Outside tappets are mostly used in the Yorkshire weaving districts, and are commonly made for weaving with about eight shafts. The top levers, with “half moons,” are centred at the cross rods EE ([Fig. 39]), and the heald is lifted from both sides of the loom. The top levers are very useful for equalizing the shed, as the connection with the upright rod can be altered without difficulty.
FIG. 39.
In a power loom there are two horizontal shafts, the top shaft A ([Fig. 38]) and the bottom shaft B. The former is used for working the slay, by means of the crank C, and the connecting rod or “crank arm” D ([Fig. 38]). The bottom shaft is used for “picking,” and for this purpose it is necessary that the shaft should revolve at one-half the speed of the top or crank shaft. The toothed wheel on the bottom shaft must therefore contain twice the number of teeth in the wheel on the crank shaft which drives it. As a plain cloth contains two picks to the round, and the bottom shaft makes one revolution for two picks, the tappets are fixed to the bottom shaft. Each tappet acts upon treadle bowl E, and therefore the size of the bowl will require to be taken into consideration in shaping the tappets. For weaving plain cloth four staves are usually taken, in order to prevent overcrowding the healds on each stave, the ends being drawn through the staves in the order 1, 3, 2, 4. As the staves are fastened together in pairs, this is the same as two staves.
The kind of movement to be given to the staves is very important, especially in quick-running looms. The staves should be moving quickest when they are level, and their speed should gradually decrease as the shed opens. It is obvious that a movement of this kind will put as little strain as possible on the warp, and therefore cause the fewest breakages. The depth of the shed should only be sufficient to allow the shuttle to pass, therefore the “lift” or stroke of the heald is dependent upon the depth of the shuttle used. The shed when opened should remain open only long enough to allow the shuttle to pass through.
Example.—What lift should a tappet have to make a plain cloth, the other arrangements in the loom being as follows: Sweep of slay 5½ inches, distance of healds from cloth 8 inches, heald connected to treadle 24 inches from fulcrum, distance from fulcrum to centre of treadle bowl 16 inches, size of shuttle 1½ inch broad, 1¼ inch deep?
Assuming that the tappets are under the loom, as in [Fig. 38], the treadle bowl E is 16 inches from M, and the heald connected 24 inches from M. If slay moves back from cloth 5½″, and the shuttle is 1½″ broad and 1¼″ deep, it follows that the shed must be 1¼″ deep, or a little over, at a point 4″ from the cloth (5½-1½ = 4). Then if the heald is 8″ from cloth, the stroke of heald may be obtained—4 : 8
To obtain the proper shape of the tappets for a plain cloth, the lift or stroke of the tappets to give the required lift to the healds must be obtained. If the lift of the heald is required to be 4 inches, and the centre of the treadle bowl E ([Fig. 38]) is situated 12 inches from the fulcrum of the treadle M, the heald being connected to the treadle at, say, 18 inches from the fulcrum, the lift or stroke of the tappet will be obtained as follows:—