The system has a toothed rack (3) slidably mounted in a steering box (2), and comprising two ends that are exterior to the box and connected to steering wheels via rods. An assistance module (M) has an assistance motor (6) acting on the rack via a ball screw device (8) that transmits assistance force to the rack. A sealing unit has an annular lip seal (17) and an annular seal (18) mounted between the box and a toothed rack portion (10). The seal internally comprises a helical structured rotary part engaged with a thread of the rack portion and maintained in a fixed plane relative to the box.
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