SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
UNIT SEVEN
HYDRAULIC TRANSMISSION
LESSON ONE INTRODUCTION
For many years a hydraulic system was
considered the result of plumbing together a
number of components,Also,a hydraulic
system has been considered merely the source
of power for control servos,Each of these
concepts can lead to basic errors,Following
either of these guide lines can result in
systems,which are inadequate,unstable,or a
combination of the two.
hydraulic 液力的
transmission 传动
plumbing
管件总称
component
组成部分
servo 伺服
伺服系统
inadequate
不充足的
unstable 不稳定的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
From an operational standpoint,any
hydraulic system can be divided into four
logical segments,The power input segment,the
power transmission system,the control devices
and the power output portion,The power input
segment usually consists of pumps and
accumulators,The power transmission system
includes tubing,tubing connectors,swivel
joints and flexible hose,Flow control valves,
pressure control valves,And relief valves make
up the control elements,The power output
portion usually consists of rotary and
oscillatory motor or linear actuators,Because
almost all practical hydraulic systems can be
divided into these same four basic segments,
they will be considered in detail later.
accumulator 蓄能器
tube connector
管接头
swivel joint
转节，旋转接头
flexible hose 软管
relief valve 溢流阀
oscillatory
摆动的，振荡的
actuator
执行机构
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Paralleling the improvement in hydraulic
fluids and other materials has been an
increase in the complexity and sophistication
of hydraulic systems,Most early hydraulic
systems were no more complex than the
system in an automobile hydraulic jack,The
jack contains a hand operated pump which
draws oil from a reservoir,Oil is pumped past
a check or non-return valve into a linear
actuator,The oil flowing into the actuator
forces the piston to move,The moving piston
can do work on an outside load,A valve
controls flow from the piston back to the
reservoir.
sophistication
改进，复杂化
pump 泵
draw 吸（入）
reservoir 油箱
check valve 单向阀
piston 活塞
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Through the use of power driven
pumps and directional control valves these
simple systems have grown so that a
number of loads can be handled
independently or sequentially from the same
power source,Power hydraulics is a power
amplifier in which a small effort applied to
a control device results in a large amount of
work performed on a load,With boost
servos,hydraulic power aids the operator’s
effort,Further refinement has led to the
irreversible servo-mechanism in which the
operator’s effort is separated from the load
by the rigidity of the oil in the actuating
cylinder or motor.
sequentially
相继的，结果的
amplifier 放大器
boost 促进
refinement
精确，精心安装
irreversible
不可逆的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
ADVANTAGES OF HYDRAULIC SYSTEMS
What are the reasons for using hydraulic power in
heavy-duty industrial,mobile and aircraft designs? Even
though pneumatic control systems preceded hydraulic
servos,and electrical power is in more widespread use
than hydraulic,systems which require high power
amplification or which must pack great energy into a small
space are almost invariably hydraulic.
If hydraulic power transmission and amplification
were to be compared with pneumatic,electrical and
mechanical systems,these are some of the reasons for
specifying hydraulic power.
amplification 放大，加强 invariably 不变的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Hydraulic motors possess an extremely high ratio of torque
to inertia,Because of this,very little of the available torque is
dissipated in accelerating the motor; most of the torque can be
applied directly to the load,Acceleration rates for unloaded
hydraulic motors,for example,vary from about 0.3?105radians
per second squared for larger sizes at 3000psi to 3.4?105radians
per second squared for smaller motors,No other transmission
system produces motors having acceleration rates anywhere
near these values,Higher supply pressures develop accordingly
higher acceleration rates.
Hydraulic motors develop no counter electro-motive force
as do electrical motors,Therefore,the acceleration rate is
essentially constant over their design speed ranges.
惯
性
消
除
加速度
psi=pound per square inches 每平方英寸磅
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Hydraulic devices can generate much more power in the
same volume than can electrical devices,The reason for this
lies in the characteristics of the materials used in electrical
equipment,The very best armature steels can develop only
the equivalent of bout 200 psi in tractive power before they
become saturated,By comparison,3000 psi hydraulic
equipment is very common and pressures up to 5000 psi are
not rare.
Because of the greater pressure density,hydraulic
equipment in power sizes greater than l hp,tends to be mach
smaller than electrical motors and somewhat smaller than
pneumatic motors.
Hydraulic motors support heavy or springy loads much
better than do electrical or pneumatic motors,Looking from
the load back into the motors,electrical fields or compressed
gasses are quite soft,The relatively incompressible oil is,
however,stiff.
使饱和 电枢，转子
horsepower
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Hydraulic transmission systems efficiently transmit
power farther than do mechanical systems,but not as far as
electrical systems.
Hydraulic transmission systems may be less hazardous
than electrical systems and are far safer than mechanical
power transmission systems.
Hydraulic oil is a good heat conductor so that the heavily
loaded working area stays relatively cool,Heat generated at
the working interface is conducted away rapidly,The
working areas can therefore be made small because heat
absorbing mass is not needed,Because of this and because
of the high-pressure density,hydraulic motors weigh less
than one pound per horsepower,Horsepower to weight
ratios of up to 2.5 are not uncommon.
危险的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Hydraulic power systems also have some
drawbacks,Hydraulic oils are messy,Leaks
are inevitable,The great lubricity of most
hydraulic oils makes leaks and spillage very
hazardous,Certain oils are excellent
removers of paint and electrical insulation.
Hydraulic systems,particularly the high
performance servo systems which use
hydraulic power so efficiently,are extremely
vulnerable to dirt and contamination,They
must constantly be watched to prevent the
addition or generation of contamination and
hydraulic systems need expensive filtration
to keep them clean.
drawback 欠缺
inevitable
不可避免的
spillage 溢出
insulation 绝缘
vulnerable 脆弱的
contamination
污染
filtration 过滤
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
A rupture or burst of an element of a high-pressure
hydraulic system can be very dangerous,The high velocity
streams of oil,which results can damage eyes,pierce skin,
and harm other equipment,The author has seen a piece of
tubing three feet long and 1.5 inches in diameter thrown
whirling for more than fifty feet when a rupture occurred in a
3000 psi line.
