In belt conveyors the driving power is transmitted to the belt by the driving pulley which is rotated by an electric motor. The basic mechanism of transmission of power from the pulley to the belt is based on the theory of friction drive.
The fundamental equation for a belt conveyor drive is given by: (The Euler’s equation)
T1 ≤ T2.eµα
T1 and T2 are the tight side and slack side tensions of the belt at the driving pulley
α = wrap angle of the belt in radiation
e = Naperian base
µ = Friction factor
The peripheral effective pull TE from a driving pulley, neglecting losses on the driving pulley due to belt stiffness is determined from the following reaction:
Te = T1 – T2
T2min > T E Max (1 / (eµα -1))
T E Max is the maximum effective peripheral pull in N, which often occurs when starting up or when braking the completely loaded conveyor.
In other conditions Te is the average effective pull. Maximum effective pull is usually 20% to 50% more than the average effective pull, depending on the type of motor starter and coupling.
Table: Coefficient of friction between driving pulley and rubber belting type of pulley lag
|Operating conditions||Smooth bare steel pulley||Rubber lagging with herring bone grooves||Polyurethane lagging with Herring bone grooves||Ceramic lagging with Herring bone grooves||PVC belt|
|Dry||0.35 to 0.4||0.4 to 0.45||0.35 to 0.4||0.4 to 0.45||0.25 to 0.35|
Clean wet (Water)
|0.1||0.35||0.35||0.35 to 0.4||0.15 to 0.3|
Wet and dirty ( Clay or Loam)
|0.05 to 0.1||0.25 to 0.3||0.2||0.35||Less than 0.25|
The value of α depends on the particular drive system selected and may range from 180° to maximum 440°.
Minimum belt tension
T = 4.2 Pc ( Wb + Wm )
Pc = idler spacing on the carrying side
Wb, Wm are the weights of belt and pay load per meter length of belt respectively.