This is an arrangement for cutting off the steam at any portion of the stroke, and for varying the quantity of steam admitted to the cylinder, without stopping the engine; for which in connexion with other improvements, Mr. Bourne of Dublin obtained a patent in 1838.
a a represents a section of a portion of the steam cylinder; b the upper, and c the lower steam passages; d the steam jacket or belt, e e the slide case connected at bottom with the condenser, and fa long D slide, by which the passages b and c are opened and closed in the usual way. That portion of the slide by which the steam is admitted to the passages, is divided into two parts by a partition g, which is faced and ground upon the face of the steam belt. The steam is admitted to the slide from the belt, by the apertures h h, to which are admitted the sliding valves i i; these valves are moved by a rod k, which passes through the rod of the D slide (made hollow for that purpose), and through a stuffing-box in the division g of the D slide. Upon this rod are cut two screws, one above and the other below the stuffing-box, which screws work in nuts on the back of the slides i i, and being one a right-handed, and the other a left-handed screw, upon turning the rod by means of the handle m, fixed on to its upper end, the valves are made to approach or recede from each other, according to the direction in which the rod is turned.
The D slide is worked by the eccentric in the usual way, and opens and closes the two passages b and c at exactly the same time, but the rod k by which the slides i i are worked is connected to an arm n fixed on the shaft of the radius bar o, which governs the parallel motion of the engine, so that the D slide and the slides i i move independently of each other, and whilst the steam passages into the cylinder (b and c) remain open throughout the stroke, the steam passages into the slide (h h) can be closed at any portion of the stroke which may be decided on. This is effected as follows: the depth of the apertures hh is equal the length of the stroke of the arm n, but the slides i i are so arranged upon the rod k, as to contract them in proportion to the degree of expansion at which the steam is to be worked; thus if it be required to cut off the steam at half stroke, allowing it to expand through the remaining half, the slides i i are so set upon the rod k that each aperture alternately shall be but half open at the commencement of the stroke, and as the slides begin to close with the commencement of the stroke, the passages, h h, will be closed when the slides have made half the stroke, as in the figure, and no more steam will be admitted to the cylinder, and the remainder of the stroke will be performed by the expansion of the steam already admitted.
If the steam is to be cut off at 1/4 of the stroke, the passages h h must open only 1/4, and if it is desired to work at full pressure throughout the stroke, the slides must be so arranged that the passages shall be fully open at the commencement of the stroke.