Circular manhole cover and frame - uses oblique tapered ribs at say third points on cover perimeter and entering frame grooves via screw action

摘要

The circular manhole cover and frame is for carriageway installation, using a thread to provide axial interlock. Oblique ribs at 120 degree intervals on the cover circumference form spiral ramps and act as screw threads, entering cover bore grooves (G) of the reciprocal form. Ribs may have an isosceles trapezium shape in horizontal cross section, the faces subtending approximately 28 and 45 degrees respectively with the horizontal. The groove provides circumferentially offset ramps limited by faces in a common oblique diametrical plane. An alternative tongue may have a half round cross section.