Chapter 8: Lubrication, Friction and Wear

1. Chapter 8: Lubrication, Friction and Wear “...among all those who have written on the subject of moving forces, probably not a single one has given sufficient attention to the effect of friction in machines...” Guillaume Amontons (1699)

2. Conformal and Nonconformal Surfaces Figure 8.1 Conformal surfaces. Figure 8.2 Nonconformal surfaces.

3. Contacting Solids Figure 8.3 Geometry of contacting solids.

4. Radii of Curvature Figure 8.4 Sign designations for radii of curvature of various machine elements. (a) Rolling elements; (b) ball bearing races; (c) rolling bearing races.

5. Ellipsoidal Contact Figure 8.5 Pressure distribution in an ellipsoidal contact.

6. Elliptic Integrals Figure 8.6 Variation of ellipticity parameter and elliptic integrals of first and second kinds as function of radius ratio. Table 8.1 Simplified equations.

7. Hydrodynamic Lubrication Figure 8.8 Mechanism of pressure development for hydrodynamic lubrication. (a) Slider bearing; (b) squeeze film bearing; (c) externally pressurized bearing. Figure 8.7 Characteristics of hydrodynamic lubrication.

8. Elastohydrodynamic Lubrication Figure 8.9 Characteristics of hard elastohydrodynamic lubrication. Figure 8.10 Characteristics of soft elastohydrodynamic lubrication.

9. Regimes of Lubrication Figure 8.11 Regimes of lubrication. (a) Fluid film lubrication - surfaces separated by bulk lubricant film. This regime is sometimes further classified as thick or thin film lubrication; (b) partial lubrication - both bulk lubricant and boundary film play a role; (c) boundary lubrication - performance depends essentially on boundary film.

10. Friction and Lubrication Condition Figure 8.12 Bar diagram showing coefficient of friction for various lubrication conditions.

11. Wear Rate and Lubrication Figure 8.13 Wear rate for various lubrication regimes.

12. Surface Roughness Centerline average roughness Root-mean-square roughness Figure 8.14 Surface profile showing surface height variation relative to mean reference line.

13. Typical Surface Roughness Table 8.2 Typical arithmetic average surface roughness for various manufacturing processes and machine components.

14. Viscosity Figure 8.15 Slider bearing illustrating absolute viscosity.

15. Viscosity Conversion Factors Table 8.3 Absolute viscosity conversion factors.

16. Viscosity Data Figure 8.16 Absolute viscosities of a number of fluids for a wide range of temperatures.

17. Viscosity of Selected Fluids Table 8.4 Absolute and kinematic viscosities of various fluids at atmospheric pressure and different temperatures.

18. Piezoviscous Properties of Fluids Table 8.5 Pressure-viscosity coefficients of various fluids at different temperatures.

19. Viscosity of Single Grade SAE Oils Figure 8.17 Absolute viscosities of SAE lubricating oils at atmospheric pressure. (a) Single grade oils.

20. Viscosity of Multigrade SAE Oils Figure 8.17 Absolute viscosities of SAE lubricating oils at atmospheric pressure. (b) Multigrade oils.

21. Viscosity of SAE Single-Grade Oils Table 8.6 Curve fit data for SAE single-grade oils.

22. Friction Coulomb Friction Law: Figure 8.18 Friction force in (a) sliding and (b) rolling.

23. Coefficient of Friction Data Table 8.7 Typical coefficients of friction for combinations of unlubricated metals in air.

24. Abrasive and Adhesive Wear Archard Wear Law: Figure 8.19 Conical asperity having mean angle θ plowing through a softer material. This action simulates abrasive wear. Figure 8.20 Adhesive wear model.

25. Wear Coefficient Data Table 8.8 Coefficients of rubbing friction and adhesive wear constant for selected rubbing materials.

26. Fatigue Wear Figure 8.21 Fatigue wear simulation. (a) Machine element surface is subjected to cyclic loading; (b) defects and cracks develop near the surface; (c) the cracks grow and coalesce, eventually extending to the surface until (d) a wear particle is produced, leaving a fatigue spall in the material