An adjusting device for a grading apparatus with a plurality of endless ridge-belts arranged side by side for moving continuously in a forward direction, forming a gap there in-between, the ridge-belts being longitudinally supported by plurality of guide rails, said adjusting device being arranged underneath said ridge-belts and supporting guide rails, for adjusting the distance between said ridge-belts, said adjusting device comprising; • a plurality of support members, said support members comprising a cylindrical central section further comprising a first screwing section, the first screwing section being a double action inner elevated portion of the cylindrical central section forming a screw thread structure extending from the centre of the cylindrical central section and towards each opening, • a plurality of adjusting screwing members, said adjusting screwing member being having a cylindrical shape and a second screwing section, the second screwing section being a double action outer elevated portion of a cylinder forming a screw thread structure extending from the centre of the cylinder and towards each end, said adjusting screwing member further comprising an inner hollow structure providing a grip for rotation, and • an adjusting screwing shaft, said adjusting screwing shaft being adapted to rotate said adjusting screwing member by penetrating the hollow structure of the adjusting screwing member having a shape providing gripping structure fitting in the circumference of the inner hollow structure of the adjusting screwing member, wherein support members and adjusting screwing members are alternately arranged in a perpendicular line, where each adjusting screwing member is screwed into the support member on each side and where each support member is positioned underneath and corresponds to each guide rails, wherein said second screwing section of the adjusting screwing member, interacts with said first screwing section of the support members upon rotation of said adjusting screwing shaft for increasing or decreasing the distance between adjacent support members and thereby said guide rails and said rigid belts, characterised in that the first screwing section of the support member is a square screw thread where the corners of the crest rim have been rounded off, in that the second screw section of the adjusting screwing member is a square screw thread where the corners of the crest rim have been rounded off, and in that root corner of the first screw section of the support member and the root corner of the second screw section of the adjusting screwing member are rectangular to provide less friction when the adjusting screwing shaft is being rotated. • a plurality of support members, said support members comprising a cylindrical central section further comprising a first screwing section, the first screwing section being a double action inner elevated portion of the cylindrical central section forming a screw thread structure extending from the centre of the cylindrical central section and towards each opening, • a plurality of adjusting screwing members, said adjusting screwing member being having a cylindrical shape and a second screwing section, the second screwing section being a double action outer elevated portion of a cylinder forming a screw thread structure extending from the centre of the cylinder and towards each end, said adjusting screwing member further comprising an inner hollow structure providing a grip for rotation, and • an adjusting screwing shaft, said adjusting screwing shaft being adapted to rotate said adjusting screwing member by penetrating the hollow structure of the adjusting screwing member having a shape providing gripping structure fitting in the circumference of the inner hollow structure of the adjusting screwing member,