09 Flex Me Chele

7/28/2019 09 Flex Me Chele

1/19

1

09 Flexible Mechanical Elements9.0 Introduction

9.1 Belts

9.1.1 Flat and Round-Belt Drives

9.1.2 The Contact Angles

9.1.3 The Steps in Analysing a Flat-Belt Drive9.2 Chains

9.2.1 The Advantages of Chains

9.2.2 The Application of Chain Drives

9.2.3 Drive Design9.2.4 Chain and Sprocket Design


2/19

2

Introduction

Belts, ropes, chains and other similar elastic orflexible machine elements are used in conveying

systems and in the transmission of power over

comparatively long distances.

The use of these elements simplifies the designof a machine and substantially reduces the cost.

These elements are elastic and usually quite

long, they play an important part in absorbing

shock loads and in damping out and isolating the

effects of vibration.


3/19

3

Belts

Characteristics of some common belt types

Belt Type Figure Joint Size range Centre

distance

Flat Yes t = { 0.75 to 5

mm

No upper limit

Round Yes d = 3 to 20 mm No upper limit

V None b = 8 to 19 mm Limited

Timing None p = 2 mm and

up

Limited


4/19

4

The characteristics of belts are:

They may be used for long centre distances. Except for timing belts, there is some slip and

creep, and so the angular-velocity ratio

between the driving and driven shafts isneither constant nor exactly equal to the ratio

of the pulley diameters.

In some cases an idler or tension pulley can

be used to avoid adjustments in centredistance that are ordinarily necessitated by

age or the installation of new belts.


5/19

5

(b) Crossed belt

(a) Open belt


6/19

6

(a) Non-reversing open belt

(b) reversing belt drive

(c) reversing open-belt drive


7/19

7

Quarter-twist belt

drive; an idler guide

pulley must be used if

motion is to be in both

directions

This drive eliminates the

need for a clutch. Flat belt

can be shifted left or right

by use of a fork.


8/19

8

Variable speed belt drives


9/19

9

Flat and Round-Belt Drives

Modern flat-belt drives consist of a strong elastic

core surrounded by an elastomer; these drives

have distinct advantages over gear drives or V -

belt drives.

A flat-belt drive has an efficiency of about 98percent, which is about the same as for a gear

drive. On the other hand, the efficiency of a V-belt

drive ranges from about 70 to 96 percent.

Flat-belt drives produce very little noise andabsorb more torsional vibration from the system

than either V-belt or gear drives.


10/19

10

The contact anglesCdDd

2sin2 1

C

dD

D 2sin2 1

where D= diameter of large pulley

d= diameter of small pulley

C = center distance= angle of contact

The length of the belt

L = [4C2

- (D - d)2

]1

/2

+1/2(DD + dd)


11/19

11

For these the angle of wrap is the same for both

pulleys and is

= + 2 sin-1C

dD

2

The belt length for crossed belts is found to be

L = [4C2 - (D + d)2]1/2 + /2(D + d)


12/19

12

Figure 9.6

Free-body of an

infinitesimal element

of a flat belt in

contact with a pulley d F

F + dF

dN

fdN

dS

dS due to centrifugal forcedN - normal force between the belt and pulleyf dN - shearing traction due to friction at the point of slipbelt width bthickness t


13/19

13

Belt speed,

1000

dnV

m/sec

The weight of

meter of belt 6

10

bt N/m

where = weight density (N/m3)

Hoop tension due to centrifugal force,

Nv

Vg

Fc81.9

)(2

2


14/19

14

whereFi = initial tensionF = tension due to the transmitted torque TD = diameter of the pulley


15/19

15

F1 F2 = 2T / D = T / D/2

ci FFFF

2

21

1)exp(1)exp(

ff

DTFi

1)exp(

)exp(2

f

fFFF ici (F1)a = b Fa Cp Cv

where (F1)a = allowable largest tension, Nb = belt width, mmFa =manufacturers allowed tension,

N/mm or N/mCp = pulley correction factorCv

= velocity correction factor


16/19

16

The Steps in Analysing a Flat-Belt Drive

1. Find exp(f) from belt-drive geometry and friction2. From belt geometry and speed find Fc3. From Nm find necessary torque

4. From torque T find necessary (F1)aF2= 2T/D5. Find F2 from (F1)a[(F1)aF2]

6. From eq. find necessary initial tension Fi

7. Check the friction development, f< f. Solve forf:

8. Find the factor of safety from fos = Ha/(HnomKs)

n

nHKT ds

159

ci FFF

F

2

21

c

ca

FF

FFf

2

1)(ln1

'


17/19

17

Chains

A chain drive consists of endless chain wrappedaround two sprockets

The chain consists of a number of links connected by

pin joints, while the sprockets are toothed wheels with

a special profile of teeth.

The chain drive is intermediate between belt and geardrives. It has so some features of belt drives and

some of gear drives.


18/19

18

The Advantages of Chains

Cost effectiveness and economy of chains and sprockets.

Large speed ratios;

Sufficient elasticity to absorb reasonable shocks;

A constant speed ratio between the driving and driven shaft;

Long life without excessive maintenance; Mechanical understandability regarding installation and

functionality;

Coupling and uncoupling with simple tools; and

A simple means to get power from its source to the location where

needed.

Th A li i f Ch i d i


19/19

19

The Application of Chain drives

Chain drives are popular in the transportation industry, such as

bicycles, motor cycles and automobile vehicles. They are used in

metal and wood working machinery for the transmission ofpower. They are used in textile, building construction and

materials handling machinery.

Drive Design

The steps to design the drive as follows:

Selection of the chain and sprocket sizes.

Determination of chain length, center distance, method of

lubrication, and in some cases the arrangement of chain casingsand idlers.

Chain and sprocket selection is based on the

horsepower and type of drive;

the speeds and sizes of the shafting;

the surrounding conditions.

09 Flex Me Chele

Documents

Transcript of 09 Flex Me Chele