An analysis is presented of the ef fect of disc geometry in relation to its areas of contact with soil at the working depth , treating the disc as a segment of a thin spherical shell . In addition , the ef fects of the disc angle of tilt , the disc angle of inclination to its direction of motion and the working depth are examined in detail . Shallow and deep disc concavities are considered . A presentation is made of formulae derived for disc critical angle and depth and for bearing and pressure areas of contact with the soil on vertical and horizontal planes . In addition , a study was made of the overlapping of soil working areas when adjacent discs are working in a gang arrangement . The ef fects of inside and outside sharpening of the circumferential edge of the disc are also examined . For the practical range of tilt angle (15 Њ to 25 Њ ) and disc angle (35 Њ to 55 Њ ) it is shown that the bearing area of the rear spherical area of discs is zero , so there is no soil contact with the rear surface of the disc . The vertical pressure area is only slightly af fected by tilt angle and there is little dif ference for the two disc concavities . Disc angle and working depth have significant ef fects on this area . The horizontal presssure area is not af fected by disc angle over its practical range . It has larger values for the 81 mm concavity than for the 51 mm concavity and is significantly influenced by tilt angle and working depth . Discs working in a gang , have overlapping of the areas of soil cut for disc angles and spacings (180 to 300 mm) adopted in practice . The area cut by an individual disc is not markedly af fected by disc angle . Inside and outside sharpening does not significantly af fect the overall findings concerning critical disc parameters . The flat bevelled surface formed by outside sharpening will generally be in contact with soil over the practical range of tilt and disc angles
