A friction drive clutch comprising: a hub adapted to be engaged with and rotated about an axis by a drive mechanism; a first housing surrounding a portion of the hub, the hub being rotatable relative to the first housing; a thrust plate engaging the hub so as to rotate with the hub and to be slidable with respect to the hub in the axial direction; a second housing surrounding a portion of the hub and located adjacent to the first housing, the thrust plate and the hub being rotatable with respect to the second housing, the second housing being rotatable with respect to the first housing, the second housing engaging the thrust plate so that the second housing and the thrust plate move axially together with respect to the hub during engaging and disengaging of the clutch; a third housing surrounding a portion of the hub and being rotatable with respect to the hub, the third housing being located adjacent to the thrust plate, the third housing being adapted to be engaged with and to rotate a driven mechanism; at least two first ramped members extending axially from a surface of the first housing, each first ramped member having a sloped axial face; and at least two second ramped members extending axially from a surface of the second housing toward the first ramped members, each second ramped member having a axial sloped face, each second ramped member being at least partially aligned with a corresponding one of the first ramped members; wherein when the second housing is rotated in one direction with respect to the first housing, the sloped faces of the second ramped members slide along the sloped faces of the corresponding first ramped members to urge the second housing in the axial direction away from the first housing, thereby causing the thrust plate to move axially into frictional engagement with the third housing, characterised in that an adjustable sheave is mounted on the third housing, the adjustable sheave configured to be slid axially on the third housing for permitting adjustments for speed and/or tensioning of a belt driven by the adjustable sheave; and wherein a spring assembly biases the thrust plate and the second housing in the axial direction toward the first housing; and wherein the axial movement of the thrust plate and the rotation of the second housing in the one direction are opposed by the biasing of the spring assembly. a hub adapted to be engaged with and rotated about an axis by a drive mechanism; a first housing surrounding a portion of the hub, the hub being rotatable relative to the first housing; a thrust plate engaging the hub so as to rotate with the hub and to be slidable with respect to the hub in the axial direction; a second housing surrounding a portion of the hub and located adjacent to the first housing, the thrust plate and the hub being rotatable with respect to the second housing, the second housing being rotatable with respect to the first housing, the second housing engaging the thrust plate so that the second housing and the thrust plate move axially together with respect to the hub during engaging and disengaging of the clutch; a third housing surrounding a portion of the hub and being rotatable with respect to the hub, the third housing being located adjacent to the thrust plate, the third housing being adapted to be engaged with and to rotate a driven mechanism; at least two first ramped members extending axially from a surface of the first housing, each first ramped member having a sloped axial face; and at least two second ramped members extending axially from a surface of the second housing toward the first ramped members, each second ramped member having a axial sloped face, each second ramped member being at least partially aligned with a corresponding one of the first ramped members; wherein when the second housing is rotated in one direction with respect to the first housing, the sloped faces of the second ramped members slide along the sloped faces of the corresponding first ramped members to urge the second housing in the axial direction away from the first housing, thereby causing the thrust plate to move axially into frictional engagement with the third housing, characterised in that an adjustable sheave is mounted on the third housing, the adjustable sheave configured to be slid axially on the third housing for permitting adjustments for speed and/or tensioning of a belt driven by the adjustable sheave; and wherein a spring assembly biases the thrust plate and the second housing in the axial direction toward the first housing; and wherein the axial movement of the thrust plate and the rotation of the second housing in the one direction are opposed by the biasing of the spring assembly.