FARM MACHINERY 189 



Lubrication. To reduce friction between two sliding sur- 

 faces and to reduce the wear and heating, it is common 

 practice to apply some substance which will adhere to each 

 of the surfaces in a thin layer, smoothing them, and pre- 

 venting them from coming in such close contact. Such a 

 substance is called a lubricant. The friction really takes 

 place between two surfaces of the lubricant. 



Oils and greases are generally used as lubricants. 

 Graphite, which is carbon in a very finely divided state, is 

 often used in connection with oils, and has the property of 

 smoothing the surfaces. Mica finely divided is used in the 

 same way in axle grease. 



Choice of a Lubricant. It is desirable that lubricating 

 oil be as light and thin as possible, and still heavy enough, 

 or having enough "body," to prevent being squeezed out 

 from the surfaces in contact. Heavy oils and grease, being 

 more viscous, give a higher coefficient of friction, and are 

 not adapted to surfaces moving over each other at high speed. 

 Thus light oils are chosen for machines running at high 

 speeds and where the pressures between the lubricated sur- 

 faces is not great, as in the case of cream separators. Heavy 

 oils and greases are used where the pressure is great and the 

 motion slow, as on axles. Manufacturers provide special^ 

 lubricants for nearly every purpose, and it is well that special 

 oils be used as far as possible. Gas engine cylinder oil is so 

 made as to stand high temperature; and although other oils 

 may be as good a lubricant at normal temperature, they 

 would be worthless at the temperatures prevailing in the gas 

 engine cylinder. 



Table of Coefficient of Friction. The following table* 

 indicates in a general way the influence of surfaces of different 

 materials and different lubricants upon friction. 



*From "Bearings and Their Lubrication," by L. P. Alford. 



