机械专业毕业设计外文翻译--振动的定义和术语

翻译：

英文原文

Definitions and Terminology of Vibration

vibration

All matter-solid, liquid and gaseous-is capable of vibration, e.g. vibration of

gases occurs in tail ducts of jet engines causing troublesome noise and sometimes

fatigue cracks in the metal. Vibration in liquids is almost always longitudinal and can

cause large forces because of the low compressibility of liquids, e.g. popes conveying

water can be subjected to high inertia forces (or

“

water hammer

”

) when a valve or tap

is suddenly closed. Excitation forces caused, say by changes in flow of fluids or

out-of-balance rotating or reciprocating parts, can often be reduced by attention to

design and manufacturing details. Atypical machine has many moving parts, each of

which is a potential source of vibration or shock-excitation. Designers face the

problem of compromising between an acceptable amount of vibration and noise, and

costs involved in reducing excitation.

The mechanical vibrations dealt with are either excited by steady harmonic

forces ( i. e. obeying sine and cosine laws in cases of forced vibrations ) or, after an

initial disturbance, by no external force apart from gravitational force called weight ( i.

e. in cases of natural or free vibrations). Harmonic vibrations are said to be

“

simple

”

if

there is only one frequency as represented diagrammatically by a sine or cosine wave

of displacement against time.

Vibration of a body or material is periodic change in position or displacement

from a static equilibrium position. Associated with vibration are the interrelated

physical quantities of acceleration, velocity and displacement-e. g. an unbalanced

force causes acceleration (a = F/m ) in a system which, by resisting, induces vibration

as a response. We shall see that vibratory or oscillatory motion may be classified

broadly as (a) transient; (b) continuing or steady-state; and (c) random.

Transient Vibrations

die away and are usually associated with irregular

disturbances, e. g. shock or impact forces, rolling loads over bridges, cars driven over

pot holes-i. e. forces which do not repeat at regular intervals. Although transients are

temporary components of vibrational motion, they can cause large amplitudes initially

and consequent high stress but, in many cases, they are of short duration and can be

ignored leaving only steady-state vibrations to be considered.

Steady- State Vibrations

are often associated with the continuous operation

of machinery and, although periodic, are not necessarily harmonic or sinusoidal. Since

vibrations require energy to produce them, they reduce the efficiency of machines and

mechanisms because of dissipation of energy, e. g. by friction and consequent

heat-transfer to surroundings, sound waves and noise, stress waves through frames

and foundations, etc. Thus, steady-state vibrations always require a continuous energy

input to maintain them.

Random Vibration

is the term used for vibration which is not periodic, i. e.

has no made clear-several of which are probably known to science students already.

Period, Cycle, Frequency and Amplitude

A steady-state mechanical

vibration is the motion of a system repeated after an interval of time known as the

period. The motion completed in any one period of time is called a cycle. The number

of cycles per unit of time is called the frequency. The maximum displacement of any

part of the system from its static-equilibrium position is the amplitude of the vibration

of that part-the total travel being twice the amplitude. Thus,

“

amplitude

”

is not

synonymous with

“

displacement

”

but is the maximum value of the displacement from

the static-equilibrium position.

Natural

and

Forced

Vibration

A

natural

vibration

occurs

without

any

external force except gravity, and normally arises when an elastic system is displaced

from a position of stable equilibrium and released, i. e. natural vibration occurs under

the action of restoring forces inherent in an elastic system, and natural frequency is a

property of he system.

A

forced

vibration

takes

place

under

the

excitation

of

an

external

force

(or

externally applied oscillatory disturbance)

which is usually a function of time, e. g.

in

unbalanced rotating parts, imperfections in

manufacture of

gears and

drives. The

frequency

of

forced

vibration

is

that

of

the

exciting

or

impressed

force,

i.

e.

the

forcing frequency is an arbitrary quantity independent of the natural frequency of the

system.

Resonance

Resonance describes the condition of maximum amplitude. It

occurs

when

the

frequency

of

an

impressed

force

coincides

with,

or

is

near

to

a

natural

frequency

of

the

system.

