374 FARM MOTORS 



excessive friction balanced valves are now made. Some 

 have on the back a friction ring, which is held against 

 the steam chest by coil springs or live steam in such a 

 manner that the steam does not get behind the valve. 

 Other valves are so constructed that the high pressure steam 



TJL^ ^^ is kept from the back of the 



^m///M//////////I^^^^^ valve by means of pieces of 



r JL ~Jjf~ strap steel working in grooves 



^^ <fw/m i n the back of the valve. These 



pieces of steel are generally 



held out against the steam chest cover by means of coil 

 springs. Fig. 253 illustrates this type of valve. 



520. Piston valve. The piston valve is probably the 

 most effectually balanced valve. The principle of this 

 valve is the same as that of the common slide valve, but 

 instead of having a seat it is cylindrical in form and has 

 packing rings the same as a piston, making it steam 

 tight (Fig. 254). 



FIG. 254 PISTON VALVE 



