| 摘要 |
<p>An improved expansion unit made from heavy sheet metal for use in supporting a ceiling or roof, particularly a mine roof, comprises a wedge and a cylindrical expansion shell having a taper provided by a plurality of lateral serrations. The serrations are of gradually diminishing height designed to ensure a uniformly parallel expansion of the shell in the pre-drilled hole of the roof when the wedge is driven downwardly.</p> |
