light机械运动和动力学中英文对照外文翻译文献

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文档含英文原文和中文翻译

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中英文对照翻译

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Kinematics and dynamics of machinery

One princple aim of kinemarics is to creat the designed motions of the subject mechanical

parts and then mathematically compute the positions, velocities ,and accelerations ,which those

motions

will

creat

on

the

parts.

Since

,for

most

earthbound

mechanical

systems

,the

mass

remains essentially constant with time,defining the accelerations as a function of time then also

defines the dynamic forces as a function of time. Stress,in turn, will be a function of both applied

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and inerials forces . since engineering design is charged with creating systems which will not fail

during

their

expected

service

life,the

goal

is

to

keep

stresses

within

acceptable

limits

for

the

materials

chosen

and

the

environmental

conditions

encountered.

This

obvisely

requies

that

all

system

forces

be

defined

and

kept

within

desired

limits.

In

mechinery

,

the

largest

forces

encountered are often those due to the dynamics of the machine itself. These dynamic forces are

proportional to acceletation, which brings us back to kinematics ,the foundation of mechanical

design.

Very

basic

and

early

decisions

in

the

design

process

invovling

kinematics

wii

prove

troublesome and perform badly.

Any

mechanical

system

can

be

classified

according

to

the

number

of

degree

of

freedom

which it systems DOF is equal to the number of independent parameters which are

needed to uniquely define its posion in space at any instant of time.

A rigid body free to move within a reference frame will ,in the general case, have complex

motoin,

which

is

simultaneous

combination

of

rotation

and

translation.

In

three-dimensional

space

,

there

may

be

rotation

about

any

axis

and

also

simultaneous

translation

which

can

be

resoled

into

componention

along

three

axes,

in

a

plane

,or

two-dimentional

space

,complex

motion becomes a combination of simultaneous along two axes in the plane. For simplicity ,we

will limit our present discusstions to the case of planar motion:

Pure

rotation

the

body

pessesses

one

point

(center

of

rotation)which

has

no

motion

with

respect to the stationary frame of reference. All other points on the body describe arcs about that

center.

A

reference

line

drawn

on

the

body

through

the

center

changes

only

its

angulai

orientation.

Pure translation all points on the body describe parallel paths. A reference line drawn on the

body changes its linear posion but does not change its angular oriention.

Complex

motion

a

simulaneous

combination

of

rotion

and

translationm

.

any

reference line drawn on the body will change both its linear pisition and its angular orientation.

Points on the body will travel non-parallel paths ,and there will be , at every instant , a center of

rotation , which will continuously change location.

Linkages

are

the

bacis

building

blocks

of

all

mechanisms.

All

common

forms

of mechanisms (cams , gears ,belts , chains ) are in fact variations of linkages. Linkages are made

up of links and kinematic pairs.

A link is an (assumed)rigid body which possesses at least two or more links (at their nodes),

which connection allows some motion, or potential motion,between the connected links.

The term lower pair is used to describe jionts with surface contact , as with a pin surrounded

by a hole. The term higher pair is used to describe jionts with point or line contact ,but if there is

any clerance between pin and hole (as there must be for motion ),so-called surface contact in the

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pin jiont actually becomes line contact , as the pin contacts actually has contact only at discrete

points , which are the tops of the surfaces

’

asperities. The main practical advantage of lower

pairs over higher pairs is

their better ability to

trap

lubricant

between their envloping surface.

This ie especially true for the rotating pin joint. The lubricant is more easily squeezed out of a

higher pair .as s result , the pin joint is preferred for low wear and long life .

When designing machinery, we must first do a complete kinematic analysis of our design ,

in order to obtain information about the acceleration of the moving parts .we next want te use

newton

’

s second law to caculate the dynamic forces, but to do so we need to know the masses of

all the moving parts which have these known acceletations. These parts do not exit yet ! as with

any design in order to make a first pass at the caculation . we will then have to itnerate to better

an better solutions as we generate more information.

A first estimate of your parts

’

masses can be obtained by assuming some reasonable shapes

and size for all the parts and choosing approriate materials. Then caculate the volume of each

part and multipy its volume by material

’

s mass density (not weight density ) to obtain a first

approximation of its mass . these mass values can then be used in Newton

’

s equation.

How will we know whether our chosen sizes and shapes of links are even acceptable, let

alone optimal ? unfortunately , we will not know untill we have carried the computations all the

way through a complete stress and deflection analysis of the parts. It it often the case ,especially

with

long

,

thin

elements

such

as

shafts

or

slender

links

,

that

the

deflections

of

the

parts,

redesign them ,and repeat the force ,stress ,and deflection analysis . design is , unavoidably ,an

iterative process .

