Machinery's Handbook, 31st Edition
2502 Lubricating Films Rolling element bearings (point contact) and the rolling section of mating gear teeth (line contact) also favor full hydrodynamic lubrication film. They differ from sliding elements in that rolling elements require considerably less lubrication than their sliding counterparts and that the load is concentrated over a much smaller footprint on a non-conforming sur face—small diameter ball or roller “rolling” over or within a much larger diameter raceway. As the ball or roller “rolls” through the load zone, the point of contact experiences a rapid pressure rise causing momentary micro distortion of both the rolling element and race. This area of deformation is named the Hertzian Contact Area (Fig. 5) and is analogous to the contact patch of a properly inflated tire on a moving vehicle. As the loaded section of rolling element moves out of the Hertzian contact area the deformed surface elastically returns to its original shape. The lubricant trapped in the Hertzian contact area benefits greatly from a phenomenon in which a lubricant under pressure will experience a dramatic rise in viscosity and act as a solid lubricant, allowing small amounts of lubricant to provide full film separa - tion under extreme loading conditions. Under these conditions the hydrodynamic film is termed elastohydrodynamic lubrication (EHDL), and is unique to point/line contact situa- tions typically found with rolling element bearings and mating gear teeth.
Load
A
Trapped EHDL Lubricant Film
Hertzian Contact Area (Deformation Area of Ball and Race)
Enlarged Section A.
Fig. 5. Hertzian Contact Area Found in Rolling Element Bearing Surfaces The wheels of industry run literally on a microfilm of lubricant; practical examples of typical oil film thicknesses expressed in machine dynamic clearance are stated in the following Table 1. Table 1. Typical Oil Film Thicknesses Expressed as Machine Dynamic Clearances Machine Component Typical Clearance in Microns Plain Journal Bearings 0.5–100 Rolling Element Bearings 0.1–3 Gears 0.1–1 Hydraulic Spool to Sleeve 1–4 Engine Piston Ring to Cylinder 0.3–7 Engine Rod Bearing (Plain) 0.5–20 Engine Main Bearing (Plain) 0.5–80 Pump Piston to Bore 5–40 1 micron = 0.00003937 inch; 25.4 microns = 0.001 inch. Lubricants A lubricant’s primary function is to reduce friction; in doing so it reduces wear and en- ergy consumption. Secondary functions are to reduce temperature, impact shock, corro sion, and contamination.
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