A fan includes a bracket, a stator, and a rotor. The bracket includes a tray, and a frame circling the tray. The stator is mounted to the tray and circled by the frame. The rotor includes a shaft, and a shell covering the stator. The rotor shell includes a top board, and a cylindrical hollow sleeve extending down perpendicularly from the top board. The shaft extends down perpendicularly from a center of the top board, and is rotatably mounted to the stator. A lower portion of the sleeve bounds a circumference of the tray. An assembly clearance is formed between the lower portion of the sleeve and the tray.