Setting Out Tappet.
The setting out of a tappet is an important problem. Two circles must be described round the same centre, the difference in the radii being the stroke of the tappet—thus, say A B or C D in [Fig. 28] is 3-1/2 inches, then that is the stroke of the tappet. This is also the radius of the smaller circle. The large circle must now be apportioned according to the number of picks to the round in plain cloth, say two. The circle is then divided into two parts by E F. Suppose we wish the healds to be still during two-thirds of a revolution of the crank-shaft, then as E A F represents a whole revolution, divide it into six parts where marked, place four of these parts from G to H for the dwell, and leave E G to form part of the lifting, and F H part of the depression of the healds.
FIG. 28.
The movement of the rising healds commences, however, before the falling heald has come to a pause; therefore we must trespass into the lower half to an extent equal to E G and H F, thus obtaining by drawing a diameter through G and the centre, also through H and the centre, the parts G J and H K for the rise and fall respectively. Divide C J now into six parts by radii to the centre, also divide by arcs of circles transversely, to these radii, the space between the small and large circles into six parts, then by drawing a line diagonally through the figures thus formed, we get a line G M giving an easy fall from the large circle to the small one, and by similar treatment a rise on the opposite side from N to H.
This is the theoretical construction of a plain tappet, from M to N being the dwell from the heald when up. N to H the depression, and H to G the dwell for the heald when down; and it will be noted, that although G H is apparently larger than N M, the time of dwell is the same in each case, in consequence of the arc N M being so much nearer the centre. In practice, the hollows N and M may be found rather fuller than shown here. This method of construction applies to tappets with an increased number of picks to the round, a point which will be found described in Chapter VI.
The speed of tappets is an important subject for calculation in connection with the loom. When the tappets are on the tappet shaft they are always plain, and are driven at half the speed of the crank-shaft, in consequence of the latter shaft only representing one pick, while the tappet shaft,[3] carrying two picking plates, represents two picks. Should the tappets be on the twill shaft, and driven from the tappet shaft, then the calculation is simple—e.g., say four picks to the round are required, then the crank-shaft must revolve four times for the twill shaft once. The tappet shaft must revolve half as many times as the crank-shaft—that is, twice—and the ratio between the speeds is as 2 to 1, which is also the ratio of the wheels, say 16 and 32, or 20 and 40, the larger wheel being on the twill shaft. In this case the rule is to divide the number of picks to the round by 2, which will give the ratio of the wheels gearing the tappet and twill shafts. Occasionally, the shaft carrying the tappets is driven from the crank-shaft direct, and the ratio of the gearing will be as one is to the number of picks to the round—say 6 picks to the round—then such a pair as 12 and 72, or 15 and 90, will be required. Obviously, for a large number of picks, an intermediate pair would have to be inserted—say 13 picks to the round must be woven, the wheel on the crank-shaft being 25 and the last wheel on the same shaft as the tappets is 65. Then, to get the size of the pair of intermediate wheels on the stud, multiply 13 by 25 and divide by 65, which will give the ratio of the size of the two wheels:—
(13 × 25)/65 = 325/65 = 65/13
These, or a multiple of these, are the wheels required, which may be proved:—
(65 × 65)/(25 × 13) = 13 picks to a round.