arcuate path 171 is depicted in fig4 it will be recognized that as each shoe moves along its arcuate path it is imperative that the pivotal movement of the shoes be restrained to the extent that the arcuate surfaces of the shoes continue to collectively define a circumferential surface that is concentric with the central axis of the frame failure of the shoes to maintain this circularity of orientation over the full range of possible diameters of the circumferential surface defined by the shoes results in uneven pressure being applied to the object being grasped by the apparatus the several links 38 and the drive link 40 are interconnected for simultaneous and equal rotation of the links about their respective rotational axes of mounting to the frame 20 as effected by the drive unit 42 in the embodiment depicted in fig3 and 4 specifically the respective distal extensions 70 of the several links 38 are interconnected by means of a plurality of rigid connecting rods 210 each of which is provided with a turnbuckle 212 for adjusting the length of the rod 210 one end 214 of each rod 210 is pivotally connected as by a pin 216 typical fig4 to a through bore 74.