190 AGRICULTURAL ENGINEERING 



Coefficient of friction of various surfaces. 



Surface in contact 



Condition of the 

 surface 



Mutual arrange- 

 ment of the fibers 



Coefficient 

 of friction 



Oak on oak 

 Oak on oak 

 Beech on oak 

 Cast-iron on oak 

 Cast-iron on cast-iron 

 Cast-iron on cast-iron 

 Cast-iron on wrought iron 

 Cast-iron on bronze 

 Balls on hardened steel 

 Rollers 



Dry 



Oily 



Coated with tallow 



Coated with tallow 



Dry 



Oily 



Coated with lard 



Coated with lard 



Perpendicular 

 Parallel 



0.336 



0.108 



0.055 



0.078 



0.152 



0.144 



0.053 



0.070 



0.002* 



0.0099* 



Fig. 100. A plain 

 bearing. 



♦Approximate values; coefficient of friction varies with speed and load. 



Bearings. The bearings are the parts of a machine which 

 contain the rotating parts. When the bearings are a sepa- 

 rable part of the machine they are often called boxes. 

 Bearing should be designed, first, from 

 material which will give a low coefficient 

 of friction; second, so that the surfaces 

 may be thoroughly lubricated; third, from 

 materials that will resist wear or which 

 can be easily replaced; and fourth, in most cases they 

 should be adjustable for wear. 



A bearing which is made in one piece and is separable 

 from the rest of the machine is styled a solid box. A bearing 

 supported on pivots or in a socket 

 which will permit its axis to be moved 

 easily is called a self-aligning bearing. 

 The rotating part which comes in 

 contact with a bearing is usually desig- 

 nated as the journal. The journal is 

 generally made of a harder material 

 than the bearing. Thus the journal 

 is usually made of steel and the bearing of brass, bronze, 



Fig. 101. A self-align- 

 ing bearing. 



