Abstract: An overpressure valve for a hydraulic pit prop comprises a housing, a valve seat provided in the housing, a valve body movable in axial direction in the housing between a closed position engaging the valve seat and an open position, and bellows means filled with a gas under pressure arranged for biasing the valve body to the closed position.

Abstract: A rotatable pressing spindle is mounted in a housing which surrounds it and which has an upper section, a lower section and a middle section. The middle section includes a self-supporting outer jacket and a tubular screen received with radial clearance in the outer jacket. Mounting elements are provided on the outer jacket, extending inwardly thereof and mounting the screen on the outer jacket, and connecting elements connect the outer jacket with the upper and lower housing sections, respectively. Other features are also disclosed.

Abstract: A centrifuge drum is mounted for rotation about an upright axis and has a bottom wall provided with an opening. A closure member is mounted for movement relative to the bottom wall between a first position in which it sealingly blocks the opening, and a second position in which it unblocks the opening. A fluid-pressure actuated arrangement is operable to move the closure member to its first position and maintain it there as long as desired. A connecting head is provided which connects the fluid pressure-actuated arrangement with a source of pressure fluid.

Abstract: A centrifuge drum is mounted for rotation about an upright axis and has a bottom wall provided with an opening. A closure member is mounted for movement relative to the bottom wall between a first position in which it sealingly blocks the opening, and a second position in which it unblocks the opening. A fluid-pressure actuated arrangement is operable to move the closure member to its first position and maintain it there as long as desired. A connecting head is provided which connects the fluid pressure-actuated arrangement with a source of pressure fluid.