Vee belt drive Standards

Classical vee belts have been available since 1920 and until the 1970s were manufactured to the various editions of BS1440.

The later, narrow wedge vee belts introduced around 1960 were covered by BS 3790:1973. More recently both types of vee belt have been manufactured to BS 3790:1995 and ISO 4184, it being recognised that as the included angle of the ropes are the same, the width of the belt or rope merely deter­mines exactly where it sits in the groove and thus defines the ef­fective pitch angle of the pulley.

Both types of vee belt have a trapezoidal cross-section consist­ing of a tension member contained within a rubber base and surrounded by a rubber-impregnated fabric cover. They are variously known as belts or ropes being a compromise between each.

To meet the wide range of speeds and powers, various rope sections have been standardised as shown in Table 11.1.

Type

Section

Pitch

Width

(mm)

Top width (mm)

Height

(mm)

Angle

(degrees)

Classical

У

5.3

6.5

4

40

Z

8.5

10

6

40

A

11

13

8

40

В

14

17

11

40

С

19

22

14

40

D

27

32

19

40

Wedge

SPZ

8

9.5

8

40

SPA

11

13

10

40

SPB

14

16

14

40

SPC

19

22

18

40

Table 11.1 Standardised vee belt sections

An indication of the likely belt section is shown in Figures 11.3 and 11.4. However it is recommended that a reputable manu­facturer be consulted for the most appropriate selection.

It should also be noted that vee belts continue to be made to other standards such as the American RMAIP20 and DIN 2215 etc.

Whilst most of the requirements for classical and wedge type vee belt drives are contained in BS 3790:1995, it should be noted that the list of ISO Standards in Section 11.7 Bibliogra­phy, is extensive and also encompasses the specification of synchronous (toothed or timing) belts as well as some of the other alternatives mentioned in Section 11.5.

‘Y’ and ‘Z’ section belts should be used for design powers lower than those shown, or when pulley diameters are smaller that the recommended minimum for A-section belts

подпись: 
'y' and 'z' section belts should be used for design powers lower than those shown, or when pulley diameters are smaller that the recommended minimum for a-section belts
Power transmitted by a vee rope or belt

The powertransmitted by a vee rope or belt may be calculated from the effective tension Te = Ti — T2 and the belt speed

-t2 =

подпись: -t2 =Power P (watts) per rope Rope speed vb(m / s) kW x 1000

Equ 11.7

K xdP(nm) N(Rev/mIn) 1000 60

Ti

подпись: tiAnd

Ц0cosec ;

Equ 11.8

These tensions and powers are for one rope. By utilizing multi­ple ropes, the powertransmitted is directly proportional to their number i. e. three ropes will transmit three times the power.

When the pulleys are rotating, the belts tend to leave the pulley grooves due to the effects of centrifugal force. An additional tension is therefore given to the belts to overcome this effect. Thus the static load e on the bearings will be greater than the running load. It should be especially noted that bearing loads for correctly tensioned drives are the same for classical and wedge belts when the belt speed, pulley diameter, and power are the same.

Fig 11.4 Selection of wedge belt cross-section

подпись: 
fig 11.4 selection of wedge belt cross-section
With wedge belts, however, due to their smaller section and therefore greater flexibility, it is possible to use smaller pulleys. This then results in lower belt speeds and correspondingly in­creased tensions. There has therefore been reluctance by some users to employ wedge belts. Provided that the minimum pulley diameters and maximum pulley widths specified in Ta­bles 11.2 and 11.3 are followed and that drives are correctly ten­sions, both classical and wedge belts will function satisfactorily and will give acceptable motor and fan bearing lives.

0.175X71X2.9238

подпись: 0.175x71x2.9238

Ti

подпись: ti

As previously noted, powers beyond the capacity of a single belt are covered by using multi-grooved pulleys and a matched set of belts. Since both classical and wedge belts are manufac­tured from the same materials and have the same included an­gle, it follows that the tension ratio is not influenced by belt sec­tion.

British and International Standards effectively assume that n =

0. 175, in both cases, i. e. well below the limiting coefficient of friction and thus if the angle of wrap is 180° (n radians).

T,

= 2.7183′

Or

= 5

Or

T

2~ 5

It is repeated that n = 0.175 is a very pessimistic value and was chosen to give a margin of safety on the frictional grip between the rope and pulley.

