US Patent 6851135 Foldable skeleton for playpen

A foldable skeleton for a playpen foldable skeleton includes an upper frame, four upright rods, and a bottom frame. The upper frame includes two pairs of first rods, two pairs of second rods, four joints, and four connecting seats. Each of the joints includes a main base, a press button, a cover, a locking block, an elastic member, and two linking blocks. The bottom frame includes four support seats, four bottom rods, two linking gears, a connecting base, a pull member, and an operation member. Thus, the teeth of the two linking gears mesh with each other, so that the two linking gears are rotated relative to each other in a synchronous manner, and the bottom rods are folded synchronously.