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Drive Belts

Timing Belts

Important Note:..The notes below are intended to be concise informative guidance notes. Manufacturers literature

and the relevant standards provide the necessary detailed information required for detail design.

I have included links to sites providing good quality information on this topic.

Introduction

Synchronous / Timing belts are basically endless flat belts which pass over pulleys- the belts having grooves which

mate with teeth on the pulleys. These belt drives, unlike flat and vee belt drives are positive. Any slip of the belt

relative to the pulleys is minor in degree and is due to belt stretch, or erosion of the grooves. These belts are used

for power transfer and for synchronised drives to ensure that the driven pulley is always rotating at a fixed speedratio to the driving pulley.

The first synchronous belts had a trapezoidal tooth profile, and is identified as timing belts. The belt tooth profile is

a trapezoidal shape with sides being straight lines The profile of the pulley teeth which mates with the belt is

involute. These belts are based on imperial (inch) pitch sizes and can provide power transmission up to 150 kW.

The development of the classical timing belt with has a rounded tooth (curvilinear tooth profile) and is identified as

as the high torque drive, or HTD. Advantages of this belt design include..

y Proportionally deeper tooth; hence tooth jumping or loss of relative position is less likely

y Lighter construction, with consequent reduced centrifugal loss.

y Smaller unit pressure on the tooth since area of contact is larger.

y G reater shear strength due to larger tooth cross section.

y Lower cost as a narrower belts will handle larger load.

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y Installation tension is reduced resulting in lower bearing loads.

HTD sprockets have metric pitches (3 5 8 14 & 20) and can transmit up to 1000 kW.

The most advanced synchronous belts, has a modified rounded tooth profile with a higher tooth angle and

shallower tooth. These belts e.g G ates Powergrip G T have available pitch sizes of 2mm, 3mm & 5mm and can

powers up to transmit up to 600 kW . The belts have the advantages that they provide a smoother drive at higher

accuracy,

A correctly designed and installed synchronous belt drive should operate successfully for between 8000 and 12000

hrs and have an operating efficiency of about 98%.

Synchronous belts have a number of advantages such that they are often used for applications not requiring shaft

synchronization. Their section and flexibility enable timing belts to operate very well on miniature drives and in

applications involving high speeds or small pulleys. They are extremely efficient when correctly installed. They

can also be specified to continuous high loads. For these reasons , synchronous belts have proved to be cost

effective in non-synchronous applications as drives for power saws, motorcycles, and domestic appliances.

The disadvantages of synchronous belt drives are that they are generally more costly compared to other belt drive

options and the require accurate alignment of the pulleys for efficient reliable operation

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Construction

Belts

Synchronous belts are made with elastomer e.g natural rubber,neoprene, polyurethane, polychioroprene, core with

reinforcement to provide increased tensile strength. These belts were originally reinforced with steel to provide thenecessary strength. In modern drives the most common reinforcement is glass fiber, but aramid is used if

maximum capacity is required. Synchronous belts are often provided with nylon facings to provide the necessary

wear resistance and can include conductive coatings.

Pulleys

Synchronous drive pulleys are often made of ductile or cast iron. Aluminum is a often selected for drives that

require low weight. These applications can include high speed drives with low inertia. Steel(and Stainless Steel

)is preferred to iron when the drive will exceed the safe operating limits for cast iron (2000 mpm) or ductile iron(2500 to 3,000 mpm).

Plastic pulleys e.g. nylon are low-cost options when power requirements are low as in office machines or home

appliances such as vacuum cleaners. Plastic gears may also be acceptabl e when it is acceptable that the belt

service life is short, as in some power tools, or lawn and garden equipment.

Pulleys are mounted to shafts using pins, keyways or by using proprietory shaft locking bushes such taperlock

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bushes. Pulleys can have one or two flanges to ensure the belts are retained in place. For drives with horizontal

pulley axes it is normal to have two flanges to retain the belt (two flanges on one pulley or one flange on each

pulley on opposite sides). On pulleys with vertical shaft axes the lower face of each pulley should include a

flange and one pulley should include two flanges.