Unless fire-resistant fluids are used,hydraulic leaks create
very real fire hazards,Most petroleum based fluids are highly
flammable,High temperatures can be generated where a high
velocity stream of oil from a rupture impacts on a flat surface.
Finally,hydraulic power is so compact that developing
and controlling fractional horsepower is difficult unless the
pressure density is reduced to levels where electrical power is
extremely competitive.
破
裂
难燃工作液
易
燃
的 冲击
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
POWER SYSTEM ANALOGIES
Hydraulic systems within the laminar flow regime at least
function in ways,which are quite analogous to mechanical and
electrical systems,Later in these discussions the equations of
analogy will be developed,At this point,illustrations show the
analogies between systems,Fig.l.1 shows the analogies relating
hydraulic parameters to those of electrical systems and
translatory and rotary mechanical systems,Similar analogies
could be developed for pneumatic,thermal or other energy
systems,These elements can be combined in ways which at least
in electrical systems are termed "passive" networks,They are
passive because no energy is added to the system in the networks
though energy is lost through conversion to heat in the resistance
elements,These networks affect the stability of transmission
systems at various frequencies when subjected to periodic
oscillation such as sinusoidal cycling.
层流 范围，领域
图
解
平
动
的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Because of the increasing importance of the dynamics
of the hydraulic system in designing for high performance,
this book will relate-in a practical way-those factors which
affect the dynamical operation of the system to the design
of components and to the disposition of the several
components within the system,No attempt will be made to
develop design techniques other than to show previous
solutions to typical problems,Instead,this book will relate
the discussion of the several parts of the hydraulic system to
basic engineering principles,A sincere attempt will be made
to show how these basic principles can be brought to bear
on the solving of typical component and system design
problems.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Where relevant,recourse will be
made to the calculus to support the
development of equations but,in most
cases,a knowledge of the calculus will
not be necessary in order to solve
problems,Certain books may be of value
in gaining additional detailed
understanding of specific subjects,This
book,however,is intended to stand on
its own and all reasonable efforts have
been made to obviate the need for
additional resources.
relevant 有关的
recourse 求助
calculus 微积分
obviate 消除
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Some of the practices,which have developed in the
hydraulics industry make problem-solving difficult,Among
these are the use of feet head of water as a measure of
pressure and the use of Saybolt Universal seconds or metric
units for viscosity,In this book most such units have been
converted into their English equivalents,Through personal
preference,the author avoids confusion in conversion by
putting everything into units of
pound-force,inches and seconds.
Wherever this might conflict with
established practice such as using
units of Newts for kinematics
viscosity-conversion formulae are
provided.
粘
性
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Where the text material is appropriate,problems are
given to allow the student to practice the achieving of
solutions,These problems nearly always represent forms
of difficulty,which have faced the author at various
times,They are practical problems in advanced
hydraulics,In many cases,a solution is possible only
because simplifying assumptions were made,Because of
these assumptions,the hydraulics engineer should
beware of blind application of the formulae,Careful
consideration of effect of practical deviations from the
assumed conditions is necessary if success in
overcoming real difficulties is to be achieved.
assumption 假定 overcome 克服
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
HYDRAULIC FLUIDS
Clearly,the single most important material in the hydraulic
system is the working fluid,Modern hydraulic fluids are
often complex formulations carefully prepared to meet their
exacting tasks,In addition to a base fluid they may contain
additives to improve viscosity characteristics,to minimize
system corrosion,to make the fluids more stable to their
environments and to reduce foaming and flammability,Dr.
Roger Hatton,in his excellent book,Introduction to Hydraulic
Fluids”,gives the following as characteristics of importance to
a hydraulic fluid:
Good lubricity
Chemically and environmentally stable
Possesses favorable viscosity
Compatible with other system materials
泡沫 易燃性润滑性
一致的，兼容的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Heat transfer capability
High bulk modulus
Low volatility
Low foaming tendencies
Low weight
Fire resistant
Good dielectric properties
Nontoxic or allergenic
Nonmalodorous
Low in cost
Easily available
This is a formidable list,No working fluid possesses all of
these good characteristics,The task of system designer is to
choose or to formulate the fluid most suitable to his
requirements.
体积（容积）弹性模量
挥发性
绝缘的，不导电
无毒的
无恶臭的
过敏的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Lubricity has several connotations,some of which are in
conflict with others,Lubrication,in general,is concerned
with minimizing the shearing action of two surfaces between
which relative motion exists.
All surfaces of working parts,no matter how smoothly
made,consist-if enlarged sufficiently-of hilly contours,In the
absence of lubrication,the hills and valley of mating parts
tend to mesh,Were it not for the presence of oxides or fatty
acids on the surfaces,the mating faces would actually weld
together.
The function of lubricants is to minimize the effect of
shearing off the hills-or more properly,the asperities-of the
surfaces as they move relative to each other,This is done in a
number of ways.
含义，内涵
asperity 粗糙
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Dry lubricants such as lead powder
and silver plating interpose a greasy
metal between the asperities,The low
shear forces required by the greasy
metals help in reducing the sliding
forces,Other dry lubricants used are
graphite and molybdenum disulphide.
Each of these has crystal faces or
molecular bonds which are weak,The
relative movement then breaks these
bonds rather than shears asperities.
Properly applied,these are quite
effective.
lead powder 铅粉
silver plating 镀银的
interpose 插入
graphite 石墨
molybdenum 钼
molybdenum disulphide
二硫化钼
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Fluid lubrication,as usually provided by hydraulic
fluids,consists of interposing the liquid between the mating
faces,If enough fluid can be forced between the two
surfaces to separate them completely,hydrodynamic
lubrication is achieved,In this condition,the only forces
resisting the relative movement of the two surfaces are
those needed to shear the fluid,In hydrodynamic lubrication,
wear is negligible and the coefficient of friction is very low.
