Fig 41. N = number of ports on each side of expansion valve. D = greatest distance of expansion valve from its relative central position, when crank is on dead centre. Section through DE. Fig. 42. Automatic Expansion Gear, In practice, B would be found to exceed the dimension given by the above expression. The ports are often divided equally along the face of the valve. The rules just given can then be employed to ascertain whether the action is satisfactory or not. Ruston-Proctor's Expansion Gear.-Fig. 42 illustrates the automatic expansion gear made by Messrs. Ruston, Proctor & Co., Lincoln. The governor is of the Porter type; and is actuated from the crank shaft by means of bevel gear. The radius link is pivoted on the bottom end, and the lifting link from the governor raises or lowers the radius rod as circumstances require. A dashpot is fitted to give steadiness to the governor, and the balance weight relieves the latter from all encumbrance due to the radius rod. Crank-Shaft Governor Expansion Gear. It has been remarked that the grade of expansion can be varied by altering the angular advance of the expansion eccentric. Fig. 43 is a diagram Fig 43. D for this class of gears. OA is the position and throw of the main or distribution eccentric; and OB the expansion eccentric. CD is the lap circle, which, in this instance, is negative. Cut-off is at OE. Suppose B moved round the shaft to position OB', then OF is the cut-off line. Fig. 44 illustrates a device whereby the angle of O B can be varied according to circumstances. The figure is self-explanatory, and it is only necessary to remark that the design admits of considerable adjustment. In the first place, the point of attachment of the springs to the levers carrying the weights B B can be altered; the position of the weights themselves can be adjusted by means of the nuts on the levers; and the connecting links A A are also adjustable. The The third method of obtaining a range of expansion is by varying both the throw and position of a simple slide valve eccentric. principle of this class of gears is set forth in the accompanying diagram. Let OA (Fig. 45), represent the throw and position of the eccentric relative to the true position of the crank OB. CD is the outside lap circle, and cut-off is at O E. Let the eccentric be altered so that its throw and position are given by the line O A. The lap circle will remain the same, and the cut-off will therefore be at O F. Westinghouse Governor.-As a typical example of this class of gears, the Westinghouse governor and eccentric is selected. Fig 46. Assumed Crank Position Real Crank Position Westinghouse Governor (position for latest cut-off). Fig. 46 shows the position of the parts for the latest cut-off, which is at OA. In Fig. 47 the gear is in position for earliest cut-off, which is at OA. The real and assumed crank positions are indicated on each figure, as is also the direction of rotation. The valve diagrams show that as the governor moves from latest to earliest cut-off, the lead increases. This governor operates upon a piston valve, which is the most suitable form for crank-shaft governors, as the work required to move the valve passes partly through the springs. Crank-shaft governors find much favour in America. The |