Abstract: A drive transmission apparatus is disclosed in which a driven shaft (18) carries two freely rotatable gear wheels (52) which are in mesh with gear wheels (50). The gear wheels (50) are fast in rotation with threaded spindles (32) which extend radially with respect to the shaft (18). Each spindle (32) passes through a tapped bore in a block (38). Each block (38) carries a sprocket (36). There are one-way clutches between the sprockets (36) and blocks (38). When viewed from the side the axes of the sprockets (36) are located on the circumference of a circle the size of which changes when the spindles (32) are rotated to drive the blocks (38) and hence the sprockets (36) radially inwardly or radially outwardly. A chain (72) is trained around the sprockets (36).