| 摘要 |
The jack (V) comprises a tubular primary screw (10) movable in rotation and translation and provided with an outer screw thread (10a) and an inner screw thread (10b) which have opposite thread hands, a fixed secondary screw (11) engaged in the inner screw thread (10b), a nut (9) connected to the movable element and engaged with the outer screw thread (10a) of the primary screw (10). The nut (9) is composite and comprises a first nut (9a) composed of a plastics material having a good coefficient of friction and a second nut (9b) composed of a mechanically strong material. Axial forces of normal amplitude are absorbed by the plastics nut (9a), while the exceptional overloads, resulting for example from an impact on the front of a vehicle whose seat structure is provided with the jack of the invention, are absorbed by the second nut (9b). In normal operation, the friction between the plastics nut (9a) and the screw (10) are considerably reduced, which provides a jack having an excellent efficiency.
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