Page 224 - Industrial Power Engineering and Applications Handbook

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toothshaped belts seat into pulley-matching grooves. By convention, this is provided by the belt manu-
For transmission of smaller loads, flat belts and for facturer per 25 mm of belt width, at different speeds
small and medium loads, V-belt drives are normally of the faster shaft (smaller pulley) for different
employed. recommended widths of belts, number of plies and
type and grade of duck, etc. We show Type I and
8.5.1 Flat belts Type 11 in Table 8.1 for a general illustration. These
ratings may vary marginally from one manufacturer
These are long centre drives with small slips. The slack to another, depending upon their product mix and
side of the belt is kept on the top side to increase the quality of curing.
angle of contact with the pulleys by sag on the top side.
This is essential for an efficient transfer of load. The SF (Service factor) - as in Table 8.5.
recommended maximum power that can be transmitted Correction for arc of contact - as in Table 8.7.
by one belt of different cross-sectional areas is provided
by the belt manufacturer and some ratings are given in In the following example we illustrate a brief procedure
Table 8.1. When selecting these drives, the following to select a flat belt for transmission of a load.
parameters should be borne in mind:
Example 8.1
1 The ratio of the diameter of the pulleys should not Determine the width of a heavy-duty double-leather flat belt
exceed 6: 1 unless a jockey (idler ) pulley or similar for transmitting a load of 18.5 kW at 1480 r.p.m. from a
arrangement is made to press the top side to indirectly squirrel cage motor to be switched directly on line. Diameters
of pulleys are 250 mm on the motor side and 200 mrn on the
increase the arc of contact. load side. The centre distance between the pulley may be
2 Similarly, for shorter centre distances between the considered as 800 mm.
drive and the driven pulleys, the arc of contact will
decrease. To ensure a good arc of contact, the centre P= 18.5 kW
distance C (Figure 8.10), should be kept as much as
possible, otherwise the provision of a jockey pulley, SF = 1.4 for heavy-duty Class 3, as shown in Table 8.5,
presuming operations for 10 hours/day.
as noted above, will also be necessary. A higher arc
of contact will ensure a better grip of the belt on the Correction for arc of contact:
pulley and hence a smaller slip during transmission
of the load. A smaller slip would mean a higher Corresponding to D-d - 250 - 2oo (Figure 8.10)
transmission of load and vice versa. C 800
3 Vertical and right-angle drives should be avoided. = 0.0625
4 Belts should not be tightened more than necessary, The correction factor from Table 8.7 = 0.99
otherwise the drive and the driven shafts will come
. w= 18.5 x 1.4
under torsion and excessive bending moment. The ..
bearings would also be subjected to excessive 0.99
stresses. = 26.17 kW
The belt must be rated at least for this load.
Specification of flat belts
1480 x 250
These belts are made of cotton duck (a cotton fabric Speed of the faster pulley = 200
used in making canvass and tents) with different mixes
to provide them with a degree of hardness, as noted in = 1850 r.p.m. (a)
Table 8.2. or Kx 2oo = 19.36 m/s, which is quite low
The duck is glued in thin layers (plies) with a rubber 60 x 1000
compound and vulcanized (cured). The number of plies (< 30 m/s) hence acceptable.
used to make a belt and their quality defines the strength Consider a belt width of 200 mm to transmit this load.
of the belt (e.g. 3 ply x 32 or 3 ply x 34, etc.).
25
Therefore minimum belt rating per 25 mm = - x 26.17
200
Selection of flat belts for transmission of load
= 3.27 kW (b)
The selection of flat belts is made along similar lines For parameters (a) and (b) the possible belt sizes can be
to that for V-belts (discussed later in more detail). The
load-transmission capacity of a flat belt can be defined 1 Four-ply 28 soft duck or four-ply 31 hard duck, according
by to Table 8.1 Type I, having a rating of 3.25 kW at 1800
r.p.m. and 3.45 kW at 2000 r.p.m., or
SF 2 Three-ply 32 soft duck or three-ply 34 hard duck, according
w= P. Correction for arc of contact (8.1) to Table 8.1 Type II, having a rating of 3.32 kW at 1800
r.p.m.
where
P = load to be transmitted in kW For a more judicious selection of belts, keeping economics in
W = maximum load-transmission capacity of the belt. mind, it is advisable to seek an opinion from the manufacturer.