爱错了人机械 外文翻译

2021年1月19日发(作者：牛希济)
机械

外文翻译

draulicsampPneumatics

Any media liquid or gas that flows naturally or can be forced to flow could be used to transmit
energy in a fluid power system. The earliest fluid used was water hence the name hydraulics was
applied to systems using liquids. In modern terminology hydraulics implies a circuit using mineral
oil.
Figure
1-1
shows
a
basic
power
unit
for
a
hydraulic
system.
Note
that
water
is
making
something
of
a
comeback
in
the
late
90s
and
some
fluid
power
systems
today
even
operate
on
seawater. The other common fluid in fluid power circuits is compressed air. As indicated in Figure
1-2 atmospheric air -- compressed 7 to 10 times -- is readily available and flows easily through
pipes tubes or hoses to transmit energy to do work. Other gasses such as nitrogen or argon could
be used but they are expensive to produce and process.


Of the three main methods of transmitting energy mechanical electrical and fluid fluid power is
least
understood
by
industry
in
general.
In
most
plants
there
are
few
persons
with
direct
responsibility
for
fluid
power
circuit
design
or
maintenance.
Often
general
mechanics
maintain
fluid power circuits that originally were designed by a fluid- power-distributor salesperson. In most
facilities
the
responsibility
for
fluid
power
systems
is
part
of
the
mechanical
engineers
job
description. The problem is that mechanical engineers normally receive little if any fluid power
training at college so they are ill equipped to carry out this duty. With a modest amount of fluid
power training and more than enough work to handle the engineer often depends on a fluid power
distributors expertise. To get an order the distributor salesperson is happy to design the circuit and
often
assists
in
installation
and
startup.
This
arrangement
works
reasonably
well
but
as
other
technologies
advance
fluid
power
is
being
turned
down
on
many
machine
functions.
There
is
always a tendency to use the equipment most understood by those involved.


Fluid power cylinders and motors are compact and have high energy potential. They fit in small
spaces and do not clutter the machine. These devices can be stalled for extended time periods are
instantly reversible have infinitely variable speed and often replace mechanical linkages at a much
lower
cost.
With
good
circuit
design
the
power
source
valves
and
actuators
will
run
with
little
maintenance
for
extended
times.
The
main
disadvantages
are
lack
of
understanding
of
the
equipment and poor circuit design which can result in overheating and leaks. Overheating occurs
when the machine uses less energy than the power unit provides. Overheating usually is easy to
design
out
of
a
circuit.
Controlling
leaks
is
a
matter
of
using
straight- thread
O-ring
fittings
to
make
tubing
connections
or
hose
and
SAE
flange
fittings
with
larger
pipe
sizes.
Designing
the
circuit for minimal shock and cool operation also reduces leaks.

A
general
rule
to
use
in
choosing
between
hydraulics
or
pneumatics
for
cylinders
is:
if
the
specified
force
requires
an
air
cylinder
bore
of
4
or
5
in.
or
larger
choose
hydraulics.
Most
pneumatic circuits are under 3 hp because the efficiency of air compression is low. A system that
requires 10 hp for hydraulics would use approximately 30 to 50 air-compressor horsepower. Air
circuits are less expensive to build because a separate prime mover is not required but operating
costs are much higher and can quickly offset low component expenses. Situations where a 20-in.
bore air cylinder could be economical would be if it cycled only a few times a day or was used to
hold
tension
and
never
cycled.
Both
air
and
hydraulic
circuits
are
capable
of
operating
in
hazardous
areas
when
used
with
air
logic
controls
or
explosion-proof
electric
controls.
With
certain
precautions
cylinders
and
motors
of
both
types
can
operate
in
high-humidity
atmospheres . . . or even under water.

When using fluid power around food or medical supplies it is best to pipe the air exhausts outside
the clean area and to use a vegetable-based fluid for hydraulic circuits.

Some applications need the rigidity of liquids so it might seem necessary to use hydraulics in these
cases even with low power needs. For these systems use a combination of air for the power source
and oil as the working fluid to cut cost and still have lunge- free control with options for accurate
stopping and holding as well. Air-oil tank systems tandem cylinder systems cylinders with integral
controls and intensifiers are a few of the available components.


