Page 158 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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6.4.3.1 Compute Design Power, P ca
A belt drive is designed based on the design power P , which is obtained by multiplying
ca
a service factor with the rated power of a mover, P,expressed as
P = K P (6.24)
A
ca
where K is a service factor, depending on the nature of duty of driver and driven
A
machines. Due to the inherent characteristics of belt drives, such as slippage and
damping effect of rubble, service factors for belt drives are low compared with those for
chain or gear drives. Therefore, the lower range of values from Table 2.1 can be selected
as service factors for belt drive design.
6.4.3.2 Specify Suitable Belt Types
The belt type can be tentatively selected from standards or manufacturer’s catalogues
according to the rotational speed of driving sheave n and design power P .Whena
1 ca
single V-belt has insufficient capacity, either multiple belts or a lager cross section belt
can be selected, determined from an economic point of view. For example, considering
the cost and the weight of sheave, a single Type B V-belt may be a better choice than two
Type A belts.

6.4.3.3 Determine the Sheave Size
The size of driving sheave should be greater than the minimum diameter recommended
for each belt type (see Table 6.1). Although a diameter less than the minimum diame-
ter could lead to a compact design, it will increase bending stress in belts and thereby
shorten belt life. The desirable dimension of driven sheave is computed by D = iD and
2
1
rounded to a diameter according to Table 6.1.
−1
Belt speeds should be within a range of 5–25 m s . A lower speed may cause an
increase of belt tension, the number of belts and the likelihood of slippage, while a higher
speed may increase dynamic effects, such as centrifugal force, belt whip and vibration,
and eventually affect the working capacity of belt drives.

6.4.3.4 Confirm the Centre Distance, a and Belt Datum Length, L
d
(1) Specify a trial centre distance, a
0
Shorter centre distances are preferred for stability of operation and economy of
space. However, too small a centre distance may increase the frequency the belt cycles
sheaves, increasing the likelihood of fatigue failure. The minimum centre distance is
limited by sheave dimensions and the minimum contact angle. On the other hand, too
large a centre distance will increase the size of belt drive and cause vibration if the belt
speed is high. The maximum centre distance is limited by available belt datum lengths.
The following formula is used to initially estimate a nominal acceptable range of centre
distance [6]
0.7(D + D ) < a < 2(D + D ) (6.25)
2
1
0
1
2
(2) Compute preliminary belt length, L ′ d
Belt length is obtained by summing the contact arc lengths with twice the distance
between the beginning and end of contact. As introduced previously, when a sheave