A roller gear (34, 600) comprising: a series of rollers (36a, 36b, 604, 605, 606) arranged to form a substantially circular array; at least two roller mounting members (39a, 39b, 35, 611, 612) axially spaced from one another along an axis of rotation of the roller gear, each roller mounting member comprising a substantially circular array of openings for receiving roller mounting pins therein to mount the rollers between the roller mounting members; an array of roller mounting pins (28, 68) disposed in the openings, each of the roller mounting pins comprising a roller mounting shaft, having a longitudinal axis, for entering the roller mounting members and the roller (604, 607) to retain the roller relative to the roller mounting members; at least one of the roller mounting pins comprising a roller mounting pin head (616, 718), delimited from the shaft by a change in radial dimension of the pin; the roller gear further comprising primary roller mounting pin retaining means (619, 716), arranged to engage the head of the at least one roller mounting pin to prevent axial movement of the roller mounting pin in a direction of its longitudinal axis, further comprising i) fixing means to fix the primary roller pin retaining means relative to a roller mounting member to which it is mounted, wherein: the at least one roller mounting pin has a neck of reduced diameter relative to the pin head and shaft, for providing said change in radial dimension of the pin and for engaging the primary roller pin retaining means to retain the pin in the roller gear; and the primary roller pin retaining means has a pin head retaining portion having a pair of opposed sides spaced apart by a distance greater than the diameter of the neck of the pin and less than the diameter of the head and the diameter of the shaft of the pin, or ii) fixing means to fix a secondary roller pin retaining means relative to a roller mounting member to which it is mounted, the secondary roller mounting pin retaining means being arranged to restrict rotation of at least one roller mounting pin via the roller pin head. a series of rollers (36a, 36b, 604, 605, 606) arranged to form a substantially circular array; at least two roller mounting members (39a, 39b, 35, 611, 612) axially spaced from one another along an axis of rotation of the roller gear, each roller mounting member comprising a substantially circular array of openings for receiving roller mounting pins therein to mount the rollers between the roller mounting members; an array of roller mounting pins (28, 68) disposed in the openings, each of the roller mounting pins comprising a roller mounting shaft, having a longitudinal axis, for entering the roller mounting members and the roller (604, 607) to retain the roller relative to the roller mounting members; at least one of the roller mounting pins comprising a roller mounting pin head (616, 718), delimited from the shaft by a change in radial dimension of the pin; the roller gear further comprising primary roller mounting pin retaining means (619, 716), arranged to engage the head of the at least one roller mounting pin to prevent axial movement of the roller mounting pin in a direction of its longitudinal axis, further comprising i) fixing means to fix the primary roller pin retaining means relative to a roller mounting member to which it is mounted, wherein: the at least one roller mounting pin has a neck of reduced diameter relative to the pin head and shaft, for providing said change in radial dimension of the pin and for engaging the primary roller pin retaining means to retain the pin in the roller gear; and the primary roller pin retaining means has a pin head retaining portion having a pair of opposed sides spaced apart by a distance greater than the diameter of the neck of the pin and less than the diameter of the head and the diameter of the shaft of the pin, or ii) fixing means to fix a secondary roller pin retaining means relative to a roller mounting member to which it is mounted, the secondary roller mounting pin retaining means being arranged to restrict rotation of at least one roller mounting pin via the roller pin head. i) fixing means to fix the primary roller pin retaining means relative to a roller mounting member to which it is mounted, wherein: the at least one roller mounting pin has a neck of reduced diameter relative to the pin head and shaft, for providing said change in radial dimension of the pin and for engaging the primary roller pin retaining means to retain the pin in the roller gear; and the primary roller pin retaining means has a pin head retaining portion having a pair of opposed sides spaced apart by a distance greater than the diameter of the neck of the pin and less than the diameter of the head and the diameter of the shaft of the pin, or ii) fixing means to fix a secondary roller pin retaining means relative to a roller mounting member to which it is mounted, the secondary roller mounting pin retaining means being arranged to restrict rotation of at least one roller mounting pin via the roller pin head.