Chair chassis and chair having the same

A chair chassis, comprising:

• a main tray base, a seat bearing plate, a movable support member and a pair of movable supporting plates, wherein the seat bearing plate is disposed above the main tray base, for bearing a seat, a front end of the movable support member is pivotally connected to the seat bearing plate, a lower end of the movable support member is pivotally connected to the main tray base, a rear end of the movable support member is used for connecting a chair backrest, the front end, the lower end and the rear end of the movable support member are in a triangle relationship and the front end and the rear end are pivotable by taking the lower end as a fulcrum, the pair of movable supporting plates are located in front of the movable support member, upper and lower ends of each movable supporting plate are respectively pivotally connected to the seat bearing plate and the main tray base, so that the seat bearing plate is subject to balanced support when moving with the movable support member, and a pivot where the movable support member and the main tray base are connected is mounted with an elastic member for resisting the movable support member from rotating rearward,
• further comprising a lock mechanism disposed on the main tray base, wherein the lock mechanism is connected to the seat bearing plate and the movable support member, for locking the seat bearing plate and the movable support member,
• wherein the lock mechanism comprises a screw, a nut and a line control assembly, the nut is mounted in the main tray base, an upper end of the screw is connected to the pivot where the movable support member and the seat bearing plate are connected, a lower end of the screw is close to the front relative to the upper end and inserted into the nut, part of positions in an outer periphery of the screw have teeth, part of positions in an inner periphery of the nut have teeth, in a first rotational position of the nut, the teeth of the nut and the screw are engaged to fixed the screw, in a second rotational position of the nut, the teeth of the nut and the screw are dislocated so that the screw can move freely in the nut, and the line control assembly is connected to the nut, for controlling the nut to switch between the first rotational position and the second rotational position.