214 



OF THE PARTS OF 



[SECT. vn. 



Pistons may be rendered tight by an elastic packing of vegetable or animal 

 matter ; but the latter kind of packing cannot be used for steam, on account of 

 the heat destroying it. 



Pistons may also be made wholly of metal, constructed so as to admit of a 

 certain degree of elasticity. 



After considering some particulars common to all pistons, we will treat of 

 pistons, as below, dividing them into two classes. 



Packed 



Pistons 



Metallic 



Common leathered 

 Atmospheric engine piston. 



Hemp 



Cartwright's 1797. 

 Barton's 1816. 

 Jessop's 1823. 



Perkins' 1823. 



on wood. 

 on metal. 



Common. 

 Woolfs. 



463. When a piston rod is to be pushed as well as drawn, unless it be of a 

 certain thickness in proportion to the diameter, it is liable to stick if there be the 

 slightest inequality in the friction, or in the centring of the rod. If it were a thin 

 plate, nothing but its connexion to the rod would prevent it turning with the 

 slightest inequality of its friction, on being pushed ; and as we make it thicker, 

 the thickness interferes more and more with any tendency to turn. The propor- 

 tions which will secure us from the risk of this evil are not difficult to ascertain. 



Let the pressure on the piston A B move the rod C D. 

 Then, in order that the piston may move steadily, its friction 

 at the circumference multiplied by half the diameter of 

 the piston, should be equivalent to the pressure producing 

 that friction, multiplied by half the thickness of the piston ; 

 consequently, the thickness should be to the diameter, as the 

 friction is to the pressure of the rubbing surfaces. 



The friction of brass on iron is at an average one-eighth 

 of the pressure ; hence the thickness of metallic pistons 

 should not be less than one-eighth of the diameter. 



The friction of hemp packing on iron is about one-sixth of the pressure, hence 

 the thickness of the packing should be one-sixth of the diameter. Practice is 



