Machinery's Handbook, 31st Edition
Lubrication Theory
2499
LUBRICATION Lubrication Theory
Whenever a solid surface moves over another, it must overcome a resistive, opposing force known as solid friction . The first stage of solid friction, known as static friction , is the frictional resistance that must be overcome to initiate movement of a body at rest. The second stage of frictional resistance, known as kinetic friction , is the resistive force of a body in motion as it slides or rolls over another solid body. It is usually smaller in mag- nitude than static friction. Although friction varies according to applied load and solid surface roughness, it is unaffected by speed of motion and apparent contact surface area. When viewed under a microscope a solid surface will appear rough with many asperi- ties (peaks and valleys). When two solid surfaces interact without a lubricating medium, full metal-to-metal contact takes place in which the asperity peaks of one solid interferes with asperity peaks of the other solid. When any movement is initiated the asperities col- lide causing a rapid increase in heat and the metal peaks to adhere and weld to one another. If the force of motion is great enough the peaks will plow through each other’s surface and the welded areas will shear causing surface degradation, or wear. In extreme cases, the resistance of the welded solid surfaces could be greater than the motive force causing mechanical seizure to take place. Some mechanical systems designs, such as brakes, are designed to take advantage of friction. For other systems, such as bearings, this metal-to-metal contact state and level of wear is usually undesirable. To combat this level of solid friction, heat, wear, and con sumed power, a suitable lubricating fluid or fluid film must be introduced as an intermedi ary between the two solid surfaces. Although lubricants themselves are not frictionless, the molecular resistive force of a gas or fluid in motion known as fluid friction is signifi - cantly less than solid friction . The level of fluid friction is dependent on the lubricant’s Viscosity (see page 2504.) Film Thickness Ratio, λ .— For all bearings, the working lubricant film thickness has a direct relationship to bearing life. The “working” or specific film thickness ratio lambda λ is defined by dividing the nominal film thickness by the surface roughness, as depicted in Fig. 1. (1) where λ = specific film thickness T = nominal film thickness R = surface roughness R T λ =
Bearing
Nominal Lubricant Film Thickness (T)
Lubricant
Maximum Surface Roughness (R)
Shaft
Fig. 1. Determining the Working Film Thickness Ratio Lambda The Lubrication Film.— Whenever a plain journal style bearing operates with a fluid film, the coefficient of friction μ or extent of friction reduction will depend on which one of three lubricant film conditions exists between the facing surfaces.
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