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Lesson 16 Basic Concepts of Mechanisms
A
combination
of
interrelated
parts(
相关零件的组合体
)
having
definite
（确
定的）
motions and capable of performing
useful work may be called machine.
具有确定的运动轨迹
,
并且能做有用的功联
接在一起的零件的组合称为机器
.
A
mechanism is
a
component
of
a
machine
consisting
of
two
or
more
bodies
arranged
so that the motion
of one compels the motion of the others.
机构是机器的组成要素
.
机器由二个及以上的机构组合而成
.
因此其中一个构件的运动会
带动其它
的构件的连锁运动
.
Kinematics is the
study of motion in mechanisms without reference to
the forces
通常意义所说的杆件包括凸轮
,
齿轮以及它的机器零件的曲柄
,
连杆机构和其
它的销件连
接部分
.
Frame
The fixed or
stationary link in a mechanism is called the frame
机架
机械装置中固定或静止不动的构件称为机架
.
When there is no link that is actually
fixed, we may consider one as being fixed
and determine the motion of the other
links relative to it
如果实际中确实没有构件是固定的
,
我们可以假设其中某个是固定的
,
这样可以判断出其
它与之相连构件的运动
.
In an automotive engine, for example,
the engine block (see Fig.14.1) is
that
act on the mechanism
运动学仅研究机构的运动而不考虑作用在机构上的力
.
Dynamics is the study of motion
individual bodies and mechanisms under the
influence of forces and torques.
动力学研究独立构件的运动及力和扭矩作用下对机构的影响
.
The study of forces and torques in
stationary systems (and systems with
negligible inertial effects) is called
statics
研究平衡中的力和扭矩
,
该系统忽略惯性影响
,
称之为静力学
.
Synthesis is the procedure by which
a product (a mechanism, for example) is
developed to satisfy a set of
performance requirements
合成是一个程序
,
通过该程序能够不断改进一个产品
(
例如一个机构
)
直到满足一系列的
性能要求
.
If a product
configuration is tentatively specified and then
examined to
determine whether the
performance requirements are met, the process is
called analysis.
如果需要探索性的详细阐
述产品的构造结构
,
然后检验
,
判断该产品是否满足性能要求
,
这个过程称为
分解
.
The design of mechanisms
involves both synthesis and analysis.
机构设计包括合成和分解
.
The
design process begins with the recognition of a
need. A set of requirements
is then
listed
设计从对于需求的确认开始
,
< br>罗列出一系列的要求
.
Detailed
analyses of displacements, velocities, and
accelerations are usually
required
通常需要对位移
,
速度
,
加速度进行具体分析
.
This
part
of
the
design
process
is
followed
by an
analysis
of
forces
and
torques.
设计过程中的这个部分是紧跟在对力和扭矩的分析之后
.
The
design
process
may
continue
long
after
（很久以后）
the
first
models
have
been
produced and may
include redesigns of components that affect
velocities,
accelerations, forces, and
torques
在制造完毕第一个模型之后
,
该设计过程仍有可能持续很长一段时间
.
并且有可能
会重
新设计这些会影响速度
,
加速度及
所知扭矩的零部件
In order to
successfully compete from year to year, most
manufactures must
continuously modify
their product and their methods of production
为了在年复一年的技术中取得优势
,
大多数制造
商必须不断调整修改他们的产品及加工
方式
.Increases in the production rate,
upgrading of product performance, redesign
for cost and weight reduction are
frequently required
,
为了增加出产率
,
提高产品的性能
,
< br>经常需要重新设计来降低成本以及减轻重量
.
Success
may
hinge
on
（决定于）
the
accuracy
of
the
kinematic
and
dynamic
analysis
对问题进行正确的运动学和动力学分析是成功的关键
.
Many of the basic linkage
configurations have been incorporated into
< br>（成
为……的一部分）
machines
designed
centuries
ago,
and
the
terms
we
use
to
describe
them
have changed over the years
很久以前
,
许多基本的连杆构造就已经运用在机器设计中去了
,
并且我们用来称呼它们的
术语在不断的变化
,.
Thus, definitions and terminology
are not consistent throughout thetechnical
literature.
因此技术供应文献中的定义和说明不是一成不变的
In
most
cases,
however,
meanings
will
be
clear
from
the
context
of
the
descriptive
matter.
在大多数的情况下
,
然而定义
,
义意可以在的解释性的描述内容中清晰的获
得
,
A few terms of particular
interest to the study of kinematics and dynamics
of
machines are defined below
下面定义了在学习
,
研究机构运动学和动力学中一些特别重
要的术语
.
Link
A link is one of the rigid bodies or
members joined together to form
a
kinematic chain.
杆件
是一个刚体或者连接在一起形成运动链的零部件
,
因此
被称为刚性构件或者是简
单构件
.
The term rigid link, or sometimes
simply link, is an idealization used in the
study of mechanisms that does not
consider small deflections due to strains in
machine members
刚性构件是在机械装置中的
理想化模型
.
这样就不无需考虑由机器零部件所产生的应力
造成的微小偏移量
.
A
perfectly
rigid
or
inextensible
link
can
exist
only
as
a
textbook
type
of
model
of a real machine
member.
一个绝对刚性或不可延展的构件只能存在于教科书上的机械零部件的模
型
.
