370 



FARM MOTORS 



b fa 



FIG. 247 



Object of lap. Lap is put on the slide valve to secure 

 the benefit of working steam expansively. If a valve 

 has no lap, steam will be admitted the full length of the 

 stroke and allowed to escape to the exhaust at boiler 

 pressure. By the application of lap, steam is cut off from 

 the boiler when the piston has traversed from three- 

 eighths to five-eighths of the stroke, and as the piston 

 completes the stroke the steam does work by expanding. 



515. Lead is given to a valve to admit steam to the 

 cylinder just before the piston reaches the end of the 

 stroke. By so doing a cushion is produced in the cylin- 

 der upon which the piston acts and this saves a jar. 

 Lead not only produces this cushion effect, but also 

 causes the port to be partly opened so that a full amount 

 of steam can be admitted to the cylinder the instant the 

 piston starts on its return stroke. Lead affects the ex- 



g 



GranK 



FIG. 248 