The
reason
fluids
can
transmit
energy
when
contained
is
best
stated
by
a
man
from
the
17th
century named Blaise Pascal. Pascals Law is one of the basic laws of fluid power. This law says:



Pressure
in
a
confined
body
of
fluid
acts
equally
in
all
directions
and
at
right
angles
to
the
containing surfaces. Another way of saying this is: If I poke a hole in a pressurized container or
line I will get PSO. PSO stands for pressure squirting out and puncturing a pressurized liquid line
will get you wet. Figure 1-3 shows how this law works in a cylinder application. Oil from a pump
flows into a cylinder that is lifting a load. The resistance of the load causes pressure to build inside
the cylinder until the load starts moving. While the load is in motion pressure in the entire circuit
stays nearly constant. The pressurized oil is trying to get out of the pump pipe and cylinder but
these mechanisms are strong enough to contain the fluid. When pressure against the piston area
becomes
high
enough
to
overcome
the
load
resistance
the
oil
forces
the
load
to
move
upward.
Understanding
Pascals
Law
makes
it
easy
to
see
how
all
hydraulic
and
pneumatic
circuits
function.


Notice
two
important
things
in
this
example.
First
the
pump
did
not
make
pressure
it
only
produced flow. Pumps never make pressure. They only give flow. Resistance to pump flow causes
pressure.
This
is
one
of
the
basic
principles
of
fluid
power
that
is
of
prime
importance
to
troubleshooting hydraulic circuits. Suppose a machine with the pump running shows almost 0 psi
on its pressure gauge. Does this mean the pump is bad Without a flow meter at the pump outlet
mechanics
might
change
the
pump
because
many
of
them
think
pumps
make
pressure.
The
problem with this circuit could simply be an open valve that allows all pump flow to go directly to
tank. Because the pump outlet flow sees no resistance a pressure gauge shows little or no pressure.
With a flow meter installed it would be obvious that the pump was all right and other causes such
as an open path to tank must be found and corrected. Another area that shows the effect of Pascals
law is a comparison of hydraulic and mechanical leverage. Figure 1-4 shows how both of these
systems work. In either case a large force is offset by a much smaller force due to the difference in
lever- arm length or piston area.



Notice that hydraulic leverage is not restricted to a certain distance height or physical location
like
mechanical
leverage
is.
This
is
a
decided
advantage
for
many
mechanisms
because
most
designs
using
fluid
power
take
less
space
and
are
not
restricted
by
position
considerations.
A
cylinder rotary actuator or fluid motor with almost limitless force or torque can directly push or
rotate
the
machine
member.
These
actions only
require
flow
lines
to
and
from
the actuator
and
feedback
devices
to
indicate
position.
The
main
advantage
of
linkage
actuation
is
precision
positioning and the ability to control without first look it may appear that mechanical
or hydraulic leverage is capable of saving energy. For example: 40000 lb is held in place by 10000
lb in Figure 1-4. However notice that the ratio of the lever arms and the piston areas is 4:1. This
means by adding extra force say to the 10000-lb side it lowers and the 40000-lb side rises. When
the 10000-lb weight moves down a distance of 10 in. the 40000-lb weight only moves up 2.5 in.
Work
is
the
measure
of
a
force
traversing
through
a
distance.
Work
Force
X
Distance..
Work
usually is expressed in foot-pounds and as the formula states it is the product of force in pounds
times
distance
in
feet.
When
a
cylinder
lifts
a
20000-lb
load
a
distance
of
10
ft
the
cylinder
performs 200000 ft-lb of work. This action could happen in three seconds three minutes or three
hours without changing the amount of work.


When
work
is
done
in
a
certain
time
it
is
called
power.
Power
Force
X
Distance
/
Time.
A
common measure of power is horsepower - a term taken from early days when most persons could
relate to a horses strength. This allowed the average person to evaluate to new means of power
such
as
the
steam
engine.
Power
is
the
rate
of
doing
work.
One
horsepower
is
defined
as
the
weight in pounds force a horse could lift one foot distance in one second time. For the average
horse this turned out to be 550 lbs. one foot in one second. Changing the time to 60 seconds one
minute
it
is
normally
stated
as
33000
ft-lb
per
minute.
No
consideration
for
compressibility
is
necessary
in
most
hydraulic
circuits
because
oil
can
only
be
compressed
a
very
small
amount.
Normally liquids are considered to be incompressible but almost all hydraulic systems have some
air trapped in them. The air bubbles are so small even persons with good eyesight cannot see them
but these bubbles allow for compressibility of approximately 0.5 per 1000 psi. Applications where
this
small
amount
of
compressibility
does
have
an
adverse
effect
include:
single- stroke
air-oil
intensifiers
systems
that
operate
at
very
high
cycle
rates
servo
systems
that
maintain
close-tolerance
positioning
or
pressures
and
circuits
that
contain
large
volumes
of
fluid.
In
this
book when presenting circuits where compressibility is a factor it will be pointed out along with
ways to reduce or allow for it. Another situation that makes it appear there is more compressibility
than stated previously is if pipes hoses and cylinder tubes expand when pressurized. This requires