Air bearings,Kingsbury thrust bearings and well-designed
journal bearings achieve hydrodynamic lubrication.
Kingsbury thrust bearing 推力轴承
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
If the lubricating fluid does not separate the surfaces
completely,some shearing of the asperities will occur.
The ability of the fluid to maintain some separation,the
tendency for the fluid to be dragged between the shearing
surfaces,and the capability of the fluid to form no
welding surfaces on the newly-sheared faces,all
contribute to the lubrication
effect,The first two of these
factors relate strongly to fluid
viscosity,The last one depends
on the chemical natures of the
fluid and the moving surfaces.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The chemical nature of boundary lubrication is
described by the chemical polishing theory,Under
pressure,two mating metal surfaces will have areas of
contact,If relative movement occurs,local high
temperatures will develop at the points of contact,At these
high temperatures,certain fluids such as the phosphate
esters react chemically with the metal to form an alloy
having a melting temperature lower than that of the base
metal,The low melting alloy will flow away from the
pressure and generate a new distribution of forces,Thus,
galling or welding is avoided at the cost of relatively high
wear,Because the mechanism relates directly to fluid-
metal chemistry,one fluid can lubricate only a limited
range of metals in this way.
phosphate ester 磷酸脂 gall 擦伤，咬住
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
These forms of lubrication are known as boundary
layer lubrication,Most practical lubrication problems in
power hydraulic systems more nearly approximate
boundary layer lubrication than they do hydrodynamic
lubrication,Hydraulic fluid choice must be considered as
a boundary layer problem,The fluids have the proper
viscosity over the working temperature range to allow it to
stay in the working area,It should coat newly sheared
surfaces with fatty acids or metal salts in order to avoid
welding,It should have a low internal shear forces.
boundary layer 边界层
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Low viscosity gives low shear forces but allows the
fluid to be squeezed from between heavily loaded surfaces.
A viscous but chemically inert fluid such as a silicone may
minimize wear but not prevent galling and seizing caused
by pressure welding,A phosphate ester base or a fatty acid
base fluid may prevent welding but allow a high wear rate.
The lubricating aspect of the choice of a fluid must then be
a compromise among the factors of low coefficient of
friction,low wear and anti-galling.
Lubricity is of primary concern where moving parts
are in contact,Examples of these areas are pump and motor
gear teeth and piston-cylinder piston heads and rod ends,
valve spool and accumulator pistons.（ c）
折衷
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
（ c） Tests for oil lubricity attempt to relate these requirement
areas to standardized testing methods,Running heavily loaded
gears with the test oil,as a lubricant is a good test for gear
pumps but may not have much relevance for piston pumps.
Perhaps the most widely used test for oil lubricity is the 4-ball
test,In this,three l/2-inch diameter steel balls are held in a cup
with the test oil,A fourth ball is pressed down onto the other
three with a known load and then rotated at a known speed,The
torque to rotate the fourth ball can be measured to give the
coefficient of friction for the particular composition of balls.
After running for a specified period of time at a controlled
temperature,the wear scar on the balls is measured,The size of
this scar can be related-at least subjectively-to the wear resisting
qualities of the oil,If the load were increased until welding
occurred,the test could give a measure of the anti-seizing
quality of the oil,防胶合
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Stability of a fluid is no more precise a term than is
lubricity,A fluid is stable if its important characteristics
change little with use or during storage.
In use,the fluid is subjected to heavy mechanical shear
forces,The fluid will be exposed to entrapped air and
moisture,It will be in contact with organic materials with
which it may combine,The fluid may be subjected to each of
these while at high temperature or when under nuclear
radiation,While no real fluid is truly stable under all of
these conditions,some fluids meet combinations of
conditions better than do others.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Heavy shear forces tend to break down fluid molecular
bonds and reduce viscosity,If the fluid is not resistant to this
tendency,then flow computations and pump designs must
account for expected changes in viscosity,Agitation of
hydraulic fluids in the presence of oxygen and water
accelerates the chemical breakdown of the fluid and the
formation of gums and varnishes,If the fluid reacts with active
metals such as silver and copper alloys,pump and valve
designs must change,or wear and early breakdown will result.
Where organic materials such as elastomers or electrical
insulations are in contact with the fluid,any reaction must be
beneficial or non-existent,As an example of a beneficial
reaction,many hydraulic fluids contain additives,which
promote some swealing in elastomers,This swealing aids the
sealing action of O-rings,Obviously,a shrinking action would
be very harmful.
搅
动
清
漆
合
成
橡
胶
燃烧，烧焦 使缩短
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
High operating temperatures accelerate each of the
chemical reactions,Nuclear irradiation tends to promote the
growth of polymeric chains and leads to greater viscosity
and eventual hardening.
When two surfaces,which have relative motion,are
separated by a real fluid,the fluid immediately adjacent to
each surface moves with that surface,Fig.7.2,The velocity
varies in proportion to the distance from the moving surface
film,Relative motion between adjacent laminae of fluid
means that a shearing stress exists between them,Newton's
law states that this shearing stress between adjacent laminae
of fluid of infinitesimal thickness is proportional to the rate
of shear in the direction perpendicular to the motion.
聚合的
无限小的 垂直的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
If a very small portion of velocity gradient is considered,
the velocity gradient is a straight-line having a slope of Δv/Δy.
In accordance with Newton's statement,the shear force along
the infinitesimally small distance Δy,which lies in a direction
perpendicular to the velocity vector is proportional to the
partial derivative of the velocity which respect to the lamina
thickness in the y direction.
S=μR 7.l
Where S = shear stress
R=v/y
μ is the constant of proportionality,μ is then the measure of
viscosity,It is called the absolute viscosity,μ has dimensions
of mass/length× time,In English units,absolute viscosity is
measured in units of lb-sec/in2 called Reyns,In metric units,
viscosity is given in Poise units,dyne-sec/cm2,Both of the
Reyn and the Poise are very large units,(c)
梯
度
偏导数
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Fluids,which obey Newton's law,7.1,are called
"Newtonian",At least during laminar flow conditions most
working hydraulic fluids are essentially Newtonian.