In

this

critical

condition,

dangerously

large

amplitudes and stresses may occur in mechanical systems but, electrically, radio and

television receivers are designed to respond to resonant frequencies. The calculation

or estimation of natural frequencies is, therefore, of great importance in all types of

vibrating

and

oscillating

systems.

When

resonance

occurs

in

rotating

shafts

and

spindles, the speed of rotation

is

known as

the

critical

speed.

Hence, the prediction

and

correction

or

avoidance3

of

a

resonant

condition

in

mechanisms

is

of

vital

importance

since,

in

the

absence

of

damping

or

other

amplitude-limiting

devices,

resonance

is

the

condition

at

which

a

system

gives

an

infinite

response

to

a

finite

excitation.

Damping

Damping

is

the dissipation

of energy from

a vibrating system,

and

thus prevents excessive response. It is observed that a natural vibration diminishes in

amplitude

with

time

and,

hence,

eventually

ceases

owing

to

some

restraining

or

damping

influence.

Thus

if

a

vibration

is

to

be

sustained,

the

energy

dissipated

by

damping must be replaced from an external source.

The

dissipation

is

related

in

some

way

to

the

relative

motion

between

the

components or elements of the system, and is caused by frictional resistance of some

sort, e.g. in structures, internal friction in material, and external friction caused by air

or fluid resistance called

viscous

”

damping if the drag force is assumed proportional

to

the

relative

velocity

between

moving

parts.

One

device

assumed

to

give

viscous

damping is the

“

d ashpot

”

which is a loosely fitting piston in a cylinder so that fluid

can flow from one side of the piston to the other through the annular clearance space.

A dashpot cannot store energy but can only dissipate it.

Basic Machining Operations and Machine Tools

Basic Machining Operations

Machine tools have evolved from the early foot-powered lathes of the Egyptians and

John Wilkinson

’

s boring mill. They are designed to provide rigid support for both the

workpiece and the cutting tool and can precisely control their relative positions and

the velocity of the tool with

respect

to

the workpiece.

Basically, in

metal

cutting, a

sharpened wedge-shaped tool removes a rather narrow strip of metal from the surface

of a ductile workpiece in the form of a severely deformed chip. The chip is a waste

product that is comsiderably shorter than the workpiece from which it came but woth

a corresponding increase in thickness of the uncut chip. The geometrical shape of the

machine

surface

depedns

on

the

shape

of

the

tool

and

its

path

during

the

machinig

operation.

Most machining operations produce parts of differing geometry. If a rough cylindrical

workpiece revolves about a central axis and the tool penetrates beneath its surface and

travels

parallel

to

the

center

of

rotation,

a

surface

of

revolution

is

producedand

the

operation

is

called

turning.

If

a

hollow

tube

is

machined

on

the

inside

in

a

similar

manner,

the

operation

is

called

boring.

Producing

an

external

conical

surface

of

uniformly varying diameter is called taper turning. If

the tool point travels in a path

of varying radius,a contoured surface like that of a bowling pin a can be produced; or,

if the piece is short enough and the support is sufficiently rigid, a contoured surface

could

be

produced

by

feeding

a

shaped

tool

normal

to

the

axis

of

rotation.

Short

tapered or cylindrical surfaces could also be contour formed.

Flat or plane surfaces are frequently required. The can be generated by adial turning

or facing, in which the tool point moves normal to the axis of rotation. In other cases,

it is more convenient to hold the workpiece steady and reciprocate the tool across it in

a

series

of

straight-line

cuts

with

a

crosswise

feed

increment

before

each

cutting

stroke.

This

operation

is

called

planing

and

is

carried

out

on

a

shaper.

For

larger

pieces

it is

easier to

keep the tool stationary

and draw the workpiece under it as in

planing. The tool is fed at each reciprocation. Contoured surfaces can be produced by

using shaped tools.

