Adjustable length cylinder support pillar for chair seat

Claim 1

A cylinder of adjustable length, particularly for use as a support pillar for a chair, seat or the like, comprising: a cylinder body, two pistons slidable in the cylinder body, a rod connected to both pistons and projecting from one end of the cylinder body, a dividing wall which forms part of the cylinder body and defines a first chamber facing one of the two pistons and a second chamber separate from the first chamber and facing the other piston, the firs and second chambers being filled with a fluid, the dividing wall also having a duct which puts the first and second chambers into communication with each other, a remotely-controllable valve interposed in the duct and adapted to cut off the communication between the two chambers, preventing the passage of the fluid from one chamber to the other and consequently locking the rod relative to the cylinder body, a main helical spring interposed axially between the cylinder body and a support element which is slidable axially relative to the rod and is urged by the helical spring into abutment with a stop surface carried by the end of the rod outside the cylinder body so that the main helical spring tends to urge the rod towards the position in which it projects furthest from the cylinder body, and an auxiliary helical spring which is interposed between the support element and a further stop surface carried by the rod and is intended to damp compressive forces to which the cylinder is subject.