Page 77 - Mechanical Engineer's Data Handbook

66 MECHANICAL ENGINEER’S DATA HANDBOOK

Timing belts

Timing belts have teeth which mate with grooves on
the pulleys. They are reinforced with high strength
polymer strands to give power capacity up to three
times that of conventional belts at three times the
speed. There is no slip so a constant ratio is main-
tained. A large number of speed ratios is available.
Belts are made in several strengths and widths.

Timing belt sizes (BS 4548: 1WO)

Type Meaning Pitch (mm) Widths (mm) Constant, K

XL Extra light 5.08 6.4, 7.9, 9.6 -
L Light 9.53 12.7, 19.1, 25.4 1.53
H Heavy 12.70 19.1, 25.4, 38.1, 50.8, 76.2 5.19
XH Extra heavy 22.23 50.8, 76.2, 101.6 12.60
XXH Double extra heavy 31.75 50.8, 76.2, 101.6, 127.0 -
~~ ~

Service factor

Hours of service per day < 10 1&16 > 16
% full power 100 72 67

Class Applications % full power

1 Typewriters, radar, light domestic 100
2 Centrifugal pumps, fans, woodworking machines, light conveyors 69
3 Punching presses, large fans, printing machines, grain conveyors 63
4 Blowers, paper machines, piston pumps, textile machines 58
5 Brickmaking machines, piston compressors, hoists, crushers, mills 54

Power capacity P=KNTWx kilowatts Example: Type H belt, W=50.8mm, N=1500
revmin-’, T=20, for large fan working 12 hours per
where: day. From tables, K=5.19 service factors 72% and
K = size constant (see table) 63 Yo.
N =number of revolutions per minute
T= teeth in smaller pulley P=5.19~ 1500~20~ x ~0.63 ~0.72=3.59kW
50.8
W= width of belt (mm)

Note: at high speeds and with large pulleys the power capacity may be up to 25% less. See manufacturer’s tables.