An apparatus comprising: a reference member, a first and second carrier (21, 22), a second central member, one or more offset members, and one or more coupling members; the second central member and all offset, and coupling members are configured to engage to transmit energy; the reference member comprises a central axis and a first and second carrier axis (1, 2); the first carrier comprises a first rotation axis and a number of first offset axes equal to the number of offset members; the second carrier comprises a second rotation axis and a number of second offset axes equal to the number of offset members; each offset member comprises a first and second member axis (8, 9); and wherein: all axes (0, 1, 2, 3, 4, 6, 7, 8, 9) are parallel; the first and second carrier axes (1, 2) are spaced apart by a carrier distance; for each offset member, the first and second member axes (8, 9) are spaced apart by the carrier distance; each first offset axis and the first rotation axis are spaced apart by an offset distance; each second offset axis and the second rotation axis are spaced apart by the offset distance; all first offset axes are arranged circumferentially around the first rotation axis; all second offset axes are arranged circumferentially around the second rotation axis; the second central member is co-axial and rotatably coupled with the central axis; the first rotation axis is co-axial and rotatably coupled with the first carrier axis; the second rotation axis is co-axial and rotatably coupled with the second carrier axis; each first member axis is co-axial and rotatably coupled with a different first offset axis; each second member axis is co-axial and rotatably coupled with a different second offset axis; and each coupling member simultaneously engages the second central member and a different offset member. a reference member, a first and second carrier (21, 22), a second central member, one or more offset members, and one or more coupling members; the second central member and all offset, and coupling members are configured to engage to transmit energy; the reference member comprises a central axis and a first and second carrier axis (1, 2); the first carrier comprises a first rotation axis and a number of first offset axes equal to the number of offset members; the second carrier comprises a second rotation axis and a number of second offset axes equal to the number of offset members; each offset member comprises a first and second member axis (8, 9); and wherein: all axes (0, 1, 2, 3, 4, 6, 7, 8, 9) are parallel; the first and second carrier axes (1, 2) are spaced apart by a carrier distance; for each offset member, the first and second member axes (8, 9) are spaced apart by the carrier distance; each first offset axis and the first rotation axis are spaced apart by an offset distance; each second offset axis and the second rotation axis are spaced apart by the offset distance; all first offset axes are arranged circumferentially around the first rotation axis; all second offset axes are arranged circumferentially around the second rotation axis; the second central member is co-axial and rotatably coupled with the central axis; the first rotation axis is co-axial and rotatably coupled with the first carrier axis; the second rotation axis is co-axial and rotatably coupled with the second carrier axis; each first member axis is co-axial and rotatably coupled with a different first offset axis; each second member axis is co-axial and rotatably coupled with a different second offset axis; and each coupling member simultaneously engages the second central member and a different offset member. all axes (0, 1, 2, 3, 4, 6, 7, 8, 9) are parallel; the first and second carrier axes (1, 2) are spaced apart by a carrier distance; for each offset member, the first and second member axes (8, 9) are spaced apart by the carrier distance; each first offset axis and the first rotation axis are spaced apart by an offset distance; each second offset axis and the second rotation axis are spaced apart by the offset distance; all first offset axes are arranged circumferentially around the first rotation axis; all second offset axes are arranged circumferentially around the second rotation axis; the second central member is co-axial and rotatably coupled with the central axis; the first rotation axis is co-axial and rotatably coupled with the first carrier axis; the second rotation axis is co-axial and rotatably coupled with the second carrier axis; each first member axis is co-axial and rotatably coupled with a different first offset axis; each second member axis is co-axial and rotatably coupled with a different second offset axis; and each coupling member simultaneously engages the second central member and a different offset member.