u201c dimensioned so that it is irreversible u201d means that the different dimensions observe the following condition see fig2 and 3 the ratio between the value of the eccentricity e and the radial distance between the center o of the outer circumference 6 of the eccentric mounting 4 and the point of contact between the eccentric mounting 4 and the sliding housing 7 is less than the value of the static friction co efficient between the eccentric mounting 4 and the sliding housing 7 for low eccentricity values in the previous formulation u201c by value of the eccentricity u201d is meant the minimum distance between the longitudinal axis of the inner circumference 5 and the longitudinal axis of the outer circumference 6 of the eccentric mounting 4 i e for an angle u03b1 for which the value is close to zero r is the contact radius in other words the distance between the center o of the outer circumference 6 of the eccentric mounting 4 and the point of contact f is the static friction co efficient between both surfaces in contact the formulation according to which lt lt the average angular position of the eccentric mounting is suitably selected in order to compensate a maximum of wear along the axis b defined earlier while minimizing the movement of the rack 2 along the axis c perpendicular to this axis b and located in a plane normal to the longitudinal axis a of the rack 2 gt gt is explained by the diagram of fig3 it means that the angle u03b1 formed by the straight line passing through the center of the off center inner circumference.