A planetary gear system comprising: a sun gear forming a first part; a ring gear co-axial with the sun gear forming a second part; at least one planetary gear which meshes with both the sun gear and the ring gear forming a third part; a planetary gear carrier which rotationally supports the at least one planetary gear forming a fourth part; wherein the there is provided a torque clutch which is supported by and connects directly between two of the parts; wherein the torque clutch prevents relative rotation of the two parts below a predetermine torque value and allows relative rotation of the two parts above the predetermined value; wherein there is a torque threshold adjustment mechanism (302, 310, 322, 321, 352) by which the predetermined value of the torque at which the torque clutch slips can adjusted; characterised in that the torque clutch has a first part and a second part, the first part rotating relative to the second part when the torque clutch is slipping, the first part and second part rotating in unison when the torque clutch is not slipping; wherein one part comprises a path support comprising: a circular path formed on a surface of the path support; and a plurality of apertures formed through the path at predetermined positions along the path; wherein the other part comprises a bearing support mechanism located adjacent to the path support and which is capable of rotating relative to the path support, the bearing support mechanism comprising a plurality of bearings moveably mounted on the bearing support mechanism which are biased to extend from and engage with the path; wherein the bearings slide along the path when the bearing support mechanism rotates relative to the path support, the bearings aligning with and extending into the apertures when the bearing support mechanism is located at predetermined angular positions relative to the path support; wherein the predetermined value of the torque at which the torque clutch slips is dependent on the amount the bearings extend into the apertures; wherein there is further provided a penetration adjustment mechanism located adjacent to the path support, the penetration adjustment mechanism co-operating with the path support to adjust the amount by which the bearings can extend into the apertures when the bearings are aligned with the apertures. a sun gear forming a first part; a ring gear co-axial with the sun gear forming a second part; at least one planetary gear which meshes with both the sun gear and the ring gear forming a third part; a planetary gear carrier which rotationally supports the at least one planetary gear forming a fourth part; wherein the there is provided a torque clutch which is supported by and connects directly between two of the parts; wherein the torque clutch prevents relative rotation of the two parts below a predetermine torque value and allows relative rotation of the two parts above the predetermined value; wherein there is a torque threshold adjustment mechanism (302, 310, 322, 321, 352) by which the predetermined value of the torque at which the torque clutch slips can adjusted; characterised in that the torque clutch has a first part and a second part, the first part rotating relative to the second part when the torque clutch is slipping, the first part and second part rotating in unison when the torque clutch is not slipping; wherein one part comprises a path support comprising: a circular path formed on a surface of the path support; and a plurality of apertures formed through the path at predetermined positions along the path; a circular path formed on a surface of the path support; and a plurality of apertures formed through the path at predetermined positions along the path; wherein the other part comprises a bearing support mechanism located adjacent to the path support and which is capable of rotating relative to the path support, the bearing support mechanism comprising a plurality of bearings moveably mounted on the bearing support mechanism which are biased to extend from and engage with the path; wherein the bearings slide along the path when the bearing support mechanism rotates relative to the path support, the bearings aligning with and extending into the apertures when the bearing support mechanism is located at predetermined angular positions relative to the path support; wherein the predetermined value of the torque at which the torque clutch slips is dependent on the amount the bearings extend into the apertures; wherein there is further provided a penetration adjustment mechanism located adjacent to the path support, the penetration adjustment mechanism co-operating with the path support to adjust the amount by which the bearings can extend into the apertures when the bearings are aligned with the apertures.