. The steam-engine and other heat-motors. ; lay off cut-off points B and B and find crank positions02 and 02; lay off XA = l/4 and obtain 04 crank position atsteam admission and prolong, thus obtain 03, a tangent to the Fig. circle, head end; the maximum port-opening, head end, beinggiven, layoff arc SS with a radius = 3/4= width of port + over-travel. Three conditions for the lap circle being given, ordinarily VALVE-DIAGRAMS AXD SLIDE-VALVES. 109 the circle is easily drawn. In this case the locus of the center ison OP, the bisectrix of the angle 203, and it is not difficult, practi-cal
. The steam-engine and other heat-motors. ; lay off cut-off points B and B and find crank positions02 and 02; lay off XA = l/4 and obtain 04 crank position atsteam admission and prolong, thus obtain 03, a tangent to the Fig. circle, head end; the maximum port-opening, head end, beinggiven, layoff arc SS with a radius = 3/4= width of port + over-travel. Three conditions for the lap circle being given, ordinarily VALVE-DIAGRAMS AXD SLIDE-VALVES. 109 the circle is easily drawn. In this case the locus of the center ison OP, the bisectrix of the angle 203, and it is not difficult, practi-cally, to obtain the center P so that a circle can be drawn tangentto 02, 03, and the arc SS. The exhaust-opening at C gives crankposition 09, and as this precedes OP the exhaust lap Pe on thehead end is negative. For the crank end, the throw of the eccentricand the angular advance (since there is only one eccentric) arefixed and, therefore, the position of P is determined. With acenter at P draw a circle tangent to 02, this gives the steam. Fig. 77. lap. Find the crank position corresponding to exhaust-openingat C and draw the crank position 09 at exhaust-opening. Asthis position follows position OP the exhaust lap on crank end ispositive. The leads and port-openings differ considerable fromthose on the head end. Dimensions of Steam-ports and Pipes.—It is easy to determinethe area of a steam-port if you have enough experience to determinewhat lineal velocity the steam may have in any particular casewithout too much loss of pressure. The roughness of castingsdoes not vary much with their size, but its influence on the veloc-ity of steam in a passageway 1/8 wide is enormous when com-pared to its effects in a passageway 4 wide. The allowable steam 110 THE STEAM-ENGINE AND OTHER HEAT-MOTORS. velocity will vary then with the size of the ports, their roughness,their length, and their freedom from sharp bends. We mustdistinguish between maximum port-opening (when it is lessthan the wi
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