It is also worth nothing that ,unlike a static force situation in which a failed design might be

fixed by adding more mass to the part to strenthen it ,to do so in a dynamic force situation can

have a deleterious effect . more mass with the same acceleration will generate even higher forces

and thus higher stresses ! the machine desiger often need to remove mass (in the right places)

form parts in order to reduce the stesses and deflections due to F=ma, thus the designer needs to

have

a

good

understanding

of

both

material

properties

and

stess

and

deflection

analysis

to

properlyshape

and

size

parts

for

minimum

mass

while

maximzing

the

strength

and

stiffness

needed to withstand the dynamic forces.

One of the primary considerations in designing any machine or strucre is that the strength

must be sufficiently greater than the stress to

assure both safety and reliability. To assure that

mechanical parts do not fail in service ,it is necessary to learn why they sometimes do fail. Then

we shall be able to relate the stresses with the strenths to achieve safety .

Ideally, in designing any machine element,the engineer should have at his disposal should

have been made on speciments having the same heat treatment ,surface roughness ,and size as

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the element he prosses to design and the tests should be made under exactly the same loading

conditions as the part will experience in service . this means that ,if the part is to experience a

bending

and

torsion,it

should

be

tested

under

combined

bending

and

torsion.

Such

tests

will

provide very useful and precise information . they tell the engineer what factor of safety to use

and what the reliability is for a given service life .whenever such data are available for design

purposes,the engineer can be assure that he is doing the best justified if failure of the part may

endanger human life ,or if the part is manufactured in sufficiently large quantities. Automobiles

and refrigrerators, for example, have very good reliabilities because the parts are made in such

large quantities that they can be thoroughly tested in advance of manufacture , the cost of making

these is very low when it is divided by the total number of parts manufactrued.

You can now appreciate the following four design categories :

(1)failure

of

the

part

would

endanger

human

life

,or

the

part

ismade

in

extremely

large

quantities consequently, an elaborate testingprogram is justified during design .

(2)the part is made in large enough quantities so that a moderate serues of tests is feasible.

(3)The part is made in such small quantities that testing is not justified at all or the design

must be completed so rapidlly that there is not enough time for testing.

(4)

The

part

has

already

been

designed,

manufactured,

and

tested

and

found

to

be

unsatisfactory. Analysis is required to understand why the part is unsatisfactory and what to do

to improve it .

It

is

with

the

last

three

categories

that

we

shall

be

mostly

means

that

the

designer

will

usually

have

only

published

values

of

yield

strenth

,

ultimate

strength,and

percentage elongation . with this meager information the engieer is

expected to design against

static and dynamic loads, biaxial and triaxial stress states , high and low temperatures,and large

and

small

parts!

The

data

usually

available

for

design

have

been

obtained

from

the

simple

tension

test

,

where

the

load

was

applied

gradually

and

the

strain

given

time

to

develop.

Yet

these

same

data

must

be

used

in

designing

parts

with

complicated

dynamic

loads

applied

thousands of times per minute . no wonder machine parts sometimes fail.

To sum up, the fundamental problem of the designer is to use the simple tension test data

and relate them to the strength of the part , regardless of the stress or the loading situation.

It is possible for two metal to have exactly the same strength and hardness, yet one of these

metals may have a supeior ability to aborb overloads, because of the property called ductility.

Dutility is measured by the percentage elongation which occurs in the material at

frature. The

usual divding line between ductility and brittleness is 5 percent elongation. Amaterial having less

than

5

percent

elongation

at

fracture

is

said

to

bebrittle,

while

one

having

more

is

said

to

be

ductile.

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The elongation of a material is usuallu measured over 50mm gauge

this did not

a measure of the actual strain, another method of determining ductility is sometimes used . after

the speciman has been fractured, measurements are made of the area of the cross section at the

fracture. Ductility can then be expressed as the percentage reduction in cross sectional area.

The

characteristic

of

a

ductile

material

which

permits

it

to

aborb

largeoverloads

is

an

additional

safety

factot

in

design.

Ductility

is

also

important

because

it

is

a

measure

of

that

property

of

a

material

which

permits

it

to

be

cold-worked .such

operations

as

bending

and

drawing are metal-processing operations which require ductile materials.

When a materals is to be selected to resist wear , erosion ,or plastic deformaton, hardness is

generally

the

most

important

property.

