Pulleys and drive belts - Adhesion drive

Belts are transmission devices in the same way as gears or roller chains. Their role is to transmit power from one rotating element to another, i.e. the product of an effort and a speed or torque.

The operating principle of belts is based on two main elements:

• a contact surface, the size of which depends on its coefficient of friction, which enables torque to be taken from the drive pulley, transmitted to the traction armature and then returned to the drive pulleys;

• a frame, capable of transforming the tangential force applied to the drive pulley into a longitudinal tensile force between the pulleys.

The combination of these two elements characterizes the transmissible force, which is directly linked to the non-slip properties of the former and the tensile strength of the latter. This is why, at present, these elements are most often made, on the one hand, of an elastomer and, on the other hand, of synthetic fibers.

Although a mechanical component, the belt's flexibility allows it to be designed with a certain degree of imprecision, thus avoiding the need for highly sophisticated, and therefore much more costly, solutions. But if the belt is the least expensive solution for a transmission (no lubrication required), it is also almost universally applicable thanks to the combination of its different possibilities:

• multiplication or reduction of movement depending on pulley diameter ;

• speed variation with pulleys with movable flanges ;

• reversing the direction of rotation by crossing the wires ;

• transmission with non-parallel axes and idlers ;

• declutching with a retractable idler.

Moreover, since the belt's operating principle allows it to slip in the event of overload, it dampens vibrations and absorbs jolts, while also acting as a fuse in the event of a transmission component blockage. When a fuse blows, it is often overlooked as the cause of a manufacturing defect, whereas premature destruction can also reflect a design fault in the transmission; as in electricity, simply fitting a stronger fuse is not enough to eliminate the malfunction. A prime example of these characteristics is agricultural machinery, where virtually all movements are powered by belts.

Belts have improved considerably since their origin. First there was the flat belt, always appreciated for high linear speeds and small winding diameters. Then, the constant quest to improve the performance and compactness of drives led to the creation of...