Abstract: A displacement control assembly includes a rotatable lead screw longitudinally extending within a tubular column supporting the assembly. A nut is threaded to the screw for relative axial displacement of the screw and nut, for example, to adjust the height of a chair seat or similar fixture. A resilient brake structure is carried by either the nut or the screw for resiliently biased, alternative locking and unlocking of the nut and screw in order to selectively prevent relative rotation and relative axial displacement of the screw and nut. The releasable lock allows the screw and nut to co-rotate without relative axial displacement so that the chair seat can be swiveled without altering the desired height. The assembly can be designed with the parameters correlated so that the seat is lowered at uniform speed independent of the weight of the seat occupant.