Abstract: A locking device for adjusting an angle or height of two parts of a seat comprises a housing, a rod slidable within the housing, a push rod with a spring to urge it forward, a spring cylinder, a cylindrical collet able to be compressed inwardly thereby engaging the spring cylinder with its cleaved portion, a cone fixed at the forward end of the push rod, moveable within the collet, a second spring biasing the cone, and drive means for driving the push rod along the housing. When the cone engages the collet by action of the second spring, the collet abuts the cylinder, thereby locking the rod and housing together. When the cone disengages from the collet, the collet is released from the spring cylinder by its own resilient force thereby releasing the rod from the housing.