188 AGRICULTURAL ENGINEERING 



friction varies much with conditions and it is desirable in 

 most instances to keep it as small as possible, as it is a waste 

 of energy or power and lowers the efficiency of the machine. 

 There are instances where friction is highly essential, as in 

 the case of the transmission of power by means of a belt, or 

 the use of friction in the friction clutch in engaging a part at 

 rest with a revolving part. The shoes of the clutch slip 

 when first engaged, allowing the parts at rest to attain speed 

 slowly, thus relieving the machine of severe shocks, but 

 finally they furnish enough resistance to slipping to transmit 

 the full power of the machine. The ratio between the force 

 holding the two surfaces together and the force necessary to 

 slip one surface over the other is called the coefficient of 

 friction. Thus if a body weighing 10 pounds requires a hori- 

 zontal force of 1 pound to move it over a level surface, then 

 the coefficient of friction equals .1. In most instances it is 

 desirable to keep the coefficient of friction as low as possible, 

 which is done by making the sliding parts of the machine of 

 materials which give a low coefficient of friction, and by 

 applying a lubricant between the surfaces. 



When two surfaces in contact are at rest for a time they 

 seem to interlock, so that a greater force is required to cause 

 them to start to slide over each other than to continue the 

 movement after sliding begins. The friction of rest is there- 

 fore greater than the friction of motion. 



Rolling Friction. When a body with a circular cross- 

 section is rolled over a plane surface, some resistance is 

 offered, but not as much as in the case of sliding. This 

 resistance is due to a compression or indentation of the sur- 

 faces in immediate contact; hence rolling friction is less with 

 hard bodies. Since rolling friction is so much less than 

 sliding friction, rollers are often inserted between two sur- 

 faces which would otherwise slide over each other. 