Relevant Standards

The British Standard for timing belt drives was

BS 4548:1987 :Specification for synchronous belt drives for industrial applications . This standard is still in use but

is declared as obsolescent the current standard in europe for timing belt drives is

ISO 5294:1989: Synchronous belt drives -- Pulleys

ISO 5296-1:1989:1989: Synchronous belt drives -- Belts -- Part 1: Pitch codes MX L , XL , L , H, XH and XXH --

Metric and inch dimensions

This is not equivalent and belts and pulleys to the British Standard are not interchangeable with the ISO standard.

Basic Timing Belt Parameters

Classical Timing belts

Belt

SectionMeaning

Pitch

mm

Widths Available

mm

MXL Extra Light 2,032 3,05 4,826 6,35XL Extra Light 5,08 6,35 9,652L Light 9,525 12,7 19,05 25,4H Heavy 12,7 19,05 25,4 38,1 50.8 76,2

XH Extra heavy 22.225 50.8 76,2 50.8 76,2 101,6127101,6

XXH Doubleextra heavy 31,75 50.8 76,2 101,6 127

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HTD- Curvilinear

Belt

SectionDesignation

Pitch

mm

Widths Available

mm

3M 3mm High Torque Drive 3 6 9 155M 5mm High Torque Drive 5 9 15 258M 8mm High Torque Drive 8 20 30 50 85

14M 14mm High Torque

Drive14 40 55 85 115 170

20M 20mm High TorqueDrive 20 115 170 230 290 340

G T - Curvilinear

Belt

SectionName

Pitch

mm

WidthsAvailable

mm

2MR(Gates) 2mm High Torque Belt 2 3 6 9

3MR(Gates) 3mm High Torque Belt 3 6 9 15

5MM(Gates) 5mm High Torque Belt 5 9 15 25

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Note : The various notes below relate to the classical timing belt drives. For the more advanced drive belt design

refer to manufactures literature... I will include notes on these belt drives at a later date...

Designing a Synchronous Belt System

Belt design procedures can be based on torque calculations or they can be based on power calculations.

Power method

1) The driven speed and the maximum driven torque required (including inertia load,

shock loads, friction, etc) are used to calculate the required driven power

2) From information on the driver, driven equipment and operating period a service

factor is obtained - see below

3) A design power is obtained based on the product of the Driven Power required and

the service factor .

4) A belt section is initially selected using a graph as typically shown below

5) A drive geometry is derived selecting suitable pulleys, and belt Centre Distance -

Some Pulley sizes are provided below

6) A Basic Power for the belt is calculated and a mesh factor is calculated - see below

7) A suitable belt width is selected -Using a table as provided below- Some iteration

may be required

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Torque Method

The classical MX L belt and the Curvilinear more advanced belt options are designed based on torque levels. The

outline method for the MX L drive is provided below. The method used for the HTD and other modern belt options

will be provided at some future date...

The MX L belts operate generally at relatively low belt speeds so the torque levels are similar for the normal range

of pulley rotational speed. Torque ratings can be calculated of each of the MX L belt widths as follows: I have

converted an imperial formula to a metric formula and minor differences with the original formulae results..

Torque ratings of belts T r (Nm) at P 2 PCDs (mm)

Belt width =3.048 mm... T r = P 2(5,03 - 9,5147.10 -6.P 22).10 -3

Belt width =4.826mm... T r = P 2(8,36 - 1,586.10 -5.P 22).10 -3

Belt width =6.35 mm...T r = P 2(11,7 - 2,213.10 -5.P 22).10 -3

To design an MX L belt system using the torque method.

1) The driven speed and the maximum driven torque required (including inertia load,

shock loads, friction, etc) are calculated

2) From information on the driver, driven equipment and operating period a service

factor is obtained - see below

3) A design torque is obtained based on the product of the torque required and the

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service factor .

