with each pair of outer lower springs 50 being disposed around an elongate spring rod 52 extending between and mounted using rod mounting brackets 53 to a pair of spaced abutments 68 which in turn are mounted to and project downward from lower surface 65 of middle bearing plate 60 externally adjacent and parallel to a corresponding pair of inner lower springs 40 of the first centering means each outer lower spring rod 52 passes through an opening 28 a in a lug member 28 mounted to and projecting upward from a medial side region of lower bearing plate 20 such that for each pair of outer lower springs 50 one spring 50 is disposed around the corresponding outer lower spring rod 52 accordingly when middle bearing plate 60 is laterally displaced in either direction relative to lower roller bed 30 and lower bearing plate 20 as previously described one outer lower spring 50 on each side of lower roller bed 30 will be compressed between a corresponding lug member 28 and a corresponding abutment 68 this can be seen in fig3 c in which the compressed outer lower spring is indicated by reference number 50 a preferably each outer lower spring rod 52 carries a washer 51 on either side of.