A speed change mechanism, comprising: a. a first planetary gear set, comprising a first sun gear, a plurality of first planet pinions, a first carrier arm and a first ring gear; b. a second planetary gear set, comprising a second sun gear, a plurality of second planet pinions, a second carrier arm and a second ring gear; c. one of the first sun gear and the first ring gear comprises a first input and the other of the first sun gear and the first ring gear comprises a first output; d. one of the second sun gear and the second ring gear comprises a second input and the other of the second sun gear and the second ring gear comprises a second output; e. a power input connection for drivingly connecting the first input (111; 114) with an output from a power source; f. a drive arrangement directly couples the first output (111; 114) to the second input (121; 124) to transfer drive from the first output to the second input, the first and second planetary gear sets (11, 12) being drivingly coupled so that the first and second outputs (111, 121; 114, 124) are driven in opposite rotary directions; characterised in that: g. the first and second carrier arms (113, 123) are both fixed; h. the speed change mechanism includes a rotatable output sleeve ring surrounding the first and second planetary gear sets (11, 12), a first one-way clutch (115, 141) for transmitting drive from the first output (111; 114) to the sleeve ring and a second one-way clutch (125, 142) for transmitting drive from the second output (121; 124) to the sleeve, both one way clutches being operative to transmit drive in one and the same rotary direction only. a. a first planetary gear set, comprising a first sun gear, a plurality of first planet pinions, a first carrier arm and a first ring gear; b. a second planetary gear set, comprising a second sun gear, a plurality of second planet pinions, a second carrier arm and a second ring gear; c. one of the first sun gear and the first ring gear comprises a first input and the other of the first sun gear and the first ring gear comprises a first output; d. one of the second sun gear and the second ring gear comprises a second input and the other of the second sun gear and the second ring gear comprises a second output; e. a power input connection for drivingly connecting the first input (111; 114) with an output from a power source; f. a drive arrangement directly couples the first output (111; 114) to the second input (121; 124) to transfer drive from the first output to the second input, the first and second planetary gear sets (11, 12) being drivingly coupled so that the first and second outputs (111, 121; 114, 124) are driven in opposite rotary directions; characterised in that: g. the first and second carrier arms (113, 123) are both fixed; h. the speed change mechanism includes a rotatable output sleeve ring surrounding the first and second planetary gear sets (11, 12), a first one-way clutch (115, 141) for transmitting drive from the first output (111; 114) to the sleeve ring and a second one-way clutch (125, 142) for transmitting drive from the second output (121; 124) to the sleeve, both one way clutches being operative to transmit drive in one and the same rotary direction only.