Axes and Motion Structure

8/12/2019 Axes and Motion Structure

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Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.9-1

Mechanical Transmissions

Chains Belts Gears


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CONTENTS

GEAR

TYPES OF GEARS

NOMENCLATUREAPPLICATIONS OF GEARS

VELOCITY RATIO

GEAR TRAINSEXAMPLE PROBLEMS AND QUESTIONS


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Gears What are they?

Gears are wheels with

teeth. Gears mesh

together and make

things turn. Gears areused to transfer motion

orpowerfrom one

moving part to another.


7/30Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.9-7

Gears The Purpose

Sports cars go fast (havespeed) but cannot pull any

weight. Big trucks can

pull heavy loads (havepower), but cannot go

fast. Gears cause this.

Gears increase or decrease

the power or speed, butyou cannot generally

speaking.



Gears are generally used for one of four

different reasons:

To reverse the direction of rotation

To increase or decrease the speed of rotation

To move rotational motion to a different axis



TYPES OF GEARS

1. According to the position of axes of the shafts.a. Parallel

1.Spur Gear

2.Helical Gear

3.Rack and Pinionb. Intersecting

Bevel Gear

c. Non-intersecting and Non-parallelworm and worm gears



Types of Gears



SPUR GEAR

Teeth is parallel to axis of

rotation

Transmit power from one shaft

to another parallel shaft

Spur Gears in any transmission

system are employed for

changing the speed and the

torque and are fixed on shafts

Gear A is called the driver because this

is turned by a motor. As gear A turns itmeshes with gear B and it begins to turn

as well. Gear B is called the driven

gear.



Pinion

Gear

Spur Gears
http://upload.wikimedia.org/wikipedia/commons/1/14/Gears_animation.gif


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Spur Gears - Types

External Set:

Opposite Movement

Internal Set:

Movement in the

same direction


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Helical Gear

The teeth on helical gears are cut at an angle tothe face of the gear

This gradual engagement makes helical gears

operate much more smoothly and quietly thanspur gears


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Helical Gear

Helical Gears can be used for transmitting motion between parallel

shafts as between perpendicular shafts. Helical gears used for

transmission at an angle are called SPIRAL GEARS.


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Herringbone gears

Herringbone gears are

mostly used on heavy

machinery.

HERRING-BONE Gears are pairs of Helical Gears.

These gears are used for large power transmission.


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Rack and pinion

Rack and pinion gearsare used

to convert rotation (From the

pinion) into linear motion (of the

rack)

A perfect example of this is the

steering system on many cars

The rotary motion of pinion 1

(small gear) is transformed into

linear motion of the rack 2. Fortransmitting large power worm &

worm rack transmission is

employed.


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A rack and pinion gears system looks quite unusual. However,

it is still composed of two gears. The pinion is the normal

round gear and the rack is straight or flat. The rack has

teeth cut in it and they mesh with the teeth of the pinion gear.

The pinion rotates and moves the rack in a straight line -

another way of describing this is to say rotary motion

changes to linear motion.


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Bevel gears can be used to change the

direction of drive in a gear system by90 degrees. A good example is seen

as the main mechanism for a hand

drill. As the handle of the drill is

turned in a vertical direction, the

bevel gears change the rotation of the

chuck to a horizontal rotation

Bevel gears


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Straight and Spiral Bevel Gears


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WORM AND WORM GEAR

Worm gearsare used when large gear reductions are

needed. It is common for worm gears to have

reductions of 20:1, and even up to 300:1 or greater

Many worm gears have an interesting property that

no other gear set has: the worm can easily turn the

gear, but the gear cannot turn the worm Worm gears are used widely in material handling

and transportation machinery, machine tools,

automobiles etc


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Types of Gears

According to the position of axes of the shafts

A. Parallel

Spur Gears Herringbone

Gears

Helical

Gears

C. Planetary

Planetary

Gears

B. Perpendicular Axis

Intersecting

Bevel

Gears

Non Intersecting

Worm Gears


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Planetary Gearssuited for highly dynamic drives

With their weight advantage, planetarygearboxes are also well established for

extremely high output torque and for

mobile applications.

The maximum ratio that can be achieved in asingle stage is approximately i = 12. Larger

ratios are then achieved by adding further

downstream stages.

Alongside the increased load capacity, it is also reducenoise emissions.

N l f S G T h


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Nomenclature of Spu r Gear Teeth

= (tooth spacing)driven gear(tooth thickness)driver, measuredon the pitch circle.

Backlash

Pitch circle

gear diam.

Fillet radiusClearance

Base Circle


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NOMENCLATURE.

Face of a tooth: That part of the tooth surface lying outsidethe pitch surface.

Flank of a tooth: The part of the tooth surface lying inside thepitch surface.

Circular thickness(also called the tooth thickness): Thethickness of the tooth measured on the pitch circle. It is thelength of an arc and not the length of a straight line.

Tooth space: pitch diameter The distance between adjacentteeth measured on the pitch circle.

Backlash: The difference between the circle thickness of onegear and the tooth space of the mating gear.

Circular pitch(Pc) : The width of a tooth and a space,measured on the pitch circle.

N

DPc


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NOMENCLATURE OF SPUR GEARS


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Axes and Motion Structure

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