(288) The mechanism just described is that on which depends the driving of the whole of the parts, and its mode of action can now be easily explained. Beginning with the commencement of the outward run, when the rim band is traversing in its normal direction, the rollers commencing to deliver yarn, and the spindles revolving, the position of the parts is as follows. The strap is on the fast pulley, and the rim shaft is revolving. The backing-off friction is out of gear, the necessary motion is given to the roller shaft, and as the claw clutch is engaged the front line of rollers is revolved, roving being delivered. At the same time the back shaft is driven, the clutch on it being in gear, and the carriage is drawn out. The scrolls on the back shaft are shaped so as to allow the carriage to move at a constant velocity. While the carriage is running out the rim band is giving the required revolution to the tin roller shaft, and the spindles rotate in consequence at their normal velocity. The carrier pulley on the square shown in Fig. 152 is arranged at such an angle that the rim band passes freely on to and from the pulley on the tin roller shaft. A similarly accurate setting is given to the guide pulleys at the back of the headstock, the wear of the bands being much reduced in consequence. The velocity given to the carriage is slightly in excess of that of the surface speed of the rollers, so that the roving is a little stretched. The excess of the carriage traverse is from 1 to 3 inches, and is known as its “gain.”
(289) When the carriage reaches the termination of its outward run, or, as is commonly said, the end of its stretch, it becomes necessary, first to arrest and then to reverse its movement, these operations necessitating a complete change in the positions of the various parts. The chief agent in making these changes is the shaft =M=, placed parallel to but a little higher than the rim shaft. It is known as the “cam shaft,” and plays an important part in the operation of the machine. It is entirely distinct both by position and function from the rest of the mechanism, and a separate view of it and its connections is given in Fig. 156, which is a detached sectional elevation.