（ c） More convenient units are the microreyn (one millionth
of a Reyn) and the centipoise (one hundredth of a Poise).
Many calculations necessary to hydraulic system analysis
involve the use of the ratio of absolute viscosity to the oil
mass density,This ratio is called the kinematic viscosity ν
(nu).
ν = μ / ρ 7.2
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
although the ratio is neither kinematic nor a viscosity,The
unit of kinematic viscosity most frequently used in hydraulic
systems is the stoke,This has the dimensions of sq.cm/sec.
Because the stoke is a large unit,viscosity is often reported
in centistokes (cs = one hundredth of a stoke),The
comparable ratio in the English system is the Newt,which
has the dimensions of square inches per second,Because all
of the other parameters of interest to hydraulics engineers
are- in the United States,at least-given in English units,this
book will use the Newts as the primary measure of viscosity.
The conversion formula is
ν(Newts) = 0.001552 ν (cs) 7.8
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Industrial hydraulic fluid kinematic viscosity is usually
given in Saybolt Universal Seconds (SSU),SSU,as
determined by ASTM Procedure D445-53T,is the time
required for 60 milliliters of the oil at 100 degrees F to flow
through a standard orifice,Unlike the other units of viscosity,
SSU cannot be related to Newton's law,The conversion
formulae among Newts,centistokes and SSU are taken from
or extrapolated from ASTM Procedure D446-53 and D666-57
or ASTM Special Technical Publication No.43C.
t =seconds SSU
ν (cs) =0.226t - 195/t 32 < t < 100 7.4
ν (cs) =0.220t - 135/t 100 < t 7.5
ν (Newts) =0.00035t - 0.303/t 32 < t < 100 7.6
ν (Newts) = 0.000341t - 0.210/t 100 < t 7.7
Saybot Seconds 塞氏秒
美国材
料试验
标准
hyphen
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The largest percentage of hydraulic fluids are petroleum
derivatives,These are excellent working fluids but
unfortunately they burn vigorously,Industrial fires,aircraft
fires and shipboard fires have each been started by or
supported by burning petroleum based hydraulic oils,In order
to reduce this fire hazard,a number of fire resistant fluids
have been made available,These all tend to fall into one or
another of the following types.
l,Emulsions -water in oil
-oil in water
2,Water-glycol solutions
3,Straight synthetics
4,Blends of synthetics and petroleum oils
强有力的
乳化液
合成的
乙二醇
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
In the water-in-oil emulsion,the continuous phase is oil.
The water is present in droplets,The viscosities of such
fluids vary from about 0.0775 Newts (250 SSU) to about
0.342 Newts (1000 SSU),The largest viscosities are
associated with increased water content,These fluids are
usable from the freezing range up to about 175oF,Their
primary drawbacks are high density and high vapor pressure.
Oil-in-water emulsions contain droplets of oil in a water
base,Up to about 30-40% of oil,the viscosity is the same as
water,These emulsions are very hard on paints and
packings,Some suppliers recommend the use of nitrite
elastomers for seals,亚硝酸盐或（脂）
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Synthetic oils usually used are
either chlorinated hydrocarbons or
phosphate esters,Each of these are
good lubricants,primarily because
they react with metals to form weld
resisting surface films,These
synthetics tend to have low VI
(Viscosity Index) and to get very
viscous at low temperature,The
phosphate esters,in particular,attack
paints,organic coatings and electrical
insulation,Special formulations of
these materials are needed wherever
these fluids are used,Butyl rubber and
PTFE seals are necessary.
chlorinated hydrocarbons
烃氯化物
butyl rubber
异丁（烯）橡胶
PTFE 聚四氟乙烯
poly tetra fluoro ethylene
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Water-glycol fluids are true solutions,not merely emulsions.
A working mixture requires approximately 45% water,These
solutions have high VI's and low pour points,The viscosity
will rise as water evaporates,These fluids can be used from
about -10oF to nearly 180oF,Most available elastomers are not
affected and may be used,Metals and alloys containing zinc,
cadmium or magnesium should be avoided,Special paints are
required.
Submerged electrical coils and contacts are sometimes
used in solenoids and servo valves in hydraulic systems,The
life of these is enhanced if the fluid is a good dielectric.
Synthetic fluid should be non-toxic if ingested with food,
licked from the hands or breathed in as vapors,The fluid should
be tested with animals to assume man's safety,One problem
with such a technique is determining how many rats equal a
man!
浸
没
提
高
，
增
强
吸收
舔
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Effect of the fluid on skin and eyes is,perhaps,more
important than the effect of ingested oil,Dermatitis studies
should be made before any wide scale use of a new fluid be
attempted.
The fluid should also be non-allergenic and should have
either no odor or a pleasant odor.
Desirably the chosen fluid should be given a distinctive
color,It should not have an acid reaction,It should not form
harmful solutions or precipitates with water,It should have
high surface tension and should not be affected by nuclear
irradiation,It should flow readily at the lowest expected
operating temperature,Finally,it should be easily available and
low in cost.
皮炎过敏的
令人愉快的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
When it comes to(the greatest invention in the past
century),opinions differ from person to person,from my
point of view,(computer is the greatest invention),the
following points can well support my statement.
In the first place…in the second…finally/lastly…
For one thing…for another…last but not least
First of all…furthermore/moreover…in addition/additionally
Form what has been discussed above,I may safely come to
the conclusion that the most significant invention is
computer….
In short/all in all/in brief/in summary/to sum up/to conclude
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
When it comes to the (question),which is more important
(… or… )? some people think that … others argue that the
opposite is true.There is probably some truth in both
arguments,but I consider that … is more important.The
following points can well support my statement.
Form what had been discussed above,I may safely come
to the conclusion that …
When asked about the (question),which is more important
(…or…)?the whelming majority of people argue that…,I
hold the same view,The following points can well support
my statement.
