Variable-speed belt drive and variable-speed drive device equipped with such a variable-speed drive unit

Variable-speed drive unit, comprising:

• a first pulley and a second pulley (1, 11),
• each of the two pulleys mounted on a corresponding pulley-support shaft (2, 12),
• each of the two pulleys (1, 11) comprising a pair of variably-spaced flanges (4, 5;
• 14, 15);
• and
• at least one transmission belt (9) between the two pulleys (1, 11), the two pulleys further comprising a first loading device and a second loading device respectively equipping a first flange (4, 14) of each pair of flanges, each of the first and second loading devices tending to exert, in a permanent manner and in the active state of the transmission, an axial effort in the direction causing the flanges (4, 5;
• 14, 15) to move inwardly to clamp the belt (9) between said flanges (4, 5;
• 14, 15),
• wherein the first loading device exerts a lesser axial effort than an axial force exerted by the second loading device;
• and
• one of the two pulleys also comprises a manual control element (19) for, independently of the corresponding loading device, drawing together or separating the flanges of the one pulley by exerting, on the first flange of the one pulley, a slight control effort corresponding essentially to a power difference of the first and second loading devices.