IAT 0932 Chapter9 Rev2

7/28/2019 IAT 0932 Chapter9 Rev2

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CHAPTER 9:

Belts & Chains

Rev: 1_Jan2011
http://en.wikipedia.org/wiki/Image:Keilriemen-V-Belt.png


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Introduction to Belts

1. Consequence of FailureBelts have a distinct advantage over chains and

gears since a sudden overload will not break a belt.

Instead, the belt slips until the overload is ended.

However, even momentary overloads can breakgear teeth or chain links.

2. Versatility in Shaft Connection

Since belts are more flexible than chains and gears,

they are the more versatile in connecting two shaftswith unusual geometrical arrangements or large

center distances.

Gears are the least versatile from a practical point

of view, especially if the center distances are large.

Factors in Belting selection:


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3. Effect on Shaft Bearing Life

Belts are driven by friction, and therefore require

initial tensioning resulting in tension on the slackside of the belt and increased bearing loads.

Gears and chains are positive drives and do not

place this increased load on the bearings.

4. Speed RatioBelt drives do not provide an exact speed ratio as

do gear systems.

The slippage that protects belt drives from damage

by sudden overloads ironically prevents an exacttiming between the driving and driven shafts.

However, special timing belts that produce positive

drive are available.


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5. Cost

Belts are the least expensive of either gears or

chains, while chains are less expensive than gears.The required precision of machining and mounting

of gears is the principal reason for their higher cost.

When using chains, the alignment of the shafts

must be more precise than for belts6. Noise and Vibration

Belt drives produce the least amount of noise and

vibration and thus are used where vibration levels

must be low7. Speed and Power

Gears can operate at higher speeds and transmit

more power than chains or belts


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8. Maintenance

Chains and belts require periodic adjustment

resulting from wear and stretch, respectively.Chains and gears require lubrication.

Properly designed gear systems require the least

amount of maintenance.


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Common applications of belts and chains

Belts Chains

Compressors Bicycles

Sewing machines Motorcycles

Textile machines Power lawn mowers

Automotive devices

- water pumps- alternators

- fans

Chain saws

CranesHoists

Paper-mill machinery

Mixing machines Conveyers

Washing machines Textile machineryPrinting machinery All terrain vehicles

Pumps

Machine tools

Crushing machinery


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Types of Belts

1. Flat Belts

2. V-Belts

3. Miscellaneous Belts


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1. Flat Belts

Rectangular in cross section

Mounted on pulleys that are usually crowned to preventthe belt from running off the pulley

Used in :

a. Open-Belt drive

b. Cross-Belt drive driver & driven opposite direction

c. Serpentine drive idler pulley to increase contact

angle

d. Quarter-turn drive - drives between non parallel

shaft

Flat belting is usually made of leather, rubber or canvas


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Belts mounted on crowned pulley

Open-belt drive


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Cross-belt drive Serpentine drive

Quarter-turn drive


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2. V-Belts

Has a trapezoidal cross section which has been

standardized into two different classes.

The light-duty belts are designated as 2L, 3L, 4L or 5L

Larger horsepower ratings are designated, having 5

different sizes designated by the letters A, B, C, D and E

For higher horsepower, multiple belts are used.

Belt

Section

Width, b

(inch)

Thickness, t

(inch)

Minimum

Sheave

Diameter

(inch)

hp range

(1 or more belts)

A 1/2 5/16 3.0 - 10

B 21/32 13/32 5.4 1-25

C 7/8 17/32 9.0 15-100

D 11/4 3/4 13.0 50-250

E 11/2 23/32 21.6 100 and higher

Standard V-Belts cross section

b

t


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V-belts are usually made with synthetic or steel tensile

cords molded in rubber and encased in an outer jacket

The cords provide great tensile strength and permitflexing of the belt.

The lower, rubberized area is capable of withstanding

compression.

The outer fabric jacket protects the belt from damagefrom moisture, heat, and dust.

Construction of typical V-Belts


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A V-belt is designed to ride inside the groove of the

pulley, or sheave.

Proper mating of the pulley/sheave and V-belt size isrequired so that the V-belt does not ride on the bottom

of the groove and the top of the belt rides

approximately flush with the top of the groove.

The pulley/sheave pitch diameter is measured to the

centroids of the belt cross-sectional areas

V-Belt in groove of sheave/ pulley


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A photograph of an actual V-belt pulley/sheave is

shown in below figure

V-Belt sheave/ pulley


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Advantages of V-belts over Flat belts :

1. V-belts operate more smoothly and quietly

2. V-belts are more compact because shorter centerdistances are permitted. The center distance can be

small as the two pulleys physically allow.

3. V-belts operate with lower belt tensions which result

in smaller bearing loads.4. V-belts can absorb greater shock loads.


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3. Miscellaneous Belting

Can be divided into several types:

(a) Round Belting(b) Positive-drive belting

(c) Banded-Together Belts

(d) Link-type V-Belts


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(a) Round Belting

Used for light load application especially when the

shafts are not parallelCan be stretched by hand over the pulleys and

snapped into place

Can be made endless or come in any desired length

Application : sewing machine, vacuum cleaner


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(b) Positive Drive Belting

Sometimes called Timing Belt

Made of rubberized fabric and is reinforcedinternally with steel tensile wires

The belt has teeth that fit into grooves cut into the

periphery of the pulley

Since the belt cannot slip, power is transmitted at aconstant ratio

No initial tension is required bearing loads are

reduced

Can be run at much higher speeds than a standardV-Belt

Maintenance costs are much lower

Disadvantage : the higher costs of the toothed belt

and corresponding toothed pulley


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Fractional-hp and 600 hp positive drive belting system


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(c) Banded together Belts

Used when more than one belt is needed to

transmit the required power

Banded-together belts on multiple groove sheave


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(d) Link-Type V-Belts

Consist of a selected number of easy to install links

Eliminate the need of splicing when endless beltcannot be used

Multi-groove sheave system using link type belts

Adjacent links of a link belt


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Roller Chain used to transmit power between parallel shafts

Like gears, roller chains do not require an initial tension;hence, no slippage occurs

The power transmission efficiency of a roller chain ishigh-98% to 99%


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Roller Chains

Because of their similarity to pulleys as wrappingconnectors, roller chains are suitable for relatively

long or short center distances

Compared to belts, chains are more compact, they

can operate in a dirty and gritty (coarse) environment

They are not affected by oil and they can be used at

the high temperatures encountered in ovens


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Roller chain consists of two different types of links

which arc alternately connected:

1. Roller Links.

Each roller link contains two inner end plates, two

rollers and two bushings

2. Pin Links. Each pin link contains two outer end

plates and two pins


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Construction of typical roller chain


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A whole number (preferably an even number) of links arerequired

If a chain contains an odd number of links, it isnecessary that the last link, which is installed to makethe chain endless, be an offset link

This requirement occurs because the last link mustconnect a roller link to a pin link, rather than a roller link

to another roller link An offset link has offset plates with one end containing

only a pin and the other end containing a combination ofa bushing and roller.


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The pitch p of a roller chain is the distance between

adjacent pins

The pitch is very important roller chain parameterbecause each pin serves as a pivot between the

adjacent interconnecting roller and pin links


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Below figure is a photograph showing a portable electric-

powered roller chain hoist.

The entire hoist weights only 10 pounds and can lift up to500 pounds

It has applications in automobile repair shops,

refrigeration repair and installation, machine shops, light

production lines, stockrooms and workshops in general.


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Portable electric Powered Roller Chain Hoist


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Dimensions of Standard Roller Chains

Documents

IAT 0932 Chapter9 Rev2