A surgical instrument includes: an adjustable end effector; an elongated instrument shaft defining an internal bore; and a drive assembly including a housing coupled to an end of the instrument shaft and an input device configured to facilitate controlled adjustment of the end effector via a drive cable extending along the bore of the instrument shaft. The input device includes: a drive shaft attached to a first portion of the drive cable, the drive shaft including an upstanding stem portion having a radially tapered outer surface; and a capstan attached to a second portion of the drive cable, the capstan defining an internal bore sized to receive the stem portion of the drive shaft, at least a lower portion of the internal bore having a radially tapered inner surface. The capstan and drive shaft are configured to transition from a disengaged state to an engaged state.