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 |
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 = |
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 |
And
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 |
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 |
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 |
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 |
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Hours per day duty |
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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.
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.
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 |
Posted in Fans Ventilation A Practical Guide