| 摘要 |
A mine roof support comprises a canopy, a base section, hydraulic props for raising and lowering the canopy relative to the base section, a hydraulic valve for supplying hydraulic fluid under pressure to the props via a check valve, and a yield valve connected across the check valve. The yield valve has a valve member movable between a first position in which it co-operates with a valve seat to prevent the release of fluid from the props via the yield valve and a second position in which fluid can be released from the props via the yield valve. One side of the valve member is subjected to fluid pressure downstream of the check valve to urge the valve member towards its second position and the other side of the valve member is acted upon by a spring device to urge the valve member towards its first position. The other side of the valve member and the spring device are also subjected to fluid pressure upstream of the check valve so that more and more of the spring force applied by the spring device is substituted by hydraulic force applied to the other side of the valve member by fluid pressure upstream of the check valve as the latter increases until the fluid pressure acting on the spring device balances the urging force thereof at a nominal yield pressure of the yield valve.
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