shoe 146 are biased apart by spring 150 which bears against shoe 140 thrust face 182 and shoe 146 thrust face 198 shoe 108 is compressed in fig1 and relaxed in fig2 the limit of maximum expansion 158 between shoes 140 and 146 is set by the length of rivet 136 the limit of maximum expansion is greater than the distance between flanges 74 and back wall 90 of track 70 so that friction wall 166 presses against back wall 90 and friction wall 170 presses against flanges 74 head 154 of rivet 136 is recessed in shoe 140 so that the head does not contact wall 90 bearing collar 188 receives arm 98 opening 176 holds rivet 136 and rivet sleeve 138 which includes upper spring guide 130 and lower spring guide 132 longitudinal lip 72 strengthens flange 74 and encloses friction wall 170 in a u channel with flange 74 and wall 76 of the track wall 190 of the outer friction shoe surrounds guide wall 194 of the inner friction shoe the limit of maximum compression of shoe 108 is determined by end 208 of wall 190 and limit shoulder 204 of inner friction shoe 146 when they meet.