STEAM ENGINES 373 



In Fig. 251 let AB be the travel of the valve, OA the 

 position of the crank, and OC the position of the eccen- 

 tric. Then the angle COD, or 0, is the angle of advance. 

 A perpendicular let fall from C to OB gives the distance 

 OE, which designates the position of the valve. In this 

 instance it also gives the lead, i.e., OE, is the lead of the 

 valve. If the position of the crank is changed from OA 



FIG. 251 



FIG. 252 DOUBLE-PORTED VALVE 



to OA { the valve will move the distance OE^ or a total 

 distance of EE + OE = OE . 



518. Double-ported valves. The common slide valve 

 has to travel so far in opening a steam port that there is 

 considerable wire drawing of the steam as it enters the 

 cylinder ; also it does not permit a free release of the ex- 

 haust steam. Some manufacturers are putting in their en- 

 gines a double-ported valve (Fig. 252) which gives about 

 the same port opening as the simple slide valve and with 

 only half the travel. 



519. Balanced valve. By inspecting Fig. 243 it will be 

 noticed that there is high-pressure steam all over the out- 

 side of the valve and none on the inside. This excessive 

 pressure on the outside causes a large amount of friction 

 between the valve and the valve seat. To overcome this 



