Tham khảo tài liệu 'mechanism of steam engines part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | TYPICAL PROBLEMS OF THE SLIDE-VALVE ENGINE 6l Then V 4- average velocity of live steam X 144 will be the port area in square inches necessary for admitting steam and V 4- average velocity of exhaust steam X 144 will be the port area in square inches necessary for exhaust steam. Since the exhaust steam will require the greater area the port will be calculated for the exhaust and will then be ample for live steam. The length of the port will usually be somewhat less than the diameter of the cylinder bore and for purposes of the port calculation it may be assumed to be three-fourths of the bore diameter. The width of the port will then be equal to the port area divided by port length. In laying out the valve and a section of its seat Fig. 70 we will assume that sufficient information is at hand to draw the diagram Fig. 69. From this the laps and travel will be taken in constructing Fig. 70. 62 MECHANISM OF STEAM ENGINES Draw the line representing the surface of the valve seat and lay off at a convenient place hj the port width as calculated by the method above explained. It is better to start with the port which has the greater exhaust lap. From Fig. 69 this is seen to be the crank end. The crank-end steam and exhaust laps are now laid off from h and j in this case the exhaust lap is zero giving the valve edges a and b. Next choose a width of bridge jk which will be practicable for strength and test to see that the edge a of the valve never moves onto the bridge far enough to allow leakage past it into the exhaust. In the figure the edge a does not run onto the bridge when it has its greatest displacement as shown by the dotted line. In some cases it might hence the necessity for the above test. The width of the exhaust cavity kn is found by making the distance mn between the inside edge of the valve when it has its greatest displacement and the nearer edge of the head-end bridge equal to or a little greater than the port width. Make the head-end trial width of the .
