each other by relative movement of the structural member 2 and 4 along the central axis whereby the spring clamp 14 of the female coupling member 6 and the head section 46 of the male coupling member 8 snappingly engage each other automatically as indicated in fig2 any two diametrically opposite spring arms 38 when in a relaxed condition are spaced from each other by a predetermined minimal distance a this distance is smaller than the minimal diameter of the annular groove 56 of the head portion 46 for a predetermined amount in order to have the spring arms 38 engage the head portion 46 under a predetermined biassing force when the snap in coupling has been closed while the coupling members 6 and 8 are being inserted into each other the insertion portions 44 of the spring arms 38 initially slide along the conical surface 54 of the head portion 46 whereby the spring arms 38 are resiliently deflected in an outward direction so as to perform a centering action between the head portion 46 and the spring clamp 14 as soon as the insertion portions 44 of the spring arms 38 have been moved beyond the cylindrical surface 56 of the head portion 46 the spring.