UNIT SEVEN
HYDRAULIC TRANSMISSION
LESSON ONE INTRODUCTION
For many years a hydraulic system was
considered the result of plumbing together a
number of components,Also,a hydraulic
system has been considered merely the source
of power for control servos,Each of these
concepts can lead to basic errors,Following
either of these guide lines can result in
systems,which are inadequate,unstable,or a
combination of the two.
hydraulic 液力的
transmission 传动
plumbing
管件总称
component
组成部分
servo 伺服
伺服系统
inadequate
不充足的
unstable 不稳定的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
From an operational standpoint,any
hydraulic system can be divided into four
logical segments,The power input segment,the
power transmission system,the control devices
and the power output portion,The power input
segment usually consists of pumps and
accumulators,The power transmission system
includes tubing,tubing connectors,swivel
joints and flexible hose,Flow control valves,
pressure control valves,And relief valves make
up the control elements,The power output
portion usually consists of rotary and
oscillatory motor or linear actuators,Because
almost all practical hydraulic systems can be
divided into these same four basic segments,
they will be considered in detail later.
accumulator 蓄能器
tube connector
管接头
swivel joint
转节，旋转接头
flexible hose 软管
relief valve 溢流阀
oscillatory
摆动的，振荡的
actuator
执行机构
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Paralleling the improvement in hydraulic
fluids and other materials has been an
increase in the complexity and sophistication
of hydraulic systems,Most early hydraulic
systems were no more complex than the
system in an automobile hydraulic jack,The
jack contains a hand operated pump which
draws oil from a reservoir,Oil is pumped past
a check or non-return valve into a linear
actuator,The oil flowing into the actuator
forces the piston to move,The moving piston
can do work on an outside load,A valve
controls flow from the piston back to the
reservoir.
sophistication
改进，复杂化
pump 泵
draw 吸（入）
reservoir 油箱
check valve 单向阀
piston 活塞
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Through the use of power driven
pumps and directional control valves these
simple systems have grown so that a
number of loads can be handled
independently or sequentially from the same
power source,Power hydraulics is a power
amplifier in which a small effort applied to
a control device results in a large amount of
work performed on a load,With boost
servos,hydraulic power aids the operator’s
effort,Further refinement has led to the
irreversible servo-mechanism in which the
operator’s effort is separated from the load
by the rigidity of the oil in the actuating
cylinder or motor.
sequentially
相继的，结果的
amplifier 放大器
boost 促进
refinement
精确，精心安装
irreversible
不可逆的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
ADVANTAGES OF HYDRAULIC SYSTEMS
What are the reasons for using hydraulic power in
heavy-duty industrial,mobile and aircraft designs? Even
though pneumatic control systems preceded hydraulic
servos,and electrical power is in more widespread use
than hydraulic,systems which require high power
amplification or which must pack great energy into a small
space are almost invariably hydraulic.
If hydraulic power transmission and amplification
were to be compared with pneumatic,electrical and
mechanical systems,these are some of the reasons for
specifying hydraulic power.
amplification 放大，加强 invariably 不变的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Hydraulic motors possess an extremely high ratio of torque
to inertia,Because of this,very little of the available torque is
dissipated in accelerating the motor; most of the torque can be
applied directly to the load,Acceleration rates for unloaded
hydraulic motors,for example,vary from about 0.3?105radians
per second squared for larger sizes at 3000psi to 3.4?105radians
per second squared for smaller motors,No other transmission
system produces motors having acceleration rates anywhere
near these values,Higher supply pressures develop accordingly
higher acceleration rates.
Hydraulic motors develop no counter electro-motive force
as do electrical motors,Therefore,the acceleration rate is
essentially constant over their design speed ranges.
惯
性
消
除
加速度
psi=pound per square inches 每平方英寸磅
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Hydraulic devices can generate much more power in the
same volume than can electrical devices,The reason for this
lies in the characteristics of the materials used in electrical
equipment,The very best armature steels can develop only
the equivalent of bout 200 psi in tractive power before they
become saturated,By comparison,3000 psi hydraulic
equipment is very common and pressures up to 5000 psi are
not rare.
Because of the greater pressure density,hydraulic
equipment in power sizes greater than l hp,tends to be mach
smaller than electrical motors and somewhat smaller than
pneumatic motors.
Hydraulic motors support heavy or springy loads much
better than do electrical or pneumatic motors,Looking from
the load back into the motors,electrical fields or compressed
gasses are quite soft,The relatively incompressible oil is,
however,stiff.
使饱和 电枢，转子
horsepower
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Hydraulic transmission systems efficiently transmit
power farther than do mechanical systems,but not as far as
electrical systems.
Hydraulic transmission systems may be less hazardous
than electrical systems and are far safer than mechanical
power transmission systems.
Hydraulic oil is a good heat conductor so that the heavily
loaded working area stays relatively cool,Heat generated at
the working interface is conducted away rapidly,The
working areas can therefore be made small because heat
absorbing mass is not needed,Because of this and because
of the high-pressure density,hydraulic motors weigh less
than one pound per horsepower,Horsepower to weight
ratios of up to 2.5 are not uncommon.
危险的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Hydraulic power systems also have some
drawbacks,Hydraulic oils are messy,Leaks
are inevitable,The great lubricity of most
hydraulic oils makes leaks and spillage very
hazardous,Certain oils are excellent
removers of paint and electrical insulation.
Hydraulic systems,particularly the high
performance servo systems which use
hydraulic power so efficiently,are extremely
vulnerable to dirt and contamination,They
must constantly be watched to prevent the
addition or generation of contamination and
hydraulic systems need expensive filtration
to keep them clean.
drawback 欠缺
inevitable
不可避免的
spillage 溢出
insulation 绝缘
vulnerable 脆弱的
contamination
污染
filtration 过滤
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
A rupture or burst of an element of a high-pressure
hydraulic system can be very dangerous,The high velocity
streams of oil,which results can damage eyes,pierce skin,
and harm other equipment,The author has seen a piece of
tubing three feet long and 1.5 inches in diameter thrown
whirling for more than fifty feet when a rupture occurred in a
3000 psi line.
