| 摘要 |
PROBLEM TO BE SOLVED: To solve the problem of having difficulty in confirming whether lock is released or not since a conventional locking kelly-bar is in an excavated hole when releasing the lock. SOLUTION: Ribs 36a-36c provided at the outer periphery of kelly-bars 30-32 are provided with grooves 38a-38m arranged at specified spaces, with a distance X from the upper ends. Driving plates 37a, 37b provided at the inner peripheries of the kelly-bars 30, 31 and rotary table 27 are engaged with the grooves 38a-38m to lock the kelly-bars 30-32 and rotary table 27 to each other. When extended most, the locking kelly-bar 24 is locked at the grooves 38a, 38e, 38i of distance X from the upper ends. At the release of lock, the locking kelly-bar 24 falls by the distance X and collides with the kelly-bars 30-32 and rotary table 27. The release of lock can be positively recognized by the collision sound made at this time.
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