Tài liệu LUBRICATION OF MACHINE ELEMENTS P1 pdf

By the middle of this century two distinct regimes of lubrication were generally recognized. The first

of these was hydrodynamic lubrication. The development of the understanding of this lubrication

regime began with the classical experiments of Tower,1

in which the existence of a film was detected

from measurements of pressure within the lubricant, and of Petrov,2

who reached the same conclusion

from friction measurements. This work was closely followed by Reynolds' celebrated analytical

paper3

in which he used a reduced form of the Navier-Stokes equations in association with the

continuity equation to generate a second-order differential equation for the pressure in the narrow,

converging gap of a bearing contact. Such a pressure enables a load to be transmitted between the

surfaces with very low friction since the surfaces are completely separated by a film of fluid. In such

a situation it is the physical properties of the lubricant, notably the dynamic viscosity, that dictate

the behavior of the contact.

The second lubrication regime clearly recognized by 1950 was boundary lubrication. The under￾standing of this lubrication regime is normally attributed to Hardy and Doubleday,4

-

5

who found that

very thin films adhering to surfaces were often sufficient to assist relative sliding. They concluded

that under such circumstances the chemical composition of the fluid is important, and they introduced

the term "boundary lubrication." Boundary lubrication is at the opposite end of the lubrication

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

ISBN 0-471-13007-9 © 1998 John Wiley & Sons, Inc.

CHAPTER 21

LUBRICATION OF MACHINE

ELEMENTS

Bernard J. Hamrock

Department of Mechanical Engineering

Ohio State University

Columbus, Ohio

SYMBOLS 508

21.1 LUBRICATION

FUNDAMENTALS 512

21.1.1 Conformal and

Nonconformal Surfaces 512

21.1.2 Bearing Selection 513

21.1.3 Lubricants 516

21.1.4 Lubrication Regimes 518

21.1.5 Relevant Equations 520

21.2 HYDRODYNAMIC AND

HYDROSTATIC

LUBRICATION 523

21.2.1 Liquid-Lubricated

Hydrodynamic Journal

Bearings 524

21.2.2 Liquid-Lubricated

Hydrodynamic Thrust

Bearings 530

21.2.3 Hydrostatic Bearings 536

21.2.4 Gas-Lubricated

Hydrodynamic Bearings 545

21.3 ELASTOHYDRODYNAMIC

LUBRICATION 556

21.3.1 Contact Stresses and

Deformations 558

21.3.2 Dimensionless Grouping 566

21.3.3 Hard-EHL Results 568

21.3.4 Soft-EHL Results 572

21.3.5 Film Thickness for Different

Regimes of Fluid-Film

Lubrication 573

21.3.6 Rolling-Element Bearings 576

21.4 BOUNDARYLUBRICATION 616

21.4.1 Formation of Films 618

21.4.2 Physical Properties of

Boundary Films 619

21.4.3 Film Thickness 621

21.4.4 Effect of Operating

Variables 621

21.4.5 Extreme-Pressure (EP)

Lubricants 623