CHAPTER 6 



THEORY OF LUBRICATION 



Lubrication reduces friction between moving 

 parts by substituting fluid friction for solid fric- 

 tion. Without lubrication, it is difficult to move a 

 hundred-pound weight across a rough surface; 

 with lubrication, and with proper attention to the 

 design of bearing surfaces, it is possible to move 

 a million-pound load with a motor that is small 

 enough to be held in the hand. By reducing 

 friction, thereby reducing the amount of energy 

 that is dissipated as heat, lubrication reduces 

 the amount of energy required to perform me- 

 chanical actions and also reduces the amount 

 of energy that is dissipated as heat. 



Lubrication is a matter of vital importance 

 throughout the shipboard engineering plant. Mov- 

 ing surfaces must be steadily supplied with the 

 proper kinds of lubricants, lubricants must be 

 maintained at specified standards of purity, and 

 designed pressures and temperatures must be 

 maintained in the lubrication systems. Without 

 adequate lubrication, a good many units of ship- 

 board machinery would quite literally grind to 

 a screeching halt. 



The lubrication requirements of shipboard 

 machinery are met in various ways, depending 

 upon the nature of the machinery. This chapter 

 deals with lubrication in general— with basic 

 principles of lubrication, with lubricants used 

 aboard ship, and with the shipboard devices used 

 to maintain lubricating oils in the required con- 

 dition of purity. The separate lubrication sys- 

 tems that are installed for many shipboard units 

 are discussed in other chapters of this text. 



FRICTION 



The friction that exists between a body at 

 rest and the surface uponwhichit rests is called 

 static friction. The friction that exists between 

 moving bodies (or between one moving body and 

 a stationary surface) is called kinetic friction . 

 Static friction, which must be overcome to put 

 any body in motion, is greater than kinetic 



friction, which must be overcome to keep the 

 body in motion. 



There are three types of kinetic friction: 

 sliding friction, rolling friction, and fluid fric- 

 tion. Sliding friction exists when the surface of 

 one solid body is moved across the surface of 

 another solid body. Rolling friction exists when 

 a curved body such as a cylinder or a sphere 

 rolls upon aflat or curved surface. Fluid friction 

 is the resistance to motion exhibited by a fluid. 



Fluid friction exists because of the cohesion 

 between particles of the fluid and the adhesion 

 of fluid particles to the object or medium which 

 is tending to move the fluid. If a paddle is used 

 to stir a fluid, for example, the cohesive forces 

 between the molecules of the fluid tend to hold 

 the molecules together and thus prevent motion 

 of the fluid. At the same time, the adhesive forces 

 of the molecules of the fluid cause the fluid to 

 adhere to the paddle and thus create friction be- 

 tween the paddle and the fluid. Cohesion is the 

 molecular attraction between particles that tends 

 to hold a substance or a body together; adhesion 

 is the molecular attraction between particles that 

 tends to cause unlike surfaces to stick together. 

 From the point of view of lubrication, adhesion 

 is the property of a lubricant that causes it to 

 stick (or adhere) to the parts being lubricated; 

 cohesion is the property which holds the lubri- 

 cant together and enables it to resist breakdown 

 under pressure. 



Cohesion and adhesion are possessed by dif- 

 ferent materials in widely varying degrees. In 

 general, solid bodies are highly cohesive but 

 only slightly adhesive. Most fluids are quite 

 highly adhesive but only slightly cohesive; how- 

 ever, the adhesive and cohesive properties of 

 fluids vary considerably. 



FLUID LUBRICATION 



Fluid lubrication is based on the actual sep- 

 aration of surfaces so that no metal-to- metal 



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