Beating Up.
The shot or pick of weft often being put through the shed, is at a distance of five inches from the woven cloth, and requires pushing up into close contact with it. The motion of the slay performs this operation. As explained on page 60, the slay is a beam of wood carrying a reed, and having a reciprocating motion to and from the fell of the cloth, imparted to it by the cranks on the shaft ([Figs. 22] and [43].) The beam is supported on two vertical rods, called slay swords, attached near the bottom of the loom to a vibrating or “rocking” shaft. The slay is away from the cloth or front centre a sufficient time to give opportunity for the propulsion of the shuttle through the shed. Were the crank-shaft at the same level as the slay-sword pin, the dwell at each end of the stroke would be exactly equal—a dwell of some duration, however slight, it is obvious, there must be. However, the centre of the crank-shaft is at a lower level than that of the connection of the crank-arm with the slay-sword ears, and thus the slay dwells longer at the healds than at the cloth. As the slay makes some 200 strokes per minute, the variations of speed at the back and front centres are scarcely observable, but by means of exact measurements a certain amount of dwell may be traced at the back centre.
By describing a circle to represent the movement of the crank, and at a distance from it drawing to scale an arc of a circle to represent the movement of the slay, we can prove the foregoing remarks. In [Fig. 24] the circle referred to is shown, and also the line A E representing the stroke of the slay. By observing the position of the ends of the connecting rod E1, when in contact with the circle at the back centre, and also when the slay is at front centre, we find that it has travelled over from E1 to A1, which is more than half of the circle. Then assuming that the crank-shaft runs at one even speed, we would say that the slay takes longer time going forward than coming back. Again, by measuring the movement of the slay from A to B at the front, and from D to E at the back, these parts are found of equal length, but by extending from these points our crank-arm of equal length in every case, we find that to move the slay from B to A and back occupies a space on the circle from B to B1, while to move the slay from D to E and back occupies a space from D1 to D1 obviously greater, from which fact we assume that the slay occupies a longer time at the back than at the front, an arrangement purposely contrived so as to get theoretically a longer dwell. The difference between the arcs D1 D1 and B B1 is approximately 15°. Calculating at 180 picks per minute, we get the difference between the time of dwell at front and back to a seventy-secondth part of a second—to a practical mind not a very great consideration.
FIG. 24.
The shuttle race is made of hard wood laid on the beam, and in addition to the depression at the centre is also rather wider at the middle of the race—at that point bulging out slightly against the reed, which is kept in contact with it by the slay cap at the top and the reed case at the bottom. Care must be taken to keep the ends of the reed from projecting in front of backboard, or the shuttle will be thrown out.