Page:The Kinematics of Machinery.djvu/541

 ANALYSIS OF DIRECT-ACTORS. 519

according to the relation between the forces acting on the mechan- ism, allows the ratchet-gear to have that motion which running- gear has of itself. The director, that is to say, removes the mono- kinetic (cf. 41) properties of the ratchet-train and so removes the difference existing between it and running-trains.

In order to stop the machine the steam-column is broken by means of the stop- valve E. This valve, with its box and fittings, forms a mechanism by itself, arranged to be worked by hand. It is the stop-gear of the engine, and so forms a part of the regulator. Taking the steam-engine as a whole then, apart from keys, cocks and so on, we find it to consist of a main-train, a director, a train for feed or supply, a self-acting and a hand-regulator, five me- chanisms in. all.

Passing now to the direct-actors, let us take first a common wharf- crane with revolving platform. Such a machine has two leading-trains which can be worked by hand, independently of one another the chain-drum with its pulleys and spur-gearing for raising the load, and another wheel-train for turning the platform. No director exists, but a regulator is provided in the brake by which weights can be lowered slowly. There is also a click-train used as stop-gear, for preventing any unintended descent of the load. It forms a part of the regulator, but is self-acting, in connection always with the first mentioned main-train.

A common clock with going and striking gear has also two main trains, one for moving the hands and the other for working the striking apparatus. As a rule each train has its own driver in the form of a weighted cord or sometimes a weighted loose pulley; they stand, however, in close kinematic connection. The going-gear is a compound spur-wheel train. Its motion is determined by a regu- lator, represented by the escapement and pendulum. We have already seen (121) that escapements are ratchet- trains acting by the periodic disengagement of the clicks. The striking-wheel with its lever- train forms the director of the machine. For each t welfth of a revolution of the hour-wheel this directing gear may cause the hammer (for example) to strike once for the half-hour and to make one of a series of strikings (in arithmetical progression) for the hour. It is driven by the going-train, and its immediate action on the striking-train is the disengaging of a click which allows the hammer-train to work. In order that the latter may act uniformly,