中英文文献翻译—液压泵的简单介绍

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英文部分：

The

commonly used

sources

of

power

in

hydraulic systems

are

pumps

and

accumulators .

Similarly,accumulator connected to atmosphere will dischange oil at atmosphere

pressure until it empty. only when connected to a system having

resistance to flow

can pressure be developed.

Three types of pumps find use in fluid-power systems: 1,rotary,2,reciprocating,3,or

piston- type,and 3,centrifugal pumps.

Simple hydraulic system may use but one type of pump . The trend is to use pumps

with the most satisfactory characteristics for the specific tasks involved . In 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

to supercharge a reciprocating pump , or a rotary pump may be used to supply

pressurized oil for the contronls associated with a reversing variabledisplacement

pumps .

Most power systems require positive displacement pumps . At high pressure ,

reciprocating pumps are often preferred to rotary pumps .

Rotary pumps

These are built in many differnt designs and extremely popular in modern fluid

power system . The most common rotay- pump designs used today are spurgear ,

internal gear ,generated rotor , sliding vane ,and screew pumps . Ehch type has

advantages that make it most suitable for a given application .

Gear pumps

Gear pumps are

the simplest type of fixed displacement hydraulic pump

available . This type consists of two external gear , generally spur gear , within a

closed- fitting housing . One of the gear is driven directly by the pump drive shaft .

It ,in turn , then drives the second gear . Some designs utilize helical gears ,but the spur

gear design predominates . Gear pumps operate on a very simple principle , illustration

Fig.7.3

. As the gear teeth unmesh , the volume at the inlet port A expands , a partial

vacuum on the suction side of the pump will be formed . Fluid from an external

reservoir or tank is forced by atmospheric pressure into the pump inlet . The

continuous action of the fluid being carried from the inlet to the discharge side B of the

pump forces the fluid into the system .

Pressure rise in a spur-gear pump is produced by the squeezing action on the fluid

as it is expellde from between the meshing gear teeth and the casing . Fluid from the

discharge side is prevented from returing to

the inlet side by the clearance between

the gears and houseing .

Vane pumps

The vane pump ,illustration 7.4 , consists of a housing that is eccentric or offset

with respect to the drive shaft axis . In some models this inside surface consists of a

cam ring that can be rotated to shift the relationship between rotor are rectangular and

extend radially from a center radius to the outside diameter of the rotor and from end to

end . A rectangular vane that is essentially the same size as the slot is inserted in the

slot and is free to slide in and out .

As the rotor turns , the vanes thrust outward , and the vane tips track the inner

surface of the housing , riding on a thin film of fluid . Two port or end plates that

engage the end face of the ring provide axial retention .

Centrifugal force generally contributes to outward thrust of the vane . As they ride

along the eccentric housing surface , the vane move in and out of the rotor slots . The

vane divide the area between the rotor and casing into a series of chambers .The sides

of each chamber are formed by two adjacent vanes ,the port or end plates , the pump

casing and the rotor . These chambers change in change in volume depending on their

respective position about the shaft .

As each chamber approaches the inlet port , its vanes move outward and its

volume expands , causing fluid to flow

into the expanded chamber . Fluid is then

carried within the chamber around to the dischange port . As the chamber approaches

the discharge port , its vanes are pushed inward ,the volume is reduced , and the fluid is

forced out the discharge port .

The variable- volume vane pump can be adjusted to discharge a different volume

of fluid while running at constant speed , simply by shifting the cam ring with respect

to the rotor .When the pump components are in position such that the individual

chambers achieve their maximun volume as they reach the inlet port , the maximum

volume of fluid will be moved . If the relationship between housing and rotor is

changed such that the chambers achieve their minmum of zero volume as they reach

the inlet port , the pump delivery will be reduced to zero .

