流体
