Abstract: A device for supporting a chair at a selected height, comprising a hydraulic cylinder operative to support the chair, the cylinder being contractable to lower the chair under the weight of the chair occupant and being subject to a spring bias which extends the cylinder to raise the chair when unoccupied. The cylinder comprises two fluid-filled chambers separated by a valve. The fluid flows between the chambers via the open valve during expansion and contraction of the cylinder, and the valve when closed blocks the fluid flow whereby to hydraulically lock the cylinder. The hydraulic fluid is preferably a grease.

Abstract: An adjustable support device for a swivel chair, comprises a swivel tube supported from a vertical rotatable screw by a nut. When the chair is unoccupied a spring-loaded friction clutch responsive to the weight acting on the screw restrains the screw against rotation so that the seat height can be adjusted by rotating the swivel tube which is attached to the seat part of the chair. When the chair is occupied, the clutch is released and the screw is freed for rotation with the tube so that swivelling movements of the seat part do not result in height adjustment, a continuous, but limited, frictional drag being applied between the screw and the nut to ensure that the screw and nut rotate as a unit when the clutch is released.