Unless fire-resistant fluids are used,hydraulic leaks create
very real fire hazards,Most petroleum based fluids are highly
flammable,High temperatures can be generated where a high
velocity stream of oil from a rupture impacts on a flat surface.
Finally,hydraulic power is so compact that developing
and controlling fractional horsepower is difficult unless the
pressure density is reduced to levels where electrical power is
extremely competitive.
破
裂
难燃工作液
易
燃
的 冲击
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
POWER SYSTEM ANALOGIES
Hydraulic systems within the laminar flow regime at least
function in ways,which are quite analogous to mechanical and
electrical systems,Later in these discussions the equations of
analogy will be developed,At this point,illustrations show the
analogies between systems,Fig.l.1 shows the analogies relating
hydraulic parameters to those of electrical systems and
translatory and rotary mechanical systems,Similar analogies
could be developed for pneumatic,thermal or other energy
systems,These elements can be combined in ways which at least
in electrical systems are termed "passive" networks,They are
passive because no energy is added to the system in the networks
though energy is lost through conversion to heat in the resistance
elements,These networks affect the stability of transmission
systems at various frequencies when subjected to periodic
oscillation such as sinusoidal cycling.
层流 范围，领域
图
解
平
动
的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Because of the increasing importance of the dynamics
of the hydraulic system in designing for high performance,
this book will relate-in a practical way-those factors which
affect the dynamical operation of the system to the design
of components and to the disposition of the several
components within the system,No attempt will be made to
develop design techniques other than to show previous
solutions to typical problems,Instead,this book will relate
the discussion of the several parts of the hydraulic system to
basic engineering principles,A sincere attempt will be made
to show how these basic principles can be brought to bear
on the solving of typical component and system design
problems.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Where relevant,recourse will be
made to the calculus to support the
development of equations but,in most
cases,a knowledge of the calculus will
not be necessary in order to solve
problems,Certain books may be of value
in gaining additional detailed
understanding of specific subjects,This
book,however,is intended to stand on
its own and all reasonable efforts have
been made to obviate the need for
additional resources.
relevant 有关的
recourse 求助
calculus 微积分
obviate 消除
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Some of the practices,which have developed in the
hydraulics industry make problem-solving difficult,Among
these are the use of feet head of water as a measure of
pressure and the use of Saybolt Universal seconds or metric
units for viscosity,In this book most such units have been
converted into their English equivalents,Through personal
preference,the author avoids confusion in conversion by
putting everything into units of
pound-force,inches and seconds.
Wherever this might conflict with
established practice such as using
units of Newts for kinematics
viscosity-conversion formulae are
provided.
粘
性
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Where the text material is appropriate,problems are
given to allow the student to practice the achieving of
solutions,These problems nearly always represent forms
of difficulty,which have faced the author at various
times,They are practical problems in advanced
hydraulics,In many cases,a solution is possible only
because simplifying assumptions were made,Because of
these assumptions,the hydraulics engineer should
beware of blind application of the formulae,Careful
consideration of effect of practical deviations from the
assumed conditions is necessary if success in
overcoming real difficulties is to be achieved.
assumption 假定 overcome 克服
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
HYDRAULIC FLUIDS
Clearly,the single most important material in the hydraulic
system is the working fluid,Modern hydraulic fluids are
often complex formulations carefully prepared to meet their
exacting tasks,In addition to a base fluid they may contain
additives to improve viscosity characteristics,to minimize
system corrosion,to make the fluids more stable to their
environments and to reduce foaming and flammability,Dr.
Roger Hatton,in his excellent book,Introduction to Hydraulic
Fluids”,gives the following as characteristics of importance to
a hydraulic fluid:
Good lubricity
Chemically and environmentally stable
Possesses favorable viscosity
Compatible with other system materials
泡沫 易燃性润滑性
一致的，兼容的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Heat transfer capability
High bulk modulus
Low volatility
Low foaming tendencies
Low weight
Fire resistant
Good dielectric properties
Nontoxic or allergenic
Nonmalodorous
Low in cost
Easily available
This is a formidable list,No working fluid possesses all of
these good characteristics,The task of system designer is to
choose or to formulate the fluid most suitable to his
requirements.
体积（容积）弹性模量
挥发性
绝缘的，不导电
无毒的
无恶臭的
过敏的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Lubricity has several connotations,some of which are in
conflict with others,Lubrication,in general,is concerned
with minimizing the shearing action of two surfaces between
which relative motion exists.
All surfaces of working parts,no matter how smoothly
made,consist-if enlarged sufficiently-of hilly contours,In the
absence of lubrication,the hills and valley of mating parts
tend to mesh,Were it not for the presence of oxides or fatty
acids on the surfaces,the mating faces would actually weld
together.
The function of lubricants is to minimize the effect of
shearing off the hills-or more properly,the asperities-of the
surfaces as they move relative to each other,This is done in a
number of ways.
含义，内涵
asperity 粗糙
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Dry lubricants such as lead powder
and silver plating interpose a greasy
metal between the asperities,The low
shear forces required by the greasy
metals help in reducing the sliding
forces,Other dry lubricants used are
graphite and molybdenum disulphide.
Each of these has crystal faces or
molecular bonds which are weak,The
relative movement then breaks these
bonds rather than shears asperities.
Properly applied,these are quite
effective.
lead powder 铅粉
silver plating 镀银的
interpose 插入
graphite 石墨
molybdenum 钼
molybdenum disulphide
二硫化钼
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Fluid lubrication,as usually provided by hydraulic
fluids,consists of interposing the liquid between the mating
faces,If enough fluid can be forced between the two
surfaces to separate them completely,hydrodynamic
lubrication is achieved,In this condition,the only forces
resisting the relative movement of the two surfaces are
those needed to shear the fluid,In hydrodynamic lubrication,
wear is negligible and the coefficient of friction is very low.
