An actuator assembly comprising: a compliant actuator shaft having an axis and a first end and a second end; a drive assembly (30, 32, 34) provided at the first end of the compliant actuator shaft to cause rotation of the compliant actuator shaft about the axis; an overtravel stop mechanism comprising: a screw shaft, coaxial with the compliant actuator shaft, and mounted to and arranged to rotate with the second end of the compliant actuator shaft, the screw shaft having an outer thread extending in the axial direction; a nut threadedly mounted to the outer thread of the screw shaft and arranged to move axially relative to the screw shaft as the screw shaft rotates; a first stop fixed to and radially extending from the screw shaft to define the axial limit of movement of the nut relative to the screw shaft, in a first axial direction, as the screw shaft rotates, by abutment of the nut against the first stop; and wherein the first end of the compliant actuator shaft is provided with a first number n of splines for spline engagement with the drive assembly and the second end of the compliant actuator shaft is provided with a second number of splines, the second number being one spline more than the first number of splines, for spline engagement with the screw shaft. a compliant actuator shaft having an axis and a first end and a second end; a drive assembly (30, 32, 34) provided at the first end of the compliant actuator shaft to cause rotation of the compliant actuator shaft about the axis; an overtravel stop mechanism comprising: a screw shaft, coaxial with the compliant actuator shaft, and mounted to and arranged to rotate with the second end of the compliant actuator shaft, the screw shaft having an outer thread extending in the axial direction; a nut threadedly mounted to the outer thread of the screw shaft and arranged to move axially relative to the screw shaft as the screw shaft rotates; a first stop fixed to and radially extending from the screw shaft to define the axial limit of movement of the nut relative to the screw shaft, in a first axial direction, as the screw shaft rotates, by abutment of the nut against the first stop; and wherein the first end of the compliant actuator shaft is provided with a first number n of splines for spline engagement with the drive assembly and the second end of the compliant actuator shaft is provided with a second number of splines, the second number being one spline more than the first number of splines, for spline engagement with the screw shaft. a screw shaft, coaxial with the compliant actuator shaft, and mounted to and arranged to rotate with the second end of the compliant actuator shaft, the screw shaft having an outer thread extending in the axial direction; a nut threadedly mounted to the outer thread of the screw shaft and arranged to move axially relative to the screw shaft as the screw shaft rotates; a first stop fixed to and radially extending from the screw shaft to define the axial limit of movement of the nut relative to the screw shaft, in a first axial direction, as the screw shaft rotates, by abutment of the nut against the first stop; and wherein the first end of the compliant actuator shaft is provided with a first number n of splines for spline engagement with the drive assembly and the second end of the compliant actuator shaft is provided with a second number of splines, the second number being one spline more than the first number of splines, for spline engagement with the screw shaft.