A damper includes a rotary shaft, a sleeve, damping oil, and the damping oil is filled in a closed chamber formed between the rotary shaft and the sleeve; cooperating grooves are formed on the rotary shaft; partition parts are formed between the two cooperating grooves; oil passing holes provided on the partition parts are communicated with the two cooperating grooves; the oil passing holes are controlled by the check valves to make the damping oil flow in a single direction between the two cooperating grooves; the rotary shaft passing through the cooperating hole on the necking stop wall of the sleeve is connected by a connection member; two spacers are integrated with the sleeve; the height of the spacers locating in the cooperating grooves just fits in with the depth of the cooperating groove; an oil passing gap formed between the rotary shaft and the sleeve and is variable.