384 



FARM MOTORS 



the throw of the indicator drum is only about 3^ inches ard the 

 stroke of the piston is 8 to 20 inches, the length of stroke of the 

 piston has to be reduced to that of the indicator. There are several 

 mechanisms for this purpose, some of which come with the indi- 

 cator (Fig. 267). If a reducing motion has to be devised, probably 

 that shown in Fig. 268 is the most simple. 



FIG. 267 REDUCING MOTION 

 ATTACHED TO INDICATOR 



531. Taking indicator diagrams. To take an indicator diagram 

 the string after being hooked up should be of proper length to give 

 the indicator drum a clear movement. When the indicator is rotat- 

 ing back and forth, if the pencil is held against it the atmospheric 

 line may be drawn. The cock should then be opened and the 

 steam allowed to enter from one end of the cylinder until the indi- 

 cator has become warmed up. Then the pencil should be held 

 against the drum while the piston takes two or three strokes. A 

 diagram can be taken from the other end of the cylinder On this 

 same card by simply turning the cock over, or this card may be 

 taken out and a new one put in. 



532. Reading an indicator diagram. To read an indicator for 

 perfect valve setting it is best to compare it with a perfect diagram. 

 It is assumed that in the diagram Fig. 269 the heavy line is the per- 

 fect one and those with dotted lines are taken from engines with 

 poorly set valves. 



a shows too early compression. 



a' shows too late compression. 



b shows excess of lead. 



b' shows insufficient lead. 



c shows wire drawing. 



/ shows late release. 



s shows early release. 