Multiple-edged tools can also be used. Drilling uses a twin-edged fluted tool for holes

with depths up to 5 10times the drill diameter. Whether the dril turns or the workpiece

rotates, relative motion between the cutting edge and the workpiece is the important

factor. In milling operations a rotary cutter with a number of cutting edges engages the

workpiecem

which

moves

slowly

with

respect

to

the

cutter.

Plane

or

contoured

surfaces may be produced, depending on the geometry of the cutter and the type of

feed. Horizontal or vertical axes of rotation ma be used, and the feed of the workpiece

may be in any of the three coordinate directions.

Basic Machine Tools

Machine tools are used to produce a part of a specified geometrical shape and precise

size by removing metal from a ductile materila in the form of chips. The latter are a

waste

product

and

vary

from

long

continuous

ribbons

of

a

ductile

material

such

as

steel,

which

are

undesirable

from

a

disposal

point

of

view,

to

easily

handled

well-broken

chips

resulting

from

cast

iron.

Machine

tools

perform

five

basic

metal-removal

processes:

turning,

planing,

drilling,

milling,

and

frinding.

All

other

metal-removal processes are modifications of these five basic processes. For example,

boring is internal turning;reaming,tapping, and counterboring modify drilled holes and

are related to drilling; hobbing and gear cutting are fundamentally milling operations;

hack sawong and broaching are a form of planing and honing; lapping, superfinishing,

polishing,

and

buffing

are

avariants

of

grinding

or

abrasive

removal

operations.

Therefore, there are only four types of basic machine tools, which use cutting tools of

specific controllable feometry: , s, ng machines, and g

machines. The frinding process forms chips, but the geometry of the barasive grain is

uncontrollable.

The amount and rate of material removed by the various machining processes may be

large,

as

in

heavy

truning

operations,

or

extremely

small,

as

in

lapping

or

superfinishing operations where only the high spots of a surface are removed.

A machine tool performs three major functions: rigidly supports the workpiece or

its holder and the cutting tool; 2. it provedes relative motion between the workpiece

and the cutting tools; 3. it provides a range of feeds and speeds usually ranging from 4

to 32 choices in each case.

Speed and Feeds in Machining

Speeds feeds, and depth of cut are the three major variables for economical machining.

Other variables are the work and tool materials, coolant and geometry of the cutting

tool. The rate of metal removal and power required for machining depend upon these

variables.

The depth of cut, feed, and cutting speed are machine settings that must be established

in any metal- cutting operation. They all affect the forces, the power, and the rate of

metal removal. They can be defined by comparing them to the needle and record of a

phonograph.

The

cutting

speed

is

represented

by

the

velocity

of

the

record

surface

relative

to

the

needle

in

the

tone

arm

at

any

instant.

Feed

is

represented

by

the

advance

the

needle

radially

inward

per

revolution,

or

is

the

difference

in

position

between two adjacent grooves.

Turning on Lathe Centers

The

basic

operations

performed

on

an

engine

lathe

are

illustrated

in

Fig.

Those

operations performed on extemal surfaces with a single point cutting tool are called

turning. Except for drilling, reaming, and tapping, the operations on intermal surfaces

are also performed by a single point cutting tool.

All machining operations, including turning and boring, can be classified as roughing,

finishing, or semi-finishing. The objective of a roughing ooperation is to remove the

bulk

of

the

material

sa

repidly

and

as

efficiently

as

possible,

while

leaving

a

small

amount of material on the work-piece for the finishing operation. Finishing operations

are

performed

to

btain

the

final

size,

shape,

and

surface

finish

on

the

workpiece.

Sometimes a semi-finishing operation will precede the finishing operation to leave a

small predetermined and uniform amount of stoxd on the work-piece to be removed

by the finishing operation.

Generally, longer workpieces are turned while supported on one or two lathe centers.

Cone shaped holes, called center holes, which fit the lathe centers are drilled in the

ends of the workpiece-usually

along the axis

of the cylindrical

part.

