(340) The necessity for a diminution of the initial velocity of the spindle is strictly relative to the counts of yarn being spun. Some of the finer counts require a very slow traverse of the nut, and there may be practically none during several draws of the carriage. As the nut slowly rises and the locking point of the winding faller is elevated, the period of the engagement of the detent catch and the wheel =S= becomes shorter, and the rotation of the screw is not so prolonged. When the cop bottom is fully formed, the nut is at its most outward point, and the “governing” motion is not therefore required. At this point, the relative positions of the arms =U U^{ 1}= are such, that the chain =Y^{ 1}= will not permit the lever =Y= to fall sufficiently to allow its tooth to engage with the wheel. The motion, therefore, falls out of use until the commencement of another set of cops.
(341) The motion of the winding scroll is communicated to the tin roller by means of a catch or “click” plate shown in detail in Fig. 168. On the spindle of the winding scroll =X^{ 1}= is a spur wheel—indicated by dotted lines—which engages with a small pinion on the tin roller shaft =T=. The whole of this special mechanism is shown in longitudinal section in the right hand top corner of Fig. 168. The pinion is cast in one piece with the disc =V= which is loose upon the shaft =T=. The latter has a pin fixed in it, on which the small catch or “click” =V^{ 1}= is hinged. The click catch is ordinarily held out of position by the bent spring =W^{ 1}=, which surrounds the boss of a ratchet wheel =T^{ 1}=—to which the name of the “click wheel” is given. When the “click spring” =W^{ 1}= is slightly oscillated in the same direction as the rotation of the ratchet wheel, it allows the click catch to fall into gear with the click wheel. As the latter is keyed upon the tin roller shaft =T=, the engagement with it of the catch causes the tin roller to be revolved, and thus rotates the spindles.
(342) It was formerly the practice to allow the click catch to fall into gear when the disc =V= began to rotate upon the commencement of the inward run of the carriage. It was, however, found that the click catch engaged with the wheel earlier at one stretch than at another, and that, consequently, winding began a little more slowly than it should. The effect of such an occurrence is that a little slack yarn was produced as the carriage was running in, although winding was not taking place. Under these conditions tight winding at the nose throughout was practically impossible. It will be easily understood that, when the click catch is released, it may very readily be left either close to the tooth with which it has to engage or only just over the point of the preceding tooth. In the first case the engagement would take place at once, while in the second instance almost the distance of a tooth would have to be travelled by the click catch before engagement occurred. In hard twisted yarns this is especially objectionable, and its prevention is of importance.