For this twill there will be four treadles, each treadle being operated by a tappet of the same shape; but the tappet operating each succeeding treadle will be placed one quarter of a revolution later than the previous one.
The size of the treadle bowl has a very appreciable effect upon the shape of the tappet, more especially when there are several picks to the round. The movement imparted to the centre of the treadle bowl will be the exact movement given to the heald as far as regards dwell and eccentricity, and as the tappet acts on the treadle bowl at a distance of 1 or 2 inches from the centre, the required amount of dwell and eccentricity must be given to the centre of the bowl, and the shape of the tappet obtained accordingly. It will be noticed at [Fig. 41], that to give a dwell of half a pick to the centre of the treadle bowl, a slightly longer dwell is on the tappet at the inner circle; and as the size of the treadle bowl increases, this hollowing out of the tappet must be increased in order to keep the dwell of the heald the same.
[Fig. 42] is a drawing of a tappet for a 3 down, 1 up, 1 down, 1 up (six to the round) twill. Centre of tappet shaft to nearest point of contact with bowl 4 inches, lift of tappet 2 inches, bowl 1½ inch diameter, dwell one-third of a pick.
FIG. 42.
To construct this tappet:—At a radius of 4 inches describe the circle A. At a radius of 4¾ inches describe the circle B. At a radius of 6¾ inches describe the circle C. As there are six picks to the round, divide the circles into six equal parts by the lines D, E, F, G, H, I. As there is one-third pick dwell, divide each pick into three equal parts, and take the middle one for dwell. Rule the lines L, M, N, O, P, Q, R, S to the centre, and divide the spaces allowed for change into six equal parts, and the distance between the circles B and C into six unequal parts, as in the previous examples. From the corners of the unequal spaces describe the circles representing the movement of the treadle bowl, and obtain the shape of the tappet accordingly. It will be noticed that at point L the treadle bowl begins to dwell, and remains stationary until it reaches the point S, when it begins to go up. The heald will thus be down for the first, second, and third picks, up for the fourth, down for the fifth, and up for the sixth.
FIG. 43.—Woodcroft’s Tappet.
