Abstract: An adjustment mechanism comprising a pivoting transmission member which surrounds an inlet member having three outwardly-directed projections. Between the inlet member and the bearing surface there are placed three springs urging wedging bodies towards the projections of the inlet member, and three fixed fingers. One of the fingers is of width sufficient to come into contact with the corresponding wedging bodies to prevent these bodies from wedging against the corresponding projections when the inlet member is in its rest position, while the other two fingers are of width that is small enough for the corresponding wedging bodies to then wedge against the corresponding projections.