Page 156 - The Tribology Handbook

91 Belt drives

BELT TYPES AND MATERIALS SELECTION

Standard Vee, Vee-ribbed and synchronous (timing) belts have a high modulus wound-cord tension member (glass fibre,
polymeric or steel members are used according to application), a rubber carcass and a woven fabric backing cover. The
drive surface of synchronous belts is strengthened by a woven fabric cover, that of Vee-belts may be covered or uncovered
(raw), while Vee-ribbed belts are generally raw. The flat belts considered here have polyamide strip tension members and
either rubber or leather drive surfaces. This handbook does not consider all variations of belt section and materials that
exist. Users should consult catalogues for the full range of constructions and materials.
The operating temperature range of belts is typically -20°C to +7O"C, but materials may be formulated for both lower
(-40°C) or higher (+12OoC) temperatures. The static conductivity of belts is regulated by standards (e.g. IS0 1813, IS0
284, IS0 9563). Belts should be shielded from oil but most types are resistant to small amounts of contamination.
Power transmission efficiencies of 95% to 98% in steady operating conditions are achievable with all well maintained belt
drives.

Table 1.1 Selection of the type of belt
WEDGE AND CLASSICAL VEE BELTS These are the standard choice for large power transmissions
where slip in the event of shock loading is needed as overload
protection. Wedge belts with a larger ratio of H to W than
classical Vee belts give more compact drives but cannot he used
in layouts requiring reverse bending of the belt. Stock pulley sizes
allow speed ratios up to 8:l. Recommended maximum belt speeds
are 30 to 40 m/s.

Further information
BS 3790, IS0 1081, DIN 221 1, RMAIP 20
IS0 4184, DIN 7753, RMAIP 23
and manufacturer's catalogues

VEE-RIBBED BELTS These have been developed to combine the grip of Vee belts and
almost the flexibility of flat belts and find application where space
is confined (smaller diameter pulleys) or where some serpentine
layout capability is needed. Stock pulley sizes allow higher speed
ratios than for Vee belts, up to 25:l depending on belt section.
Recommended maximum belt speeds are 35 to 45 m/s.

Further information
IS0 8372, DIN 7876, RMAIP 26
IS0 9982 and manufacturer's catalogues

FLAT BELTS The ease ofjoining polyamide strip tension member belts enables
them to be made of virtually any length; their flexibility makes
them suitable for highly serpentine or out of plane layouts. At low
T- -T belt speeds, drives are less compact and bearing loads are higher
than for other belt types, but as speed increases above 20 rn/s.,flat
belts come into their own. Speeds up to 70 m/s are possible.
The cross-sections of flat belts are not controlled by Standards.
Flat belts are rated by k,,, the load per unit width to stretch the
belt 1%.
Further information from manufacturer's catalogues

SYNCHRONOUS BELTS Synchronous belts are now developed to a similar power capacity
as Vee belts. They are clearly essential if synchronous motion is
"1 needed but can suffer from tooth failure in conditions of extreme
shock loading. The earlier developed trapezoidal toothed belt
(illustrated) has now been displaced by curvilinear toothed belts
in new drive designs. Typical maximum speed ratios are 10: I and
belt speeds are up to 60 m/s.

Further information
BS 4548, IS0 5296, DIN 7721
and manufacturer's catalogues
B1.2