on threaded plate 4 according to fig1 which is identified here in the section by reference numeral 22 the surfaces 30 and 30 u2032 on the sectioned threaded plate serve for positioning in a y direction the projection 11 of the side strut 2 according to fig1 and the surfaces 29 and 29 u2032 for positioning in an x direction the projection being identified by reference numeral 21 in the cross section the aforedescribed positioning surfaces acting between projection 11 and threaded plate 4 and stop element 3 of fig1 are the same in shape and function as the positioning surfaces acting between projection 11 and threaded plate 4 and stop element 3 u2032 of fig2 with sufficiently large contact areas of the surfaces 23 23 u2032 24 24 u2032 25 25 u2032 and 26 26 u2032 the symmetrically arranged surfaces 27 27 u2032 28 28 u2032 29 29 u2032 and 30 30 u2032 may be eliminated since precise machining of the surfaces becomes nevertheless more difficult and costly with its increasing size the configuration shown in cross sectional view in fig5 is preferred the cooperating surfaces 23 23 u2032 or 30 30 u2032 as well as 24 24 u2032 or 29 29 u2032 reliably prevent twisting of the side strut relative to the shaft rod u2014 even when these surfaces are small in.