288 



CREAMERY BUTTER MAKING 



9, valve stem gland ; lo, valve stem stuffing box ; 1 1, valve 

 chest cover; 12, steam inlet; 13, slide valve. 



Working of Piston. The arrows in the preceding cut 

 show the course which the steam takes in the valve chest 

 and cylinder. As the steam enters at port 7' the piston is 



'^ ^■^ '-^^^^ ^^ 



PISTON RlM* 



Fig. 81.— Piston and ring. 



pushed back and the exhaust steam escapes through port 

 7. The slide valve 13 gradually moves forward while the 

 piston moves back so that both ports will be closed when 

 the piston has traveled about four-fifths of the distance 

 of the cylinder. There is, however, enough energy stored 

 in the fly wheel or drive pulley to carry the piston 

 beyond the dead center when steam will enter the cylinder 

 through port 7, causing the piston to move forward while 

 the exhaust steam escapes through port y\ When the 

 piston has traveled about four-tifths of the distance of the 

 cylinder both ports are again closed, so that at every revo- 

 lution of the crank the dead center is passed twice. 



Fig. 81 shows the piston and piston ring. 



The piston must fit the cyHnder tight enough to prevent 

 leakage of steam, yet not so tight as to cause undue 



