| 摘要 |
<p>A retractable extension jib for a crane boom (1) has a frame body (4) rotatably mounted on an end of the boom (1) so as to be rotatable about a rotary axis, a horizontal shaft (6) provided at a lower portion of the frame body and rotatably connected to a base portion of a jib body (2). A pair of oil hydraulic cylinders (9) are provided between a base portion of the jib body (2) and an upper portion of the frame body (4) so as to rotate the jib about the horizontal shaft (6). The hydraulic cylinders (9) are so arranged as to be able to rotate the jib from a retracted position to a lower position where a longitudinal axis of the jib is substantially parallel with the rotary axis.</p> |