For typical machine
parts, the maximum changes in dimension are on the
order of
only a one-thousandth of the
part length.
对于典型的零部件
,
最大的尺寸变化量仅仅是沿着该部件长度方向的千分之一
.
We
are
justified
（合理的）
in
neglecting
this
small
motion
when
considering
the
much greater motion characteristic of
most mechanisms
当考虑到大多数的机构相对较为显着的运动特征
,
忽略这点微量的偏移是合情合理的
.
The
word
link
is
used
in
a
general
sense
to
include
cams,
gears,
and
other
machine
members in addition
to cranks, connecting rods and other pin-
connected
（铰接）
components
considered the
frame, even though the automobile may be moving
例如汽车发动机
,
我们通常是把发动机气缸
视为机架
,
让它保持静止不动
,
实际上整部汽
车者是运动的
.
Degree of Freedom
The
number of degrees of freedom of a linkageis the
number
of
independent
parameters
required
to
specify
the
position
of
every
link
relative
to
the frame or fixed link
自由度
连杆机构的自由度是给定独立参数的数目，需要自由度来确定某一个与机架或
固定杆件相连接的构件的位置，
If the
instantaneous configuration of a system may be
completely defined be
specifying one
independent variable, that system has one degree
of freedom
如果系统的瞬间结构完全可以由一个独立的变量来确定，这样系
统就具有一个自由度，
An unconstrained
rigid body has six degrees of freedom:
大多数实际上运用的机构装置只有一个自由度。
translation in three coordinate
directions and rotation about three coordinate
axes
一个不受约束的刚体它有六个自由度，沿着三个坐标
平面的平移和绕着三个坐标轴的转
动。
If
the
body
is
restricted
to
motion
in
a
place,
there
are
three
degrees
of
freedom:
?
如果刚体受到约束在平面上移动，那么就有三个自由度，
?
translation in
two coordinate directions and rotation within the
plane.
沿着两个坐标方向的移动以及在平面方向的转动。
Lower and Higher Pairs Connections
between rigid bodies consist of lower
and higher pairs of elements.
低副和高副
刚体连接的构件由低副和高副组成，
The two elements of a lower pair have
theoretical surface contact with one
another
低副的两个构件理论上是通过平面接触。
while the two elements of a higher pair
have theoretical point or line contact
(if we disregard deflections).
高副的两个构件理论上是点和线的接触（如果我们忽略变形）。
Kinematic
Chain A
kinematic
chain
is
an
assembly
（集合）
of
links
and
pairs
(joints).
Each
link
in
a
closed-loop
kinematic
chain
is
connected
to
two
or more
other links
运动链
是若干个构件或运动链联接结
合在一起，
在闭式运动链中，
每个构件都有相同两
个及两个以上的其它构件相连接
,
A
linkage failing to meet the closed-loop criterion
is an open-loop kinematic
chain.
运动链没有满足闭式运动链的标准则称为开环运动链。
Linkage
Since
we
may
wish
to
examine
kinematic
chains
without
regard
to
their
ultimate
（最后）
use,
it
is
convenient
to
identify
any
assemblage
of
rigid
bodies
connected by kinematic joints as a
linkage
连杆机构
由于我
们希望检验运动链而不考虑它们的最终用途
,
可以非常方便的区
分出任何做为连接机构的刚体的集合
,
这些连杆机构刚体是通过运动链连接在一起的，
Thus,
both mechanisms and machines may be considered
linkages
因此机构与机器都被视为连杆机构
,
In some references, however, the term
linkage is restricted to kinematic chains
made up of lower pairs.
然而在某
些参考文献中
,linkage
这个词仅仅限于由低副组成的运
动链。
Planar Motion and Planar
Linkages If all points in a linkage may be
described as a planar linkage
平面移动和平面连杆机构
如果连杆
机构上所有的点都同一平面内移动
,
则说明系
< br>统正在平移
.
If we examine the
linkage made up of the crank, connecting rod, and
piston
in a piston engine, we see that
it is a planar linkage.
在活塞发动机中由曲柄
,
连杆和活塞组成的连杆机构
,
< br>我们视为平面连杆机构
.
Most of the
mechanisms in common use may be treated as planar
linkages.
大多数普遍使用的机械装置都被视平面连杆机构
.
Spatial Motion and Spatial Linkages
空间运动和空间连杆机构
The
more general case in which motion cannot be
described as taking place in
parallel
planes is called spatial motion,
在更多一般
情况下
,
不能在平面内进行运动的称为空间运动
.
and the linkage may be described
as a spatial or three-dimensional (3D)
linkage.
该连杆机构被称为空间或三维连杆机构
,
Inversion
The absolute
motion of a linkage depends on which of the links
is fixed, that is, which link is
selected as the frame
连杆机构的绝对移动取决于哪根连杆是
构件是固定的
,
简而言之就是选择哪根杆件
做为机架
,
If two otherwise
equivalent linkages(
等效连杆机构
)
have different fixed
link,
如果两根相当的连杆机构的固定机架是不同的
,
then each is an inversion of the other.
Relative motion will be the same in
both linkages.
那么一个就是另外一个的倒置<
/p>
.
这在两个连杆机构中相当的运动都是相
同的
.
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