Air bearings,Kingsbury thrust bearings and well-designed
journal bearings achieve hydrodynamic lubrication.
Kingsbury thrust bearing 推力轴承
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
If the lubricating fluid does not separate the surfaces
completely,some shearing of the asperities will occur.
The ability of the fluid to maintain some separation,the
tendency for the fluid to be dragged between the shearing
surfaces,and the capability of the fluid to form no
welding surfaces on the newly-sheared faces,all
contribute to the lubrication
effect,The first two of these
factors relate strongly to fluid
viscosity,The last one depends
on the chemical natures of the
fluid and the moving surfaces.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The chemical nature of boundary lubrication is
described by the chemical polishing theory,Under
pressure,two mating metal surfaces will have areas of
contact,If relative movement occurs,local high
temperatures will develop at the points of contact,At these
high temperatures,certain fluids such as the phosphate
esters react chemically with the metal to form an alloy
having a melting temperature lower than that of the base
metal,The low melting alloy will flow away from the
pressure and generate a new distribution of forces,Thus,
galling or welding is avoided at the cost of relatively high
wear,Because the mechanism relates directly to fluid-
metal chemistry,one fluid can lubricate only a limited
range of metals in this way.
phosphate ester 磷酸脂 gall 擦伤，咬住
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
These forms of lubrication are known as boundary
layer lubrication,Most practical lubrication problems in
power hydraulic systems more nearly approximate
boundary layer lubrication than they do hydrodynamic
lubrication,Hydraulic fluid choice must be considered as
a boundary layer problem,The fluids have the proper
viscosity over the working temperature range to allow it to
stay in the working area,It should coat newly sheared
surfaces with fatty acids or metal salts in order to avoid
welding,It should have a low internal shear forces.
boundary layer 边界层
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Low viscosity gives low shear forces but allows the
fluid to be squeezed from between heavily loaded surfaces.
A viscous but chemically inert fluid such as a silicone may
minimize wear but not prevent galling and seizing caused
by pressure welding,A phosphate ester base or a fatty acid
base fluid may prevent welding but allow a high wear rate.
The lubricating aspect of the choice of a fluid must then be
a compromise among the factors of low coefficient of
friction,low wear and anti-galling.
Lubricity is of primary concern where moving parts
are in contact,Examples of these areas are pump and motor
gear teeth and piston-cylinder piston heads and rod ends,
valve spool and accumulator pistons.（ c）
折衷
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
（ c） Tests for oil lubricity attempt to relate these requirement
areas to standardized testing methods,Running heavily loaded
gears with the test oil,as a lubricant is a good test for gear
pumps but may not have much relevance for piston pumps.
Perhaps the most widely used test for oil lubricity is the 4-ball
test,In this,three l/2-inch diameter steel balls are held in a cup
with the test oil,A fourth ball is pressed down onto the other
three with a known load and then rotated at a known speed,The
torque to rotate the fourth ball can be measured to give the
coefficient of friction for the particular composition of balls.
After running for a specified period of time at a controlled
temperature,the wear scar on the balls is measured,The size of
this scar can be related-at least subjectively-to the wear resisting
qualities of the oil,If the load were increased until welding
occurred,the test could give a measure of the anti-seizing
quality of the oil,防胶合
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Stability of a fluid is no more precise a term than is
lubricity,A fluid is stable if its important characteristics
change little with use or during storage.
In use,the fluid is subjected to heavy mechanical shear
forces,The fluid will be exposed to entrapped air and
moisture,It will be in contact with organic materials with
which it may combine,The fluid may be subjected to each of
these while at high temperature or when under nuclear
radiation,While no real fluid is truly stable under all of
these conditions,some fluids meet combinations of
conditions better than do others.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Heavy shear forces tend to break down fluid molecular
bonds and reduce viscosity,If the fluid is not resistant to this
tendency,then flow computations and pump designs must
account for expected changes in viscosity,Agitation of
hydraulic fluids in the presence of oxygen and water
accelerates the chemical breakdown of the fluid and the
formation of gums and varnishes,If the fluid reacts with active
metals such as silver and copper alloys,pump and valve
designs must change,or wear and early breakdown will result.
Where organic materials such as elastomers or electrical
insulations are in contact with the fluid,any reaction must be
beneficial or non-existent,As an example of a beneficial
reaction,many hydraulic fluids contain additives,which
promote some swealing in elastomers,This swealing aids the
sealing action of O-rings,Obviously,a shrinking action would
be very harmful.
搅
动
清
漆
合
成
橡
胶
燃烧，烧焦 使缩短
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
High operating temperatures accelerate each of the
chemical reactions,Nuclear irradiation tends to promote the
growth of polymeric chains and leads to greater viscosity
and eventual hardening.
When two surfaces,which have relative motion,are
separated by a real fluid,the fluid immediately adjacent to
each surface moves with that surface,Fig.7.2,The velocity
varies in proportion to the distance from the moving surface
film,Relative motion between adjacent laminae of fluid
means that a shearing stress exists between them,Newton's
law states that this shearing stress between adjacent laminae
of fluid of infinitesimal thickness is proportional to the rate
of shear in the direction perpendicular to the motion.
聚合的
无限小的 垂直的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
If a very small portion of velocity gradient is considered,
the velocity gradient is a straight-line having a slope of Δv/Δy.
In accordance with Newton's statement,the shear force along
the infinitesimally small distance Δy,which lies in a direction
perpendicular to the velocity vector is proportional to the
partial derivative of the velocity which respect to the lamina
thickness in the y direction.
S=μR 7.l
Where S = shear stress
R=v/y
μ is the constant of proportionality,μ is then the measure of
viscosity,It is called the absolute viscosity,μ has dimensions
of mass/length× time,In English units,absolute viscosity is
measured in units of lb-sec/in2 called Reyns,In metric units,
viscosity is given in Poise units,dyne-sec/cm2,Both of the
Reyn and the Poise are very large units,(c)
梯
度
偏导数
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Fluids,which obey Newton's law,7.1,are called
"Newtonian",At least during laminar flow conditions most
working hydraulic fluids are essentially Newtonian.
