Fig. 241.
Fig. 242.
Draw lines, as H and I, representing the tooth breadth. From W, as a centre, draw on each side of G G dotted lines, as P, representing the height of the tooth above and below the pitch line G G. At a right angle to G G draw the line J K; and from where this line meets B, as at Q, mark the arc a, which will represent the pitch circle for the large diameter of the pinion D. [The smallest wheel of a pair of gears is termed the pinion.] Draw the arc b for the height, and circle c for the depth of the teeth, thus defining the height of the tooth at that end. Similarly from P, as a centre mark (for the large diameter of wheel C,) arcs g, h, and i, arc g representing the pitch circle, i the height, and h the depth of the tooth. On these arcs draw the proper tooth curves in the same manner as for spur wheels; that is, obtain the curves by the construction shown in Figures 237, or by those in Figures 238 and 239.
To obtain the arcs for the other end of the tooth, draw line M M parallel to line J K; set the compasses to the radius R L, and from P, as a centre, draw the pitch circle k. For the depth of the tooth draw the dotted line p, meeting the circle h and the point W. A similar line, from i to W, will give the height of the tooth at its inner end. Then the tooth curves may be drawn on these three arcs, k, l, m, in the same as if they were for a spur wheel.
Similarly for the pitch circle of the inner and small end of the pinion teeth, set the compasses to radius S L, and from Q as a centre mark the pitch circle d. Outside of d mark e for the height above pitch lines of the tooth, and inside of d mark the arc f for the depth below pitch line of the tooth at that end. The distance between the dotted lines as p, represents the full height of the tooth; hence h meets p, which is the root of the tooth on the large wheel. To give clearance and prevent the tops of the teeth on one wheel from bearing against the bottoms of the spaces in the other wheel, the point of the pinion teeth is marked below; thus arc b does not meet h or p, but is short to the amount of clearance. Having obtained the arcs d, e, f, the curves may be marked thereon as for a spur wheel. A tooth thus marked is shown at x, and from its curves between b and c, a template may be made for the large diameter or outer end of the pinion teeth. Similarly for the wheel C the outer end curves are marked on the arcs g, h, i, and those for the other end of the tooth are marked between the arcs l, m.