| 摘要 |
A mine roof support unit has a floor sill, a roof cap supported above the floor sill by a pair of inclined hydraulic props, and control apparatus for controlling load-bearing capacity of the props. The inclination of the props relative to the vertical increases as the props are retracted. The control apparatus comprises a pressure-relief valve for limiting the pressure of the hydraulic fluid supplied to the hydraulic props. The pressure-relief valve has a closure member which is biased towards its closed position by a spring. The spring is backed by an abutment member which constitutes a setting device for adjusting the biasing force of the spring, thereby controlling the operating pressure of the pressure-relief valve, and hence the load-bearing capacity of the props. The setting device is operatively associated with a movable part of the roof support unit in such a manner that the operating pressure of the pressure-relief valve is reduced in response to movement of said movable part of the roof support unit which results from extension of the hydraulic props. |
