Fixing assembly with helical ramp

The fixing unit for two assembled parts (2,4) comprises a cylindrical peg (6), a collar (8) comprising a lug (28) and a support (12). There is a rotational guide (14) for the collar connected to the collar and the support. There is also a helicoidal ramp (26) in the peripheral surface of the peg for receiving a lug. The ramp comprises first (30) and second (32) parts having opposite helixes.