4) A belt section is initially selected (assuming MX L) using a graph as typically shown

below

5) A drive geometry is derived selecting suitable pulleys, and belt Centre Distance -

Some Pulley sizes are provided below

6) The design torque is divided by the teeth mesh factor (see below) to arrive at an

adjusted torque

7) The table below is used to select the belt width which has a torque value equal to or

larger than the corrected torque

Torque Rating for MX L Belt (Nm)

No Teeth

->

10MX

L

12MX

L

14MX

L 16MX L 18MX L 20MX L

22MX

L 24MXL 28MX L 30MXL

PCD(mm

) ->6.477 7.7724 9.0678

10.337

8

11.633

2

12.928

614.224

15.519

4

18.110

2

19.405

6

width

=3.05mm0.033 0.040 0.045 0.052 0.059 0.064 0.071 0.078 0.092 0.097

width =

4.83mm0.054 0.066 0.076 0.087 0.097 0.108 0.119 0.130 0.151 0.163

width =

6.35mm0.076 0.090 0.106 0.121 0.136 0.151 0.166 0.182 0.211 0.227

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Service Factors

When designing belt drives it is normal to apply a service factor to the drive operating load to compensate for allow

for different driver type, driven load types and operating periods. Typical service factor values are included on the

linked page Service Factors

Designating Classical Synchronous belts

Synchronous Belt sizes are identified by a standard number. The first digits specify the belt length to one-tenth

inch followed by the belt section (pitch) designation. The digits following the belt section designation represent the

nominal belt width times 100. For example, an L section belt 30.000 inches pitch length and 0.75 inches in width

would be specified as a 300 L 075 Synchronous Belt. A similar method is used for designating metric belt

designations

Initial selection of Timing Belt

When the design power has been determined (Power x Service Factor) a synchronous belt can be selected

generally using a graph similar to the one below..This is provided for guidance only and is copied from published

graphs generally available.

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Horsepower Rating of Timing Belt

This method is based on the method shown in Machinery's handbook. It is preferable to use the calculation tool

provided by the belt manufacturers to size the belts for detail design. Or even better let the suppliers do the

design for you...

The Power ratings of belts for the basic belt widths (in brackets) are as identified below..

y r = Rpm of faster shaft /1000y P 2 = Pitch diameter of smallest Pulley (mm)

y Z = P 2.r /25.4

For Belt (width) = X L (9.652)...... P r = 0.746. Z .(0,0916 - 7,07.10 -5.Z 2 )

For Belt (width) = L (25,4)...... P r = 0.746. Z .(0,436 - 3,01.10 -4.Z 2 )

For Belt (width) = H (76,2)...... P r =0.746. Z .(3,73- 1,41.10 -3.Z 2 )

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For Belt (width) = XH (101,6)...... P r = 0.746. Z .(7,21 - 4,68.10 -3.Z 2 )

For Belt (width) = XXH (127)...... P r =0.746. Z .(11,14 - 7,81.10 -3.Z 2 )

Determining the timing belt length

1) The Pitch dia of a pulley P = No Teeth on Pulley . Pitch / T

2) The Drop distance d = [ P 1 - P 2 ] /2

3) The belt contact angle = arcsin(d /C) ..C= Centre distance

4) The belt fall length = fl = d / tan

5) The contact length Small Pulley= C L2= P 2. T . [90 - ]/180 degrees

6) The contact length L arge Pulley = C L1=P 1.T . [90 + ]/180 degrees

7) The Belt Length L = 2.fl + C L1 + C L2

8) Total number of teeth on belt = L / Pitch

9) Number of teeth in mesh (small pulley) = C L2 /Pitch. Rounded down to nearest whole number.

Mesh Factor

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The horsepower ratings obtained above are based on the smallest pulleys having six or more teeth in mesh. For

drives with small angles of lap on the smallest pulleys the mesh factor is required.

NoTeeth

in mesh

Mesh

Factor

6 or more 1

5 0,84 0,63 0,4

2 0,2

Determination of the Belt Width required

1) First establish the design power to be transferred(kW) = Service Factor x Power.

2) Select a suitable belt and calculate the basic power using the belt size, smaller pulley speed, and smaller pulley

size.

3) If the basic belt power is less than the design power- change one or more of belt size , pulley size or speed.

3) Divide the Basic power/ Design power to obtain a belt width factor.

