Italian patent 1311501 and WO95/34765 disclose transmission devices which comprises a first cylindrical sleeve which can be mechanically coupled or uncoupled with a second cylindrical sleeve by tightening or loosening, respectively, a helical spring arranged in a substantially coaxial manner astride these two cylindrical sleeves, wherein a control collar is mechanically coupled with a tubular member, so that the mutual translation of the control collar with respect to the tubular member also causes their mutual rotation, the ends of the helical springs being arranged into seats made in the control collar and in the tubular member so that their mutual rotation tightens or loosens the coils of the helical spring, thereby causing the coupling or the uncoupling, respectively, between the first cylindrical sleeve and the second cylindrical sleeve.
The torque which can be transmitted by this known device depends on the force exerted by the helical spring on the outer walls of the two cylindrical sleeves, i.e. depends substantially on the size of the spring, so that it is necessary to enlarge the whole device for transmitting high torques, with consequent size problems of the same device.
U.S. Pat. No. 4,986,400 and U.S. Pat. No. 4,825,988 disclose transmission devices with two pairs of cylindrical sleeves, two control collars and two helical springs, which however cannot transmit high torques, since the helical springs have the same coiling direction, so that they cannot be tightened at the same time on both pairs of sleeves.