Principle of rolling office chair

How does the swivel mechanism on an office chair work?
How does it work without metal parts grating against each other from the weight of the seat's occupant? Is the center rod threaded in some way, if so how is the seat able to spin endlessly without screwing off? I know this is a dumb question but I've always wondered this.


Answer

It is a gas cylinder that has an internal piston that compresses a gas to a higher pressure when weight is put on it. A friction clamp to hold the piston's rod in a set position is the height selector. A soft spring gives a bit of cushion when a person sits on the chair. If the chair is too low, a lever releases the friction clamp, letting the piston come up, raising the chair against the chair's weight. If a person is sitting in the chair at that time, the gas pressure against the piston is not enough, and the chair goes down.

Basically, the gas cylinder and piston give it a low resistance spin and allows it to spin infinitely in any direction because it is all resting on the piston. Think of how a car wheel can spin with out coming off and conceptually its the same principle.