32a serving the primary purpose of reducing the torque peak once such occlusion has taken place it is to be understood that facing 32a although highly advantageous is optional and that its presence does not alter the fact that plate 32 of the platen assembly is rigid and provides firm support for tubing 11 the upper surface of plate 32 and the upper surface of facing 32a if provided is arcuate when viewed in transverse section specifically the upper surface 35 defines an elongated channel or trough for receiving and supporting tubing 11 the radius of transverse curvature of the trough is slightly greater than the radius of curvature of the outer surface of each outer bearing race or member 17 ideally the radius of curvature of the trough should equal the sum of the radius of each bearing assembly plus the extent of its eccentricity plus double the wall thickness of tube 11 plus the thickness of membrane 13 where the membrane is included since the membrane is relatively thin its thickness is exaggerated in the drawings for clarity of illustration it may be stated that the radius of curvature of the trough approximates the radius of each bearing assembly plus its eccentricity plus 2 times the wall thickness of the tube the result is that when tube 11 is fully compressed and its lumen is occluded by a bearing assembly as depicted in fig3 the curvatures of the bearing assembly the collapsed wall.