A linkage mechanism is disclosed that produces high travel advantage in one phase of linear actuator motion and high mechanical advantage in another phase of linear actuator motion. The linkage mechanism may be powered by a hydraulic actuator system and is capable of producing high preloads and a high amount of travel with minimal actuator motion or force. The linkage mechanism may be used to secure a hatch door to a pressure vessel housing or may be used in any environment involving high pressure differentials, for example.