if the available depth of the groove 11 is not sufficient to provide the necessary flexibility in the region of the depressions 5 then a further groove 13 can be provided as can be seen from fig6 the groove 13 is arranged a relatively small distance above the depressions 5 in the radial circumferential surface 1b and extends radially inward therefrom this additional groove 13 makes it possible to compensate for the loss of flexibility due to the reduced depth of the groove within the scope of the invention only the groove 13 can be provided instead of the groove 11 as is shown in fig6 and 8 the groove 13 also isolates a spring portion of the nut along its depth and in the region below the groove 13 i e between the groove 13 and the lower end face of the nut 1 39 the groove 13 either alone or in combination with groove 11 provides the nut with the resilient flexibility that enables the depressions 7 to ratchet into and out of engagement with the projections 5 as the nut is tightened against the counterpart 2 in the two embodiments described above the depressions 5 are constructed integrally with the nut whilst the bosses 7 are provided on the counterpart 2.