Several

methods

of

hardness

testing

are

available,

depending upon which particular property is most desired. The four hardness numbers in greatest

usse are the Brinell, Rockwell,Vickers, and Knoop.

Most hardness-testing systems employ a standard load which is applied to a ball or pyramid

in contact with the material to be tested. The hardness is an easy property to measure , because

the test is nondestructive and test specimens are not required . usually the test can be conducted

directly on actual machine element .

Virtually all machines contain shafts. The most common shape for shafts is circular and the

cross

section

can

be

either

solid

or

hollow

(hollow

shafts

can

result

in

weight

savings).

Rectangular shafts are sometimes used ,as in screw driver bladers ,socket wrenches and control

knob stem.

A shaft must have adequate torsional strength to transmit torque and not be over stressed. If

must

also

be

torsionally

stiff

enough

so

that

one

mounted

component

does

not

deviate

excessively

from

its

original

angular

position

relative

to

a

second

component

mounted

on

the

same shaft. Generally speaking,the angle of twist should not exceed one degree in a shaft length

equal to 20 diameters.

Shafts

are

mounted

in

bearing

and

transmit

power

through

such

device

as

gears,

pulleys,cams and clutches. These devices introduce forces which attempt to bend the shaft;hence,

tha shaft must be rigid enough to prevent overloading of the supporting bearings ,in general, the

bending deflection of a shaft should not exceed 0.01 in per ft of length between bearing supports.

In addition .the shaft must be able to sustain a combination of bending and torsional loads.

Thus an equivalent load must be considered which takes into account both torsion and bending .

also ,the allowable stress must contain a factor of safety which includes fatigue, since torsional

and bending stress reversals occur.

For fiameters less than 3 in ,the usual shaft material is cold-rolled steel containing about 0.4

percent carbon. Shafts ate either cold- rolled or forged in sizes from 3in. to 5 in. for sizes above 5

5

in.

shafts

are

forged

and

machined

to

size

.

plastic

shafts

are

widely

used

for

light

load

applications . one advantage of using plastic is safty in electrical applications, since plastic is a

poor confuctor of electricity.

Components such as gears and pulleys are mounted on shafts by means of key. The design

of the key and the corresponding keyway in the shaft must be properly evaluated. For example,

stress

concentrations

occur

in

shafts

due

to

keyways

,

and

the

material

removed

to

form

the

keyway further weakens the shaft.

If shafts are run at critical speeds , severe vibrations can occur which can seriously damage

a

machine .it

is

important

to

know

the

magnitude

of

these

critical

speeds

so

that

they

can

be

avoided. As a general rule of thumb , the difference betweem the operating speed and the critical

speed should be at least 20 percent.

Many

shafts

are

supported

by

three

or

more

bearings,

which

means

that

the

problem

is

statically indeterminate .text on strenth of materials give methods of soving such problems. The

design effort should be in keeping with the economics of a given situation , for example , if one

line shaft supported by three or more bearings id needed , it probably would be cheaper to make

conservative assumptions as to moments and design it as though it were determinate . the extra

cost of an oversize shaft may be less than the extra cost of an elaborate design analysis.

Another important aspect of shaft design is the method of directly connecting one shaft to

another , this is accomplished by devices such as rigid and flexiable couplings.

A

coupling

is

a

device

for

connecting

the

ends

of

adjacent

shafts.

In

machine

construction , couplings are used to effect a semipermanent connection between adjacent rotating

shafts

,

the

connection

is

permanent

in

the

sense

that

it

is

not

meant

to

be

broken

during

the

useful life of the machinem , but it can be broken and restored in an emergency or when worn

parts are replaced.

There are several types of shaft couplings, their characteristics depend on the purpose for

which

they

are

used

,

if

an

exceptionally

long

shaft

is

required

in

a

manufacturing

plant

or

a

propeller shaft on a ship , it is made in sections that are coupled together with rigid couplings. A

common type of

rigid coupling

consists of two

mating radial flanges

that

are attached by

key

driven hubs to the ends of adjacent shaft sections and bolted together through the flanges to form

a rigid connection. Alignment of the connected shafts in usually effected by means of a rabbet

joint on the face of the flanges.

In

connecting

shafts

belonging

to

separate

device

(

such

as

an

electric

motor

and

a

gearbox),precise aligning of the shafts is difficult and a fkexible coupling is used . this coupling

connects the shafts in such a way as to minimize the harmful effects of shafts misalignment of

loads

and

to

move

freely(float)

in

the

axial

diection

without

interfering

with

one

another .

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