Variable ratio epicyclic transmission

A variable ratio epicyclic drive transmission, of the kind having a fixed central axis and comprising a rotatable driving element (10) with said fixed axis as its axis of rotation, a rotatable driven element (11) also with said fixed axis as its axis of rotation, a central sun wheel (12) fixed relative to one of said elements so as to rotate in concert with said one element also with said fixed axis as its axis of rotation, a stationary outer ring member co-axial with said elements, a planet carrier (15) fixed relative to the other of said elements so as to rotate in concert with said other element with said fixed axis as its axis of rotation and defining an orbiting axis of rotation parallel to and radially spaced from said fixed axis, and at least one planet wheel (19) mounted on said carrier (15) for rotation about its own axis and for orbital movement in engagement with said ring member, said transmission being characterised;

• in that the ring member comprises an almost complete, but broken, substantially circular loop of a pliable positive drive tensile member (14) engagable by said planet wheel and supported by a contractile iris structure (13) permitting the diameter of the loop to be varied while maintaining its circularity;
• in that said iris structure includes joint means (20) comprising a tail guide defining a passage (33) extending out of the loop at one end of the break therein, through which said tensile member may be drawn from the loop or taken back into the loop, and bridging means (47) maintaining continuity of engagement between the ring member and the planet wheel (19) as it traverses the break in the loop;
• in that the planet wheel (19) is swing mounted on the planet carrier (15);
• and in that orbiting drive transfer means (53,54,55) mounted on said carrier (15) connect the planet wheel (19) to the sun wheel (12).