Since the vane pump housing or cam ring must be shifted to change the

eccentricity and vary the output , variable-displacement vane pumps cannot have the

closed end fit common to fixed- displacement vane pumps . Volumetric efficiency is in

the range of 90% to 95% . These pumps retain their efficiency for a considerable

length of time since compensation for wear between the vane ends and the housing is

automatic .As these surfaces wear , the vanes move farther outward from their slots to

maintain contact with the housing .

Vane pump speed is limited by vane peripheral speed . High peripheral speed will

cause cavitation in suction cavity . which results in pump damage and reduced flow .

An imbalance of the vanes can cause the oil film between the vane tips and the

cam ring to break down , resulting in metal-to-metal contact and subsequent increased

wear and slipage . One metheod applied to eliminate high vane thrust loading is a

dual-vane construction .

In the dual-vane construction , two independent vanes are located in each rotor

slot . Chambered edges along the sides and top of each vane from a channel that

essentially force causes the vane to follow the contour of each pair of vanes .

Centrifugal force causes the vane to follow the contour of the cam-shaped ring . There

is just sufficient seal between the vanes and ring without destroying the thin oil film .

Piston-type pump

Two basic types of piston or reciprocating pumps are the radial piston and the

axial typese , both are available as fixed or variable displacement models . Axial piston

pumps may be further divided into in-line and bent axis types .

All piston pumps operate by allowing oil to flow into a pumping cavity as a piston

retreats and then forcing the oil out into another chamber as the piston advances .

Design differences among pumps lie primarily in the methods of separating inlet from

outlet oil .

In-line piston pump

The siplest typeof axial piston pump is the swash plate in-line design , illustration

7.5 .The cylinder are connected though piston shoes and a retracting ring , so that the

shoes beat anainst an angled swash plate . As the block turns ,the piston shoes follow

the swash plate ,causing the piston to reciprocate . The ports are arranged in the valve

plate so that the pistons pass the inlet port as they are being pulled out and pass the

outlet port as they are being forcing back in .

The angle of the swash plate controls the delibery . Where the swash plate is

fixed , the pump is of the constant-displacement type . In the variable- displacement ,

inline piston pump , the swash plate is moumted on a pivoted yoke . As the swash plate

angle is increased , the cylinder stroke is increase , resulting in a greater flow . A

pressure compensator control can position the yoke automatically to maintain a

constant output pressure .

Operation of he inline pump compensator control is shown schematically in

Fig.7.6 .The control can position the yoke automatically in Fig.7.6 . The control

consists of a compensator valve balanced between load pressure and the force of a

spring , a yoke piston controlled by the compensator valve to move the yoke , and a

yoke retun spring .

With no outlet pressure , the yoke return spring moves the yoke to the full

delibery position .As

pressure builds up ,it acts against the end of the valve

spool .When the pressure is high enough to overcome the valve spring , the spool is

displaced and oil enters dis placement . If the pressure falls off , the spool moves back ,

oil is discharged from the piston to the inside of the pump case , and the spring returns

the yoke to a greater angle .

The compensator thus adjusts the pump output to whatever is required to develop

and maintain the preset pressure . This prevents excess power losses bu relief valve

operation at full pump volume during holding or clamping .

There compensator thus adjusts the pump output to whatever is required to

develop and maintain the preset pressure . This prevents excess power losses by relief

valve operation at full pump volume during holding or clamping .

There is a variation of the swash plate in-line pump . It is a design where the

swash plate turns , but the cylinder barrel remains stationary . The plate is canted so

that it wobbles as it turns . This action pushes the pistons in and out the stationary

cylingder barrel .

This type of in-line pump contains a separate inlet and outlet check valve for each

piston since the pistons do not move past the inlet and outlet port .

BENT-axis piston pump

Illustration 7.7 show a bent-axial piston pump , which contatins a cylinder block

assembly in which the pistons are equally spaced around the cylinder block axis .

Cylinder bores are parallel to the axis . The cylinder block turns with the drive shaft ,

but at an offest angle . The piston rods are attaached to the drive shaft flange by ball

joints . A universal link keys the cylinder block to the drive shaft to maintain alignment

and assure that they turn together . The link does not transmit force except to accelerate

and decceltate the cylinder block and to overcome resistance of the block revolving in

oil filled housing .

