When, according to the invention, a coupling is provided between a driving shaft (1) and a hub (6), the shaft (1) being configured with several sets of grooves (2, 3) in which an elastic cord (4) may extend, the cord (4) in the deep track (2) will have a slight or poor engagement with the hub (6), while the cord in the low track (3) will be pressed out into firm engagement with the hub (6).
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