Abstract: A hopper which contains loose material, and a door and frame assembly for attachment to the bottom of the hopper, for controlling the flow of the loose material therefrom. The assembly includes a door frame formed of spaced side and end walls, and a cross-piece which, with the side walls and one of the end walls, defines a hopper outlet opening. A door is slidably mounted to the frame so as to be across the outlet opening when in a closed position and to be clear of the opening when in an open position. A door moving device is provided for sliding the door, with respect to the opening, between the closed position and the open position. The frame particularly includes structure for attaching a plurality of different types of door moving devices, including air or hydraulic cylinder arrangements, or rack-and-pinion arrangements operated by an electric motor, or by hand or by means of a chain or cable.