Fig. 69.
The diameters of the interior and exterior describing circles obviously depend upon the diameter of the intermediate circle, and as this may, as already stated, be selected, within certain limits, at will, it is evident that the relative diameters of the interior and exterior describing circles will vary in proportion, the interior becoming smaller and the exterior larger, while from the very mode of construction the radius of the two will equal that of the axes of the wheel and pinion. Thus in [Fig. 69] the radii of s, t, equal a b, or the line of centres, and their diameters, therefore, equal the radius of the annular wheel, as is shown by dotting them in at the upper half of the figure. But after their diameters have been determined by this construction either of them may be decreased in diameter and the teeth of the wheels will clear (and not interfere as in [Fig. 62]), but the action will be the same as in ordinary gear, or in other words there will be no arc of action on the circle r. But s cannot be increased without correspondingly decreasing t, nor can t be increased without correspondingly decreasing s.
Fig. 70.
[Fig. 70] shows the same pair of gears as in [Fig. 68] (the wheel having 22 and the pinion 12 teeth), the diameter of the intermediate circle having been enlarged to decrease the diameter of s and increase that of t, and as these are left of the diameter derived from the construction there is receding action along r from the line of centres to t.
Fig. 71.