Friction and Lubrication in Mechanical Design Episode 1 Part 2 pdf

Introduction 5

1. In any situation where the resultant of tangential forces is smaller

than some force parameter specific to that particular situation, the

friction force will be equal and opposite to the resultant of the

applied forces and no tangential motion will occur.

2. When tangential motion occurs, the friction force always acts in a

direction opposite to that of the relative velocity of the surfaces.

3. The friction force is proportional to the normal load.

4. The coefficient of friction is independent of the apparent contact

area.

5. The static coefficient is greater than the kinetic coefficient.

6. The coefficient of friction is independent of sliding speed.

Strictly speaking, none of these laws is entirely accurate. Moore indicated

that laws (3), (4), (9, and (6) are reasonably valid for dry friction under the

following conditions:

For law (3), the normal load is assumed to be low compared to that

causing the real area of contact to approach the apparent area.

For law (4), the materials in contact are assumed to have a definite yield

point (such as metals). It does not apply to elastic and viscoelastic

materials.

Law (5) does not apply for materials with appreciable viscoelastic

properties.

Law (6) is not valid for most materials, especially for elastomers where

the viscoelastic behavior is very significant.

A number of workers also found some exceptions to the first friction law.

Rabinowicz [ 181 reported that Stevens [ 191, Rankin [20], and Courtney-Pratt

and Eisner [21] had shown that when the tangential force Fis first applied, a

very small displacement occurs almost instantaneously in the direction of F

with a magnitude in the order of 10-5 or 10-6 cm.

Seireg and Weiter [22] conducted experiments to investigate the load￾displacement and displacement-time characteristics of friction contacts of a

ball between two parallel flats under low rates of tangential load application.

The tests showed that the frictional joint exhibited “creep” behavior at

room temperatures under loads below the gross slip values which could

be described by a Boltzmann model of viscoelasticity.

They also investigated the frictional behaviors under dynamic excitation

[23, 241. They found that under sinusoidal tangential forces the “break￾away” coefficient of friction was the same as that determined under static

conditions. They also found that the static coefficient of friction in Hertzian

contacts was independent of the area of contact, the magnitude of the

normal force, the frequency of the oscillatory tangential load, or the ratio

6 Chapter I

of the static and oscillatory components of the tangential force. However,

the coefficient of gross slip under impulsive loading was found to be

approximately three times higher than that obtained under static or a vibra￾tory load at a frequency of l00Hz using the same test fixture.

Rabinowicz [25] developed a chart based on a compatibility theory

which states that if two metals form miscible liquids and, after solidification,

form solid solutions or intermetallic components, the metals are said to be

compatible and the friction and wear between them will be high. If, how￾ever, they are insoluble in each other, the friction and wear will be low.

Accordingly two materials with low compatibility can be selected from the

chart to produce low friction and wear.

In the case of lubricated surfaces, Rabinowicz [26] found that the

second law of friction was not obeyed. It was found that the direction of

the instantaneous frictional force might fluctuate by one to three degrees

from the expected direction, changing direction continuously and in a ran￾dom fashion as sliding proceeded.

The general mechanisms which have been proposed to explain the

nature of dry friction are reviewed in numerous publications (e.g., Moore

[17]). The following is a summary of the concepts on which dry friction

theories are based:

Mechanical interlocking. This was proposed by Amontons and de la

Hire in 1699 and states that metallic friction can be attributed to

the mechanical interlocking of surface roughness elements. This

theory gives an explanation for the existence of a static coefficient

of friction, and explains dynamic friction as the force required to lift

the asperities of the upper surface over those of the lower surface.

Molecular attraction. This was proposed by Tomlinson in 1929 and

Hardy in 1936 and attributes frictional forces to energy dissipation

when the atoms of one material are “plucked” out of the attraction

range of their counterparts on the mating surface. Later work

attributed adhesional friction to a molecular-kinetic bond rupture

process in which energy is dissipated by the stretch, break, and

relaxation cycle of surface and subsurface molecules.

Efectrostatic forces. This mechanism was presented in 1961 and explains

the stick-slip phenomena between rubbing metal surfaces by the

initiation of a net flow of electrons.

Welding, shearing and ploughing. This mechanism was proposed by

Bowden in 1950. It suggests that the pressure developed at the

discrete contact spots causes local welding. The functions thus

formed are subsequently sheared by relative sliding of the surfaces.

Ploughing by the asperities of the harder surface through the matrix