| 摘要 |
878,825. Pit props. GUTEHOFFNUNGSHUTTE STERKRADE A. G. Dec. 11, 1957 Dec. 11, 1956], No. 38570/57. Class 20(2) An hydraulic telescopic pit prop is provided in its head with a spring-pressed rod 3 adapted to engage the mine roof and compress the spring 4 when the prop is set so that on release of the prop the spring overcomes initial friction and resistance and assists in collapsing the prop. |
