Principles of naval engineering - Historic Naval Ships Association

CHAPTER 6THEORY OF LUBRICATIONLubrication reduces friction between movingparts by substituting fluid friction for solid friction.Without lubrication, it is difficult to move ahundred-pound weight across a rough surface;with lubrication, and with proper attention to thedesign of bearing surfaces, it is possible to movea million-pound load with a motor that is smallenough to be held in the hand. By reducingfriction, thereby reducing the amount of energythat is dissipated as heat, lubrication reducesthe amount of energy required to perform mechanicalactions and also reduces the amountof energy that is dissipated as heat.Lubrication is a matter of vital importancethroughout the shipboard engineering plant. Movingsurfaces must be steadily supplied with theproper kinds of lubricants, lubricants must bemaintained at specified standards of purity, anddesigned pressures and temperatures must bemaintained in the lubrication systems. Withoutadequate lubrication, a good many units of shipboardmachinery would quite literally grind toa screeching halt.The lubrication requirements of shipboardmachinery are met in various ways, dependingupon the nature of the machinery. This chapterdeals with lubrication in general— with basicprinciples of lubrication, with lubricants usedaboard ship, and with the shipboard devices usedto maintain lubricating oils in the required conditionof purity. The separate lubrication systemsthat are installed for many shipboard unitsare discussed in other chapters of this text.FRICTIONThe friction that exists between a body atrest and the surface uponwhichit rests is calledstatic friction. The friction that exists betweenmoving bodies (or between one moving body anda stationary surface) is called kinetic friction .Static friction, which must be overcome to putany body in motion, is greater than kineticfriction, which must be overcome to keep thebody in motion.There are three types of kinetic friction:sliding friction, rolling friction, and fluid friction.Sliding friction exists when the surface ofone solid body is moved across the surface ofanother solid body. Rolling friction exists whena curved body such as a cylinder or a sphererolls upon aflat or curved surface. Fluid frictionis the resistance to motion exhibited by a fluid.Fluid friction exists because of the cohesionbetween particles of the fluid and the adhesionof fluid particles to the object or medium whichis tending to move the fluid. If a paddle is usedto stir a fluid, for example, the cohesive forcesbetween the molecules of the fluid tend to holdthe molecules together and thus prevent motionof the fluid. At the same time, the adhesive forcesof the molecules of the fluid cause the fluid toadhere to the paddle and thus create friction betweenthe paddle and the fluid. Cohesion is themolecular attraction between particles that tendsto hold a substance or a body together; adhesionis the molecular attraction between particles thattends to cause unlike surfaces to stick together.From the point of view of lubrication, adhesionis the property of a lubricant that causes it tostick (or adhere) to the parts being lubricated;cohesion is the property which holds the lubricanttogether and enables it to resist breakdownunder pressure.Cohesion and adhesion are possessed by differentmaterials in widely varying degrees. Ingeneral, solid bodies are highly cohesive butonly slightly adhesive. Most fluids are quitehighly adhesive but only slightly cohesive; however,the adhesive and cohesive properties offluids vary considerably.FLUID LUBRICATIONFluid lubrication is based on the actual separationof surfaces so that no metal-to- metal112