A failsafe mechanism for a tool is described, such as a lifting or separating wedge. It comprises a housing 5 for an actuator rod and an actuator rod 10, which is preferably a piston. The actuator rod 10 is adapted to change repeatedly between active and inactive configurations to activate and deactivate the tool in response to a control mechanism. The actuator rod or piston 10 has a deformation region 15 which can change between an operative configuration and an inoperative configuration. In the operative configuration the actuator rod 10 can change repeatedly between active and inactive configurations, and in the inoperative configuration this change is resisted or prevented. In the event of a load being applied to the actuator rod 10 in excess of a load limit, the actuator rod 10 deforms preferentially at the deformation region 15, which is adapted to change from the operative configuration to the inoperative configuration to resist activation of the tool.