drive in both the rear and the front part fig1 a shows an expansion element 3 as shown for example in the illustration of fig1 although the devices for rotatably driving the cage 1 are changed here the expansion element 3 in fig1 b is notable for two rectangular plates 5 at both ends of the elongated base body 4 which have the same dimensions the bearing surfaces 9 do not terminate at the surfaces of the shorter narrow sides 33 a hexalobular socket 12 is incorporated in each plate 5 the expansion element 3 according to fig1 c is notable for bead shaped recesses 34 in the shorter narrow sides 33 of the rectangular plate 5 for improved locking in the expanded state of the cage fig1 d shows an expansion element 3 with two plates 5 of equal dimensions wherein the bearing surface 9 terminates at the surfaces of the shorter narrow sides 33 to produce an h shaped bearing lying flat in a plane the expansion element 3 according to fig1 e differs from the expansion element according to fig1 b in that additional bead shaped recesses 34 serving for improved locking are incorporated in both shorter narrow sides 33 of the plates.