| 摘要 |
During upward drilling operations the casing for the hole is, at times, supported from the drill stem from which the cutterhead is operatively driven. A collapsible spider is placed about the drill stem and supported therefrom, and has extension arms which, during one operating condition, pass underneath the lower end of the casing for providing it with vertical support, and in another operating condition are pivotally folded upwardly so that the casing may be moved vertically past both the spider and its associated drill stem.
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