372 FARM MOTORS 



where there is too much lead the exhaust port is likely to 

 open so soon that the steam is released before it expands 

 as much as possible. Again, if it has not sufficient lead 

 there will be no cushioning effect, and in addition suffi- 

 cient steam will not have entered the cylinder by the 

 time the piston starts on the return stroke to produce the 

 maximum pressure. 



516. Eccentric. The eccentric is a mechanism often 

 used where it is impossible to use a crank. The eccentric 

 of a steam engine consists of a disk or sheave fastened to 

 the crank shaft in such a manner that it is eccentric or 

 out of center with the center of the shaft. Around this 

 sheave is the eccentric strap, which is so adjusted that 

 there is a free and smooth bearing surface between the 

 two. The eccentric rod, which actuates the valve, is at- 

 tached to a strap and gives to the valve a reciprocating 

 motion similar to that of the piston, but on a reduced 

 scale. The throw of the eccentric, which is also the 

 travel of the valve, is twice the distance from the center 

 of the eccentric to the center of the shaft. In other 

 words, it is the same as that of a crank whose length of 

 arm is equal to the eccentricity of the eccentric. 



517. Angle of advance. On a slide-valve engine, with 

 the valves properly set when the engine is on dead center, 

 the center line of the eccentric will not be at right 

 angles to the crank, but will be at an angle greater than 

 a right angle. The difference between this angle and a 

 right angle is known as the angle of advance. 



The size of this angle varies with different engines, 

 but it is generally from 10 to 20. The object of the 

 angle of advance is to give the engine lead, and to vary 

 the lead means to change the position of the eccentric on 

 the shaft. Changing the position of the eccentric changes 

 the angle of advance. 



