Abstract: The present invention provides an actuator (20) for selectively displacing an output member (33) against an opposing load (L) in response to the output (24) of a motor (23). The actuator includes a first transmission mechanism (50) for displacing the output member relative to the motor at a first nominal ratio with respect to the motor output; a second transmission mechanism (60) for displacing the output member relative to the motor at a nominal second ratio with respect to the motor output; a selector (34, 39) for coupling the motor output to the output member only through the first transmission mechanism (50) when the load is less than a predetermined value, and for coupling the motor output to the output member through the first and second transmission mechanisms (50, 60) when the load is greater than the predetermined value. The force exerted by the output member on the load is the sum of the forces transmitted through the first and second transmission mechanisms.