（ c） More convenient units are the microreyn (one millionth
of a Reyn) and the centipoise (one hundredth of a Poise).
Many calculations necessary to hydraulic system analysis
involve the use of the ratio of absolute viscosity to the oil
mass density,This ratio is called the kinematic viscosity ν
(nu).
ν = μ / ρ 7.2
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
although the ratio is neither kinematic nor a viscosity,The
unit of kinematic viscosity most frequently used in hydraulic
systems is the stoke,This has the dimensions of sq.cm/sec.
Because the stoke is a large unit,viscosity is often reported
in centistokes (cs = one hundredth of a stoke),The
comparable ratio in the English system is the Newt,which
has the dimensions of square inches per second,Because all
of the other parameters of interest to hydraulics engineers
are- in the United States,at least-given in English units,this
book will use the Newts as the primary measure of viscosity.
The conversion formula is
ν(Newts) = 0.001552 ν (cs) 7.8
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Industrial hydraulic fluid kinematic viscosity is usually
given in Saybolt Universal Seconds (SSU),SSU,as
determined by ASTM Procedure D445-53T,is the time
required for 60 milliliters of the oil at 100 degrees F to flow
through a standard orifice,Unlike the other units of viscosity,
SSU cannot be related to Newton's law,The conversion
formulae among Newts,centistokes and SSU are taken from
or extrapolated from ASTM Procedure D446-53 and D666-57
or ASTM Special Technical Publication No.43C.
t =seconds SSU
ν (cs) =0.226t - 195/t 32 < t < 100 7.4
ν (cs) =0.220t - 135/t 100 < t 7.5
ν (Newts) =0.00035t - 0.303/t 32 < t < 100 7.6
ν (Newts) = 0.000341t - 0.210/t 100 < t 7.7
Saybot Seconds 塞氏秒
美国材
料试验
标准
hyphen
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The largest percentage of hydraulic fluids are petroleum
derivatives,These are excellent working fluids but
unfortunately they burn vigorously,Industrial fires,aircraft
fires and shipboard fires have each been started by or
supported by burning petroleum based hydraulic oils,In order
to reduce this fire hazard,a number of fire resistant fluids
have been made available,These all tend to fall into one or
another of the following types.
l,Emulsions -water in oil
-oil in water
2,Water-glycol solutions
3,Straight synthetics
4,Blends of synthetics and petroleum oils
强有力的
乳化液
合成的
乙二醇
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
In the water-in-oil emulsion,the continuous phase is oil.
The water is present in droplets,The viscosities of such
fluids vary from about 0.0775 Newts (250 SSU) to about
0.342 Newts (1000 SSU),The largest viscosities are
associated with increased water content,These fluids are
usable from the freezing range up to about 175oF,Their
primary drawbacks are high density and high vapor pressure.
Oil-in-water emulsions contain droplets of oil in a water
base,Up to about 30-40% of oil,the viscosity is the same as
water,These emulsions are very hard on paints and
packings,Some suppliers recommend the use of nitrite
elastomers for seals,亚硝酸盐或（脂）
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Synthetic oils usually used are
either chlorinated hydrocarbons or
phosphate esters,Each of these are
good lubricants,primarily because
they react with metals to form weld
resisting surface films,These
synthetics tend to have low VI
(Viscosity Index) and to get very
viscous at low temperature,The
phosphate esters,in particular,attack
paints,organic coatings and electrical
insulation,Special formulations of
these materials are needed wherever
these fluids are used,Butyl rubber and
PTFE seals are necessary.
chlorinated hydrocarbons
烃氯化物
butyl rubber
异丁（烯）橡胶
PTFE 聚四氟乙烯
poly tetra fluoro ethylene
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Water-glycol fluids are true solutions,not merely emulsions.
A working mixture requires approximately 45% water,These
solutions have high VI's and low pour points,The viscosity
will rise as water evaporates,These fluids can be used from
about -10oF to nearly 180oF,Most available elastomers are not
affected and may be used,Metals and alloys containing zinc,
cadmium or magnesium should be avoided,Special paints are
required.
Submerged electrical coils and contacts are sometimes
used in solenoids and servo valves in hydraulic systems,The
life of these is enhanced if the fluid is a good dielectric.
Synthetic fluid should be non-toxic if ingested with food,
licked from the hands or breathed in as vapors,The fluid should
be tested with animals to assume man's safety,One problem
with such a technique is determining how many rats equal a
man!
浸
没
提
高
，
增
强
吸收
舔
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Effect of the fluid on skin and eyes is,perhaps,more
important than the effect of ingested oil,Dermatitis studies
should be made before any wide scale use of a new fluid be
attempted.
The fluid should also be non-allergenic and should have
either no odor or a pleasant odor.
Desirably the chosen fluid should be given a distinctive
color,It should not have an acid reaction,It should not form
harmful solutions or precipitates with water,It should have
high surface tension and should not be affected by nuclear
irradiation,It should flow readily at the lowest expected
operating temperature,Finally,it should be easily available and
low in cost.
皮炎过敏的
令人愉快的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
When it comes to(the greatest invention in the past
century),opinions differ from person to person,from my
point of view,(computer is the greatest invention),the
following points can well support my statement.
In the first place…in the second…finally/lastly…
For one thing…for another…last but not least
First of all…furthermore/moreover…in addition/additionally
Form what has been discussed above,I may safely come to
the conclusion that the most significant invention is
computer….
In short/all in all/in brief/in summary/to sum up/to conclude
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
When it comes to the (question),which is more important
(… or… )? some people think that … others argue that the
opposite is true.There is probably some truth in both
arguments,but I consider that … is more important.The
following points can well support my statement.
Form what had been discussed above,I may safely come
to the conclusion that …
When asked about the (question),which is more important
(…or…)?the whelming majority of people argue that…,I
hold the same view,The following points can well support
my statement.
