Mechanism of Steam Engines Part 8

Tham khảo tài liệu 'mechanism of steam engines part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | RIDING CUT-OFF VALVES AND THEIR GOVERNING DEVICES 89 also the relative travel of riding valve when the eccentric is set for the riding valve to cut-off on both ends at the same time as the main valve cut-offs. The head-end clearance of the riding valve is also known. To find the absolute travel of the riding valve the crank-end clearance the position of the riding valve eccentric relative to the main eccentric when set for cut-off as above stated then assuming that the governor is to shift the eccentric so that crank-end cut-off by the riding valve occurs at a known crank angle earlier in the stroke to find the angle through which the eccentric must be shifted the relative travel for this grade and the position of head-end cut-off corresponding to the shortened crank-end cut-off. The stroke line is omitted from the figure although it was used in getting the various crank positions. The main valve circles are first drawn and the crank positions for main valve events located from the data. Since the relative travel of the plate on the valve is known for longest cut-off the diameter of the relative displacement circle for this setting must be one half this relative travel. If the riding valve is to give head-end cut-off at the crank position och the same as main valve head-end cut-off it must be displaced toward the head end relative to main valve at that time an amount equal to head-end clearance. Therefore the relative displacement circle OV2 must be drawn in such a position that its intercept on och will be equal to the head-end clearance. This can be done by drawing about 0 an arc with radius equal to the clearance and finding the center of a circle of the required diameter which will pass through o and the point where the clearance arc cuts och. In this way the diameter OV2 is found and from that 071. Then by constructing the parallelogram E071P1 and drawing its diagonal 0P1 we find the eccentricity of the plate eccentric and its setting for latest cut-off. The .