| 摘要 |
A prop comprises a central screw (3) and a nut composed of two parts (6a, 6b) disposed in a head (5) turning on the end of the support tube (1). Said two nut parts (6a, 6b) are movable between a position of cooperation with the screw (3) and a position in which it is separated from the screw (3), under the control of arms (12a, 12b). A return spring (16) urges said nut parts (6a, 6b) towards their position of engagement with the screw. The flanks of the thread of the nut parts (6a, 6b) are symmetrical, while the external thread of the screw (3) has asymmetrical flanks, the bottom flank having a slighter slope, thus making it possible for the screw (3) to be pulled by hand into the top position without the nut parts (6a, 6b) preventing this. Safety hooks (9a, 9b) limit the axial movement of the head (5) relative to the tube (1). The large displacements of the screw (3) in both directions can be achieved without the nut having to be turned. Fine adjustment is made with minimum rotation of the head (5). Two safety devices prevent the head (5) from moving out of the tube (1) in the case of one (8, 9) of these devices, and, in the case of the other (16), prevent the screw (3) from moving out of the head (5) and the tube (1), when the prop is inclined with its head downwards.
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