PLATE X. The figures of this plate are to illustrate the combinations used to produce rectilinear motion, from motion in a circular arc. Fig. 1. is the parallel motion used for steam boat engines. The beam A F is below the cylinder; from G, the end of the cross head, draw a line from G to A the centre of the axis of the beam, and it will cut the rod D B in E, and the length of the radius bar D C may be found by the rule, (art. 491;) when E B is equal to E D the length of the bar C D is equal to A B, and this is the best though not always the most convenient form. The rod D' G may be at any height, provided it be parallel to A F, and B may be at any point in A F. Fig. 2. shews the most common construction for engines with the beam above the cylinder. H is the piston rod connected at G, and C D is the radius bar. The line G A cuts the link B D in E, the proper point for the air pump rod. Fig. 3. shews a plan of the upper side of the beam, where C D, C D are the radius bars, and the beam is in two parts, as is usual in large engines. (See art. 492.) Fig. 4. is a diagram to illustrate the investigation of the properties of the combination in its most simple form. (See art. 489.) Fig. 5. is a diagram for the apparently more complicated case, when the rod is fixed to one angle of a parallelogram. (See art. 492.) Fig. 6. shews how to arrange for three piston rods to move parallel, as for Woolf's engine; the points of suspension must be all in the line A G. Fig. 7. shews another arrangement for three rods at one end, and two at the other end of the beam. (See art. 495.) In all the cases the corresponding points are marked by the same letters, and therefore by referring to the investigation of Fig. 4, the relations may be traced; the particular forms of different engine makers, will be found by turning the part upside down, altering the place of the parallel bar G D, or altering the proportions of the parts. In every combination where the bar CD is not equal to A B, the variation from rectilinear motion increases with the extent of the angle described. Other variations of the parallel motion are exhibited in Plate XI. and Plate XIX. PLATE XI. This plate represents a plan and elevation of an atmospheric engine, for raising water from a mine. The beam is supported by a frame of cast iron, designed so that it may be taken apart when it is necessary to move the engine to another mine, (art. 578.) The steam comes from the boiler by the steam pipe S, and is admitted to the cylinder C by a sliding piston in B, (see Fig. 2, Plate VI.) and then the piston in the cylinder C rises to the top of the stroke, and the piston rod forces the end ƒ of the beam up with it; as the beam rises it draws the rod F G with it, which near the end of the stroke moves the sliding frame H, (art. 483,) and by the rod O the slide in B so as to shut the steam off, and open the passage to the air pump A; into the passage a jet of water plays at I. The piston then begins to descend by the pressure of the atmosphere and raises the pump rods, and at the end of the stroke the part F of the rod F G, moves the slide and shuts the passage to the air pump, and opens that for the steam. (See Plate VI. Fig. 1.) The parallel motion is guided by the radius rod c d, attached to the frame, (art. 491 and 495,) and by connecting the rods h, i; the same rod does for both ends of the beam. The sliding frame His supported by a cross bar beneath H, and another at K, and the slide may be moved by hand by the lever M. Cold water for injection is supplied by the pump E, and water is raised from the hot well by the pump D, and passes to the boiler by the pipe Q, with a small branch pipe at P, to give water to the top of the piston. (See art. 400-405.) |