Page 76 - Mechanical Engineer's Data Handbook

APPLIED MECHANICS 65

2.2 Belt drives

2.2. I Flat, vee and timing belt drives Tension ratio for belt about to slip:
' ,F
Formulae are given for the power transmitted by a belt For pulley a >=epe*
drive and for the tensions in the belt. The effect of F2
centrifugal force is included. F
=
A table of information on timing belt drives is For pulley 'b' 2 ereb
included. F2
where: e=base of natural logarithms (=2.718).
Symbols used:
F, =belt tension, tight side Power capacity P=o(F, -F,)
F, = belt tension, slack side where: belt velocity u=2nraN,= 2nr,N, (no slip).
r, =radius of pulley a Pulley torque T,=r,(Fl -F,); T,=r,(F, -F2)
rb =radius of pulley b
(F +F )
N, = speed of pulley a Initial tension F, = -L--L
N, = speed of pulley b 2
m=mass of belt per unit length Effect of centrifugal force: the belt tensions are
P = power transmitted reduced by mu2 so that
p =codficient of friction between belt and pulley
F, = initial belt tension F, -mu2
--
6, =arc of belt contact pulley a F, -mu2 - d9
eb=arc of belt contact pulley b
L = distance between pulley centres Vee belt
s = percentage slip
u = belt velocity The 'wedge' action of the vee belt produces a higher

N, rb (100-s) effective coefficient of friction p'
Speed ratio -=-
N, r, ~ 100 p'=- P
(when pulley b is the driver) sin a
where: a=the 'half angle' of the vee (p'=2.9p for
Arc of contact (r, >rb): a = 20").
ea= 180" + 2 sin-' -
L

Ob= 180" -2 sin-' -
(ra -rb)
L