The belt-drive should transfer, with relatively low energy loss, and as little servicing as possible through retensioning, a relatively high torque from the crankshaft to more and more ancillary components.
The belt-drive it is not there to control, but rather for the transmission of torque. Before there were today's, wide ribbed belts, e.g., the ancillary components of truck or coach engines were connected with each other and driven by multiple fan belts. The costs were high, and the efficiency was low.Single (wedge) belt-drives can provide only one particular maximum transmission ratio. If it becomes too big, the looping angle and radius of the small wheel are too low. In this case, and if the driving- and the driven wheel are too far removed from each other, a multiple belt drive can make sense.
The belt drive has a long history. In the old days, only one drive source was available to manufactures, (e.g., a waterwheel) all the machinery had to be driven by flat leather belts. Thus, energy was mechanically distributed around the whole factory. The first motor cars also had to be satisfied with this form of power transfer, before the constructors changed over to the more efficient chains and gearwheels. Still today there are any number of two-wheeled vehicles using the belt-drive as a transmission element. 09/09