The total running tension, which has to be resisted by the drive end fan bearing and the nose motor bearing = Ti + T2.

Thus:

Where kv is constant or

1.2 T, = kv x 0.8 T,

Or

K = — = 1.5 v 0.8

I. e.

T, + T2 =1.5(T1-T2)

The line of action of this pull will be determined by the number and section of the belts. A moment will be produced at the bear­ing and this will be reduced by keeping the pulleys as close as possible to the bearings.

The tension is that theoretically required for running and is usu­ally exceeded in practice. Where the tensioning of the drive is in accordance with the manufacturer’s recommendations, the figure should be multiplied by a safety factor of 1.25. Poor fit­ting, however, can result in considerable over tensioning when a factor of 2 is more appropriate.

Motor frame size

Min pulley dia (mm)

Max pulley width (mm)

D63

50

50

D71

63

50

DD80

75

100

D90S

75

150

D90L

115

100

D100L

160

100

D112M

200

100

D132S

160

160

D132M

215

125

D160M

180

200

D160L

245

160

D180M

260

160

D180L

280

160

D200L

315

200

D225S

355

200

D225M

400

200

Table 11.2 Pulley dimensions for electric motors

Fan Shaft dia (mm)

Min pulley dia (mm)

Max pulley width (mm)

20

80

100

30

90

100

40

140

125

50

180

140

55

250

160

60

280

160

65

315

160

70

355

170

80

400

170

90

450

170

100

500

170

125

630

170

Table 11.3 Pulley dimensions for fan shafts

Service factors

When determining the number of ropes in a multi-vee rope drive, it is usual to apply a service factor to the calculated power thus increasing the number of ropes above that theoretically necessary. This service factor is to take account of the in­creased loads likely when starting and for more arduous condi­tions during running (see Table 11.4).

It should be noted that such factors inevitably mean that under normal running conditions the drive may be over-engineered and thus of lower efficiency. The problem may be particularly serious where low power fans may be specified with two belts

When one might be sufficient. The value of low maintenance has to be weight against lowered energy efficiency. A soft start electric solution may be an alternative.

Special Cases

Types of prime mover

“Soft” starts

“Heavy” starts

For speed increasing drive of:

Electric Motors:

Electric Motors:

Speed ratio 1,00 — 1,24 multiply service factor by 1,00 Speed ratio 1,25-1,74 multiply service factor by 1,05 Speed ratio 1,75 — 2,49 multiply service factor by 1,11 Speed ratio 2,50 — 3,49 multiply service factor by 1,18 Speed ratio 3,50 and over multiply service factor by 1,25

AC — Star Delta start DC-Shunt Wound Internal Combustion Engines with 4 or more cylinders

All prime movers fitted with Centrifugal???

AC — Direct-on-Line start

DC — Series & Compound Wound Internal Combustion Engines

With less than 4 cylinders

Prime movers not fitted with soft Start devices

Hours per day duty

Types of Fan

< 10

> 10 to 16

>16

< 10

> 10 to 16

> 16

Blowers, exhausters and fans of all types up to 7.5kW

1,0

1,1

1,2

1,1

1,2

1,3

Blowers, exhausters and fans of all types above 7.5kW

1,1

1,2

1,3

1,2

1,3

1,4

Table 11.4 Service factors

The fan inertia and these must be determined to prevent belt breakage or tooth shear, see Figure 11.5.

Micro-vee belts

These combine the simplicity and flexibility of a single flat belt with the properties of higher power and higher speed ratios of vee belts. The belt is constructed with an uninterrupted strength member of synthetic cord extending across the whole width of the belt. Unlike vee ropes they do not operate by wedg­ing action but there is continuous contact between the ribbed surface of the belt and pulley grooves. Being a single belt, there are no matching problems and they cannot turn over as a result of shock loads.

Banded belts

In applications where pulsating or shock loads can cause nor­mal vee ropes to turn over, twist or whip, then banded belts are a solution, as shown in Figure 11.6. By joining together a num­ber of vee ropes with a tie band and thus forming a compromise between the flat belt and vee ropes, the lateral rigidity is in­creased sufficiently to resist turn over etc. Also by ensuring that the underlying ropes enter the pulley grooves in a straight line, excessive jacket wear is avoided, resulting in a longer life.

Figure 11.5 Toothed belt

подпись: figure 11.5 toothed belt

Posted in Fans Ventilation A Practical Guide


Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *