P = tension in belt (1-tight, 2-slack)
r = pulley radius
T = Torque (in-lb)
w = rpm
f = coefficient of friction (leather = 0.3, rubber = 0.25 Flat belt (use .5123 for V-belts)
B = 1/2 groove angle
C = distance between shafts (in.)
D₁ = sheave diameter
D₂ = sheave diameter
L = Length
m' = mass/unit length of belt
= Angle of contact

• Angle of contact
  Note:
• Use (+) for large pulley and X-belts
• Use (-) small pulley
• Angle in radians