The end of the

workpiece adjacent to the tailstock is always supported by a tailstock center, while the

end near the headstock may be supported by a headstock cener or held in a chuck. The

headstock end of the workpiece may be held in a four-jar chuck, or in a collet type

chuck.

This

method

holds

the

workpiece

firmly

and

transfers

the

power

to

the

workpiece smoothly; the additional support to the workpiece priovided by the chuck

lessens the tendency for chatter to occur when cutting. Precise results can be obtained

with this method if care is taken to hold the workpiece accurately in the chuck.

Very precise results can be obtained by supporting the workpiece between two centers.

A lathe dog is clamped to the workpiece; together they are driven by a driver

p;ate

mounted

on

the

spindle

nose.

One

end

of

the

workpiece

is

machined;

then

the

workpiece

can

be

turned

around

in

the

lathe

to

machine

the

other

end.

The

center

holes in the workpiece serve as precise locating surfaces as well as bearing surfaces to

carry the weight of the workpiece and to resist the xutting forces. After the workpiece

has been removed from the lathe for any reason, the center holes will accurately align

the workpiece back in the lathe or in another lathe,or in a cylindrical grinding machine.

The workpiece must never be held at the headstock end by both a chuck and a lathe

center. While at first thought this seems like a quick method of aligning the workpiece

in the chuck, this must not be done because it is not possible to press evenly with the

jaws

against

the

workpiece

while

it

is

also

supported

by

the

center.

The

alignment

provided

by

the

center

will

not

be

maintained

and

the

pressure

of

the

jaws

may

damage

the

center

hole,

the

lathe

center,and

prehaps

even

the

lathe

spindle.

Compensatng or floating jaw chucks used almost exclusively on high production work

provice

an

exception

to

the

statements

made

above.

These

chucks

are

really

work

drivers and cannot be used for the same purpose as ordinary three or four=jaw chucks.

While very large diameter workpieces

are sometimes mounted on two centers,

they

are preferably held at the headstock end by faceplate jaes to obtain the smooth power

transmission; moreover, large lathe dogs that are adequate to transmit the power not

generally available, although they can be maed as a special.

Faceplate jaws are like

chuck jaws except that thet are mounted on a faceplate, which has less overhang from

the spindle bearings than a large chuck would have.

Boring

The boring operation is generally performed in two steps; namely, rough boring and

finish

boring.

The

objective

of

the

rough-boring

operation

is

to

remove

the

excess

metal

rapidly

and

efficiently,

and

the

objective

of

the

finish-boring

operation

is

to

obtain the desired size, surface finish, and location of the hole. The size of the hole is

obtained by using the trial-cut procedure. The diameter of the hole can be measured

with inside calipers and outside micrometer calipers. Basic Measuring Insteruments,

or inside micrometer calipers can be used to measure the diameter directly.

Cored holes and drilled holes are sometimes eccentric wwith respect to the rotation of

the lathe. When the boring tool enters the work, the boring bar will take a deeper cut

on

one

side

of

the

hole

than

on

the

other,

and

will

deflect

more

when

taking

this

deeper cut,with the result that the bored hole will not be concentric with the rotation

of the work. This effect is corrected by taking several cuts through the hole using a

shallow

depth

of

cut.

Each

succeeding

shallow

cut

causes

the

resulting

hole

to

be

more concentric than it was with the previous cut. Before the final, finish cut is taken,

the hole should be concentric with the rotation of the work in order to make certain

that the finished hole will be accurately located.

Shoulders, grooves,

contours, tapers, and threads are bored inside of holes.

Internal

grooves are cut using a tool that is similar to an external grooving tool. The procedure

for

boring

internal

shoulders

is

very

similar

to

the

procedure

for

turning

shoulders

are

faced

with

the

boring

tool

positioned

with

the

nose

leading, and using the cross slide to feed the tool. Internal contours can be machined

using a tracing attachment on a lathe. The tracing attachment is mounted on the cross

slide

and

the

stylus

follows

the

outline

of

the

master

profile

plate.

This

causes

the

cutting tool to move in a path corresponding to the profile of the master profile plate.