4) Use the table below and select a width with a width factor higher than the calculated width factor required

Table of Belt Width Factors -

Belt

Section

Belt Width

3,05 4,826 6,35 9,652 12,7 19,05 25,4 38,1 50.8 76,2 101,6 127

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MXL 0,43 0,73 1,00 - - - - - - - - -

XL - - 0,62 1,00 - - - - - - - -

L - - - - 0,45 0,72 1,00 - - - - -

H - - - - - 0,21 0,29 0,45 0,63 1,00 - -

XH - - - - - - - - 0,45 0,72 1,00 -

XXH - - - - - - - - 0,35 0,56 0,78 1,00

Typical Pulley Sizes

Below are listed a collection of pulley Dimensions (PCD and OD) for pulleys in the classical timing belt range. In

practice there are a vast number of pulleys available from suppliers on the belt sections shown and on other higher

specification sections . Additional data is available using the links below and preferable by contacting the

suppliers.

MXL XL L H XH XXH

Teeth

PCD OD

Teeth

PCD OD

Teeth

PCD OD

Teeth

PCD OD

Teeth

PCD OD

Teeth

PCD OD

10 6,475,96 10

16,17

15,67 10

30,32

29,56 10

40,43

39,08 18

127,34

124,54 18

181,91

178,87

11 7,116,61 11

17,79

17,29 11

33,35

32,59 11

44,47

43,12 20

141,49

138,68 20

202,13

199,09

127,76

7,25 12

19,40

18,90 12

36,38

35,62 12

48,51

47,16 22

155,64

152,83 23

232,45

219,30

14 9,068,55 13

21,02

20,52 13

39,41

38,65 13

52,55

51,20 24

169,79

167,01 25

252,66

239,50

16 10,359,84 14

22,64

22,14 14

42,45

41,68 14

56,60

55,25 26

183,94

181,15 26

262,76

259,72

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18 11,6411,13 15

24,26

23,76 16

48,51

44,72 15

60,64

59,29 28

198,08

195,30 30

303,19

300,15

20 12,9411,78 16

25,87

25,37 17

51,54

47,75 16

64,68

63,33 30

212,23

209,45 34

343,62

340,56

2113,58

13,07 17

27,49

26,99 18

54,57

50,78 17

68,72

67,37 32

226,38

223,60 40

404,25

401,19

22 14,2313,72 18

29,11

28,61 19

57,61

56,84 18

72,77

71,42 40

282,98

280,19 48

485,10

482,07

24 15,5215,02 20

32,34

31,84 20

60,64

59,88 19

76,81

75,46 48

339,57

336,78 60

606,38

603,32

28 18,1117,60 21

33,96

33,46 21

63,67

62,91 20

80,85

79,50 60

424,47

421,67 72

727,66

648,41

30 19,4018,90 22

35,57

35,07 22

66,70

65,94 21

84,99

83,54 72

509,36

506,58 90

909,57

906,53

3220,70

20,19 24

38,81

38,31 24

72,77

72,00 23

92,98

91,63 84

594,25

591,46

36 23,2922,78 25

40,43

39,93 25

75,80

75,04 25

101,06

99,71 90

636,70 0,00

40 25,8725,37 26

42,04

41,54 26

78,83

78,07 26

105,11

103,76 96

679,15

676,35

42 27,1726,67 28

45,28

44,78 28

84,89

84,13 28

113,19

111,84 120

848,93

846,15

44 28,4627,94 30

48,51

48,01 30

90,96

90,19 30

121,28

119,93

4831,05

30,53 32

51,74

51,24 32

97,02

96,26 32

129,36

128,01

60 38,8138,30 36

58,21

57,71 36

109,15

108,39 33

133,40

132,05

72 46,5746,05 40

64,68

64,18 40

121,28

120,51 34

137,45

136,10

42 67,9167,41 42

127,34

126,58 35

141,49

140,14

44 71,1570,65 44

133,40

132,64 36

145,53

144,18

4877,62

77,12 48

145,53

144,77 38

153,62

152,27

50 80,8580,35 50

151,60

150,83 40

161,70

160,35

54 87,3286,82 54

163,72

162,96 42

169,79

168,44

60 97,0 90,5 60 181, 181, 44 177, 176,

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2 2 91 15 87 52

72 116,43115,93 72

218,30

220,57 48

194,04

192,69

1.