| 摘要 |
A mine roof support includes a first liquid supply means having a first control valve for connecting a source of liquid under pressure to the legs of the support. The support is also provided with a second control valve which is actuatable, when the pressure of the liquid being supplied to the legs attains a value below the setting pressure for the legs, to connect the legs to a source of liquid under pressure by way of a second liquid supply means. Time-delay means is operable on the second control valve to cause it to close when a predetermined time has elapsed after the pressure in the legs has reached the setting pressure. |