As the shaft roates , distance between any one piston and the valving surface

changes continually . Each piston moves away from the valving surface during one

half of the revolution and toward the valving surface during the other half . The inlet

chamber is in line as the pistons move away , and the outletr chamber is in line as the

pistons move closer , thus drawing liquiring in during one half of the inlet chamber as

the pistons are moving away from the pintle . Thereforce , during rotation

, pistons

draw liquid into the cylinder bores as they pass the inlet side of the pinntle and force

that liquid out of the bores as they pass the outlet side of the pintle . The displacement

of this pump varies with the offset angle , the maximum angle being 30 degree ,the

minimum zero . Fixed displacement models are usually avaiable with 23 degree

angle .In the variable displacement construction a yoke with an external control is used

to change the angle . With some contronls , the yoke can be moved over center to

reverse the direction of flow from the pump .

Pump/system interaction

Frequently , hydraulic system designers choose off-the-shelf pumps with little

cocern other than supplying sufficient flow at available input power . Early enphasis

that positive displacement pumps supply only flow and that pressure is developed by

the system suggests that , as a minmum , the pump should be chosem in light of several

overall requirements and with system detailed design and the nature of the working

fluid well in mind .

Positive displacement pumps generate flow . In a fixed delivery pump , provisions

must be made to dissipate flow or system pressure will rise until a rupture occurs . The

usual means of accomplishing flow control is to place a relief valve inthe high pressure

line . When the pressure rise above an established amoumt ,the relief valve will vent

excess flow back to the reservoir . In such systems , pump flow and relief valve

capacity must be carefully matched to assure proper venting . Flow from a high

pressure line through a relief valve to a low pressure element is wasted hydraulic

horsepower , which can be calculated from the following relationship :

hp=PQ/1714

Where : Q = flow in gpm

This wasted horsepower is converted to heat in the hydraulic system . If not

properly removed , the heat can damage the fluid , elastomer seals , and other organic

material in the system .

Pressure-compensated variavle delivery pumps do not require a relief valve in the

high pressure line . The pressure compensation feature eliminates the need for the

relief valve . In nearly all working systems ,however , at least one is used on

just-in-case basis . The use of a pressure compensator , while avoiding dependence on

a relief valve , brings on its own problems . The actuator -spring-spool arrangement in

the compensator is a dynamic , damped-mass-spring arrangement . However , when the

system calls for a chang in axhieve their maxmum volume as they reach the inlet port ,

the maximum volume of fluid will ve moved . If the relationship between housing and

rotor is changed such that the chambers achieve their minimum of zero volume as they

reach the inlet port , the pump delivery will be reduced to zero .

Since the vane pump housing or cam ring must be shifted to change the

eccentricity and vary the output , variable-displacement vane pumps cannot have the

closed end fit common to fixed- displacement pumps . Volumetric efficiency is the

range of 90% to 95% . These pumps retain their efficiency for a considerable length of

time since compensation for wear between the vane ends and the housing is automatic .

As these surfaces wear , the vanes move farther outward from their slots to maintain

contact with the housing .

Vane pump speed is limited by vane peripheral speed . High peripheral speed will

cause cavitation in suction cavity , which results in pump damage and reduced flow .

An imbalance of the vanes can cause the oil film between the cane tips and the

cam ring to break down , resulting in metal-to-metal contact and subsequent increased

wear and slipage . One method applied to eliminate high vane thrust loading is a

dual-vane construction .

In the dual- vane

construction , tow independent vanes are located in each totor

slot chmbered edges along the sides and top of each vane from a channel that

essentially balances the hydraulic pressure on the top and bottom of each pair of

vanes .

Centrifugal force cause the vane to follow the contour of the cam-shaped

ring .There is just sufficient seal between the vanes and ring without destroying the

thin oil film

.