机械类毕业设计外文翻译（中英对照）

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GEAR AND SHAFT INTRODUCTION——齿轮和轴的介绍

GEAR AND SHAFT INTRODUCTION

Abstract: The important position of the wheel gear and shaft

can''''''''''''''''t falter in traditional machine and modern machines.The

wheel gear and shafts mainly install the direction that delivers the dint at

the principal axis box.The passing to process to make them can is divided into

many model numbers, useding for many situations respectively.So we must be the

multilayers to the understanding of the wheel gear and shaft in many ways .

Key words: Wheel gear;Shaft

In the force analysis of spur gears, the forces are assumed to act in a single

plane. We shall study gears in which the forces have three dimensions. The

reason for this, in the case of helical gears, is that the teeth are not

parallel to the axis of rotation. And in the case of bevel gears, the

rotational axes are not parallel to each other. There are also other reasons,

as we shall learn.

Helical gears are used to transmit motion between parallel shafts. The helix

angle is the same on each gear, but one gear must have a right-hand helix and

the other a left-hand helix. The shape of the tooth is an involute helicoid. If

a piece of paper cut in the shape of a parallelogram is wrapped around a

cylinder, the angular edge of the paper becomes a helix. If we unwind this

paper, each point on the angular edge generates an involute curve. The surface

obtained when every point on the edge generates an involute is called an

involute helicoid.

The initial contact of spur-gear teeth is a line extending all the way across

the face of the tooth. The initial contact of helical gear teeth is a point,

which changes into a line as the teeth come into more engagement. In spur gears

the line of contact is parallel to the axis of the rotation; in helical gears,

the line is diagonal across the face of the tooth. It is this gradual of the

teeth and the smooth transfer of load from one tooth to another, which give

helical gears the ability to transmit heavy loads at high speeds. Helical gears

subject the shaft bearings to both radial and thrust loads. When the thrust

loads become high or are objectionable for other reasons, it may be desirable

to use double helical gears. A double helical gear (herringbone) is equivalent

to two helical gears of opposite hand, mounted side by side on the same shaft.

They develop opposite thrust reactions and thus cancel out the thrust load.

When two or more single helical gears are mounted on the same shaft, the hand

of the gears should be selected so as to produce the minimum thrust load.

Crossed-helical, or spiral, gears are those in which the shaft centerlines are

neither parallel nor intersecting. The teeth of crossed-helical fears have

point contact with each other, which changes to line contact as the gears wear

in. For this reason they will carry out very small loads and are mainly for

instrumental applications, and are definitely not recommended for use in the

transmission of power. There is on difference between a crossed heli

cal gear and a helical gear until they are mounted in mesh with each other.

They are manufactured in the same way. A pair of meshed crossed helical gears

usually have the same hand; that is ,a right-hand driver goes with a right-hand

driven. In the design of crossed-helical gears, the minimum sliding velocity is

obtained when the helix angle are equal. However, when the helix angle are not

equal, the gear with the larger helix angle should be used as the driver if

both gears have the same hand.

Worm gears are similar to crossed helical gears. The pinion or worm has a small

number of teeth, usually one to four, and since they completely wrap around the

pitch cylinder they are called threads. Its mating gear is called a worm gear,

which is not a true helical gear. A worm and worm gear are used to provide a

high angular-velocity reduction between nonintersecting shafts which are

usually at right angle. The worm gear is not a helical gear because its face is

made concave to fit the curvature of the worm in order to provide line contact

instead of point contact. However, a disadvantage of worm gearing is the high

sliding velocities across the teeth, the same as with crossed helical gears.

Worm gearing are either single or double enveloping. A single-enveloping

gearing is one in which the gear wraps around or partially encloses the worm..

A gearing in which each element partially encloses the other is, of course, a

double-enveloping worm gearing. The important difference between the two is

that area contact exists between the teeth of double-enveloping gears while

only line contact between those of single-enveloping gears. The worm and worm

gear of a set have the same hand of helix as for crossed helical gears, but the

helix angles are usually quite different. The helix angle on the worm is

generally quite large, and that on the gear very small. Because of this, it is

usual to specify the lead angle on the worm, which is the complement of the

worm helix angle, and the helix angle on the gear; the two angles are equal for

a 90-deg. Shaft angle.

When gears are to be used to transmit motion between intersecting shaft, some

of bevel gear is required. Although bevel gear are usually made for a shaft

angle of 90 deg. They may be produced for almost any shaft angle. The teeth may

be cast, milled, or generated. Only the generated teeth may be classed as

accurate. In a typical bevel gear mounting, one of the gear is often mounted

outboard of the bearing. This means that shaft deflection can be more

pronounced and have a greater effect on the contact of teeth. Another

difficulty, which occurs in predicting the stress in bevel-gear teeth, is the

fact the teeth are tapered.

Straight bevel gears are easy to design and simple to manufacture and give very

good results in service if they are mounted accurately and positively. As in

the case of squr gears, however, they become noisy at higher values of the

pitch-line velocity. In these cases it is often go

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od design practice to go to the spiral bevel gear, which is the bevel

counterpart of the helical gear. As in the case of helical gears, spiral bevel

gears give a much smoother tooth action than straight bevel gears, and hence

are useful where high speed are encountered.

It is frequently desirable, as in the case of automotive differential

applications, to have gearing similar to bevel gears but with the shaft offset.

Such gears are called hypoid gears because their pitch surfaces are

hyperboloids of revolution. The tooth action between such gears is a

combination of rolling and sliding along a straight line and has much in common

with that of worm gears.

A shaft is a rotating or stationary member, usually of circular cross section,

having mounted upon it such elementsas gears, pulleys, flywheels, cranks,

sprockets, and other power-transmission elements. Shaft may be subjected to

bending, tension, compression, or torsional loads, acting singly or in

combination with one another. When they are combined, one may expect to find

both static and fatigue strength to be important design considerations, since a

single shaft may be subjected to static stresses, completely reversed, and

repeated stresses, all acting at the same time.

The word “shaft” covers numerous variations, such as axles and spindles.

Anaxle is a shaft, wither stationary or rotating, nor subjected to torsion

load. A shirt rotating shaft is often called a spindle.

When either the lateral or the torsional deflection of a shaft must be held to

close limits, the shaft must be sized on the basis of deflection before

analyzing the stresses. The reason for this is that, if the shaft is made stiff

enough so that the deflection is not too large, it is probable that the

resulting stresses will be safe. But by no means should the designer assume

that they are safe; it is almost always necessary to calculate them so that he

knows they are within acceptable limits. Whenever possible, the power-

transmission elements, such as gears or pullets, should be located close to the

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摘要：

GEARANDSHAFTINTRODUCTION——齿轮和轴的介绍GEARANDSHAFTINTRODUCTIONAbstract:Theimportantpositionofthewheelgearandshaftcan''''''''''''''''tfalterintraditionalmachineandmodernmachines.Thewheelgearandshaftsmainlyinstallthedirectionth...

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