压力之源——泵和压缩机外文文献翻译、中英文翻译

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Sources of Pressure -Pumps and Compressors

Fundamentally most pumps suitable for pumping hydraulic fluid will operate

with air or gas, although most oil pumps make poor air compressors, and some

compressors will not operate with incompressible fluid without modification.

General considerations. The main requirements of a hydraulic pump capable of

delivering sufficiently great a pressure to be of practical use, are:

a. Means for controlling fluid leakage either by special seals of close working

clearances.

b. A substantially uniform, non-pulsating delivery.

c. The absence of trapping of fluid during rotation, since the fluid is for all

practical purpose incompressible.

d. Mechanical balance to a sufficient degree to enable the pump to be run as fast

as possible, to reduce bulk for a given output.

e. Adequate provision for minimizing the effect of distortion of the working

parts, casings, ect., due to the internal pressure in the pump.

f. Low clearance volumes so that the pump will act momentarily during priming,

or with aerated fluid, as a compressor, to discharge the air through the delivery line,

instead of leaving It trapped in the pump.

g. Adequate inlet valve or port provision to enable the pump to creat a good

suction, or alternatively, as in the case of aircraft, to operate at high altitude, and with

relatively thick fluids. It should be noted that the time of opening of the inlet valve or

port is almost as important as the size of the port.

The main requirements of a good compressor are:

a. The smallest possible clearance volume compatible with mechanical

clearance; this is because any gas compressed in the clearance volume will expand

again instead of being discharged, and will thus adversely affect not only the output

of the pump, but the maxium pressure that can be achieved.

b. Except on low-pressure pumps, two or more stages of compression with as

much intercooling between stages as possible. This is desirable, firstly, to reduce the

wastage of work and output due to the gas being compressed adiabatically instead of

isothermally, and, secondly, to reduce the overall loss of output due to clearance

volume.

c. Pre-compression of the air in the pump up to the pressure of the delivery line.

Although the normal reciprocating pump with an automatic or spring-load delivery

valve will ensure this, some of the rotary compressors have to be ported specially to

obtain pre-compression, and with some it is nor possible at into the pump, and the

whole volume has again to be compressed. A rotary compressor with pre-compression

cannot be operated with hydraulic fluid.

d. Provision for air or water cooling of the cylinder so that the temperature of the

air in the compression cannot be operated with hydraulic pump.

e. Means for controlling gas leakages during compression either by pistonrings

on a reciprocating piston compressor, or fine clearance on the rotary types.

f. Adequate inlet valve area to prevent throttling of the incoming air, with the

consequent loss of volumetric efficiency.

The main types of pump and compressor use pistons, meshing gears or rotating

vanes, these having been found to give good high pressure performance. Other types

of pumps, such as centrifugal pumps or turbo-compressors, are not used for pressure

systems as they do not produce high enough pressure to be useful.

Variable Speed Hydraulic Systems

It is particularly important on many hydraulic systems, as on machine tools, to

be able to vary the speed of operation at will. This can be carried out in the following

ways, sometimes more than one way being combined:

a. By varying the pump output manually;

b. By using several pumps in combinations;

c. By restricting or throttling the output of a automatically variable delivery

pump, or a pump accumulator system, or by throttling the inlet;

d. By by-passing part of the pump output with a flow dividing valve;

e. By varying the volume of the operating jack.

1.Variation in pump delivery. Pump delivery can be varied by

a. Alteration in its speed;

b. Alteration of its stroke in a variable stroke type of pump;

c. Using two or more pumps of different delivery in parallel so that by stopping

and starting the pumps in various combinations different total deliveries can obtained.

The first system is an easy one when the pump is electrically driven, although

the electric motor involved is comparatively complicated for normal requirements.

Mechanical variable speed gear boxes have been used successfully with constant

speed electric drive.

Several of the pump mechanisms previously described can readily be adapted to

give a varying output by reducing the working strok manually by means of a control

wheel, etc.

The third system is simple enough, but varies the output in fixed steps. Two

pumps in parallel can give three ranges of output corresponding to

Pump A, Pump B, Pump A plus B.

Three pumps in parallel can give seven steps corresponding to

Pump A, Pump A plus B, Pump B plus C,

Pump B, Pump A plus C, Pump A plus B plus C.

Pump C.

Since, however, variable stroke pumps are readily available, such a complication

as three pumps in parallel hardly seems worthwhile although the two-pump system is

probably excellent for such duties as presses, etc. where a great of the working stroke

is at low pressure, where a pump for the final working stroke. Automatic isolation of

the low pump can be effected by a valve. Any normal type of automatic cut-out will

operate in the low-pressure system to by-pass it, without interference from the other

pump.

2. Restriction of Pump Output. With a variable delivery pump the flow of oil to

the system proper can be metered through a restriction, the delivery of the pump

automatically adjusting itself to the reduced flow. An automatic flow control valve or

throttle is to be preferred to a simple restrictor. This is an extremely simple system,

but is liable to variation of speed owing to change in viscosity of the oil, temperature

effects, etc. And the metering restriction may have to be adjusted from time to time to

keep the speed constant. On the other hand, it is possible to evolve a restriction

compensated for changes. By fitting the flow control valve in either jack line, control

in one directions only can be exercised, but note that as the volumes of the jack

returning to tank may not be same in both directions, the degree of speed control may

not be similar.

3.Use of Flow dividing Valves. The flow dividing valves of various types are

used to control the speed of a system by –passing part of the pump output, even if at

the expense of a slight wastage of power. It is possible to use a selector incorporating

several ports, which in turn control of fluid past several different flow dividing

valves, giving different rates of flow for each position of the selector.

4.Variation in hack volume. Another means of obtaining variable speed from a

constant delivery pump is use jacks of different volumes (i.e. at different pressures),

either in parallel, or using a multi-volume construction. If, for example, the machine

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摘要：

SourcesofPressure-PumpsandCompressorsFundamentallymostpumpssuitableforpumpinghydraulicfluidwilloperatewithairorgas,althoughmostoilpumpsmakepooraircompressors,andsomecompressorswillnotoperatewithincompressiblefluidwithoutmodification.Generalconsiderations.Themainrequirementsofahydraulicpumpcapableofd...

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