CLASSES OF AUTOMATIC EXPANSION GEAR. 43 crossing the main and expansion curves at K and L respectively. KL is the necessary negative lap to accomplish this grade of expansion in the front stroke. From 20 per cent. on the scale for the back stroke, draw the line vertically, to meet the piston curve at M. From M, draw the horizontal, intersecting the valve curves at O and N. ON is the negative lap for the front end of the valve, to effect this grade of expansion. If the horizontals K J and M O crossed the curves at P and Q—that is, where P and Q cross each other—there would be no lap whatever; but if the verticals crossed on the opposite side of P and Q, the lap would be a positive quantity. The equalisation of cut-off for any particular grade of expansion having been effected, it is easy to observe the inequality for the other grades; and herein lies the chief advantage of the diagram. The expansion-valve curve being drawn on a separate sheet of tracing paper, the angular advance may be altered by merely moving the position of the tracing; the effect of which is instantly manifested. This is another merit of the Reynold's diagram. Classes of Automatic Expansion Gear.-Automatic expansion gears are made to vary the cut-off:-1. By varying the travel occurs at O E. This diagram illustrates the principle of expansion gears of the first class. Hartnell's Governor.-Fig. 39 is a sketch of the well-known Hartnell governor combined with a link expansion gear. When the engine is running at the normal speed, the die in the swinging link occupies the position indicated. When an increase of speed takes place, the balls fly out, raising the radius rod, which shortens the travel of the expansion valve. Cut-off will therefore be earlier in the stroke. When the speed falls below the normal, the radius Fig 39. Hartnell's Governor. rod is lowered. This action increases the valve's travel, and cut-off is later. The governor itself is a spring-loaded one. Between certain limits, any desired speed may be maintained by adjusting the helical spring in the head. Diagram for Hartnell's Expansion Gear. -No exact construction can be given to determine all particulars of this gear. Generally, the positions of the crank at early and late cut-off, and the maximum and minimum travels of the expansion valve would be decided upon. But it is necessary to assume either the angular advance or the lap of the expansion valve before a diagram can be constructed. Let O E, Fig. 40, be the line of latest cut-off in relation to the crank position ON revolving as shown; OA the throw and position of the main eccentric, and O B and O B' the throws and position of the expansion eccentric for maximum and minimum travels respectively. OC is the resultant circle corresponding to OB; and OD the resultant circle for O B'. OE intersects the resultant circle for maximum travel, at F. OF is the required negative lap. Draw the negative lap circle FG, intersecting the resultant circle for minimum travel at G. OH is the position of the crank at earliest cut-off. It may be that this range of expansion M H Fig 40. N Diagram for Hartnell's Expansion Gear. is not sufficient. Decreasing the negative lap would not alter the total range of expansion, as far as the cut-off valve is concerned, but would cause OE and OH to fall earlier in the stroke. But OK is the cut-off by the main valve. OE can be made to coincide with OK, by giving negative lap O L. This will alter OH to OM; which alteration has increased the actual range of expansion by the angle HOM. Decreasing the angular advance of the expansion eccentric would also increase the range of expansion, as. may be shown by construction. Application of the Diagram. The valves for the Hartnell gear may assume the form shown by Fig. 41, the proportions being as follows: The sum of the width of the ports C, may be equal to one and a-half times the width of port E, to ensure a good steam line. A=D+C-L, + say 4-inch for tightness. |