This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Figs. 848, 849. Diagonal catch and hand-gear used in large blowing and pumping engines. In Fig. 848 the lower steam-valve and upper eduction-valve are open, while he upper steam-valve and lower eduction-valve are shut; consequently the piston will be ascending. In the ascent of the piston-rod the lower handle will be struck by the projecting tappet, and being raised will become engaged by the catch, and shut the upper eduction and lower steam valves; at the same time the upper handle being disengaged from the catch, the back weight will pull the handle up and open the upper steam and lower eduction valves, when the piston will consequently descend. Fig. 849 represents the position of the catches and handles when the piston is at the top of the cylinder. In going down, the tappet of the piston-rod strikes the upper handle, and throws the catches and handles to the position shown in Fig. 818.
Figs. 850, 851, represent a modification of Figs. 848, 849, the diagonal catches being superseded by two quadrants.
Fig. 852. Apparatus for disengaging the eccentric-rod from the valve-gear. By pulling up the spring handle below until it catches in the notch a, the pin is disengaged from the gab in the eccentric-rod.
Fig. 853. A mode of driving a pair of feed-rolls, the opposite surfaces of which require to move in the same direction. The 2 wheels are precisely similar, and both gear into the endless screw which is arranged between them. The teeth of one wheel only are visible, those of the other being on the back or side which is concealed from view.
Fig. 854. Link-motion valve-gear of a locomotive; 2 eccentrics are used for one valve, one for the forward and the other for the backward movement of the engine. The extremities of the eccentric-rods are jointed to a curved slotted bar, or, as it is termed, a link, which can be raised or lowered by an arrangement of levers terminating in a handle as shown. In the slot of the link is a slide and pin connected with an arrangement of levers terminating at the valve-stem. The link, in moving with the action of the eccentrics, carries with it the slide, and thence motion is communicated to the valve. Suppose the link raised, so that the slide is in the middle, then the link will oscillate on the pin of the slide, and consequently the valve will be at rest. If the link is moved so that the slide is at one of its extremities, the whole throw of the eccentric connected with that extremity will be given to it, and the valve and steam-ports will be opened to the full, and it will only be toward the end of the stroke that they will be totally shut; consequently the steam will have been admitted to the cylinder during almost the entire length of each stroke.
But if the slide is between the middle and the extremity of the slot, as shown in the figure, it receives only a part of the throw of the eccentric, and the steam-ports will only be partially opened, and are quickly closed again, so that the admission of steam ceases some time before the termination of the stroke, and the steam is worked expansively. The nearer the slide is to the middle of the slot the greater will be the expansion, and vice versa.
Figs. 855, 856. Modifications of Fig. 852.