The transmission (21) has a first coil spring (10) wound on a casing (1) and on a first drum (5). Its dia. can be increased by turning an internal-tooth gearwheel (9) in one direction. There is also a second coil spring (11) wound on the same casing and on a second drum (6) in the same direction as the first spring. The dia. of the second coil spring can be increased by turning the gearwheel the opposite way to that required to increase the dia. of the first coil spring. There are also cylindrical elements (33,34) outside the two coil springs to limit the variation in their dias. These elements can turn relative to the gearwheel.
