Drive belts, which convey forces from one pulley to another solely by means of frictional forces, require a certain tension which provides the necessary contact against the pulleys. It is previously known to start and maintain this tension either by means of adapting the distance between the pulleys or by means of a tensioning pulley, which presses against a length of the belt and deflects it in a larger or smaller arc in order to tension the belt. An adaption of the distance between the shafts requires that at least one shaft is movable, which in turn generally implies that the entire unit, of which the shaft is a part, has to be movable. Many times, such an arrangement is not possible because of the fact that such a displacement cannot be done or is not convenient. If an automatic post-tensioning of the belt is desired after mounting, something which in many cases is desirable, it is often most disadvantageous if an entire unit should have to be displaced by means of, for instance, springs or the force of gravity.
When the principle of correcting the distance between the shafts is concerned, the only remaining possibility, in many cases, is to apply a manual post-tensioning. For the remaining cases, the principle of a tensioning pulley must be applied. This, however, implies that an additional bearing has to be arranged and when automatic post-tensioning by means of a spring-loaded tensioning pulley is concerned, there is a risk that this and its suspension start to oscillate.
For this reason, there is a need of another type of tensioning device, which is not associated with the above-mentioned disadvantages. For chain drives it has been suggested to insert a shaft-less, resilient ring between the chain lengths. The ring may adopt a compressed shape between the chain lengths, when in a compressed position, in order to subsequently expand to a more circular shape whenever a post-tensioning is required. This solution, which is evident from DD, A3, 275 166, may not, however, be applied for belt drives. When chain drives are concerned, the tensioning pulley is toothed and engages with the chain and will therefore maintain its position in the system. When belt drives are concerned, on the other hand, slippage is to be expected, resulting in that the ring may "float "within the system.