| 摘要 |
A mine roof support system including a relief valve communicable with the props of each roof support through respective control valve means, and a main return pipe for receiving liquid discharged, by way of the relief valve, from the props when liquid at high pressure is no longer supplied thereto and the roof support is being advanced. A movable element of the relief valve comprises at least two relatively-movable and connected parts and is so co-operable with its seating as to ensure that when the support is so advancing a substantially constant predetermined back-pressure is maintained in the props sufficient to maintain the support in contact with the mine roof. If the pressure in the main return pipe exceeds the pressure then subsisting in the control valve means, one of the parts of said element is caused automatically to close onto the seating. |
