Friction and Lubrication in Mechanical Design Episode 2 Part 5 doc

330 Chapter 8

where

K = constant for the material lubricant

f = coefficient of friction

W,, = normal load per unit length

b = width of the contact band

Y, , V2 = surface velocities

C,, C2 = constants of materials which are the square root of the product of the

thermal conductivity, specific heat, and density

A modification of Blok’s formula was proposed by Kelly [15] for similar

materials with consideration of surface roughness. The formula is given as:

where

TT = total surface temperature

TB = material bulk temperature

S = rms surface roughness (pin.)

K = constant for the material lubricant combination

8.6.2 Mechanism for Surface Crack Initiation

It is generally accepted that the penetration of asperities causes plastic

deformation in the surface layers where the yield point is exceeded at the

real area of contact. Below the plastically compressed layer are layers under

elastic compression. As soon as the asperity moves, the elastically com￾pressed layers will exert upon the plastic layer a force, which will create in

it a state of tension. Consequently, tensile stresses will appear on the surface

in such conditions.

The sliding motion also generates a temperature field, which pene￾trates the surface layers. The maximum temperature occurs at the contact

surface and decreases with increasing distance from the surface as dis￾cussed in Chapter 5. Accordingly, the surface layer is thermally elongated

more than the subsurface layers and will experience compressive stresses

Wear 331

imposed by the bulk material. If this compressive stress exceeds the yield

stress, then a tensile residual stress will be induced in the surface after

cooling. It should also be noted that the temperature at the real area of

contact can be very high at high sliding speeds which results in reducing

the yield strength significantly and thus, increasing the stressed zone.

The tensile thermal stress on the surface can be calculated from [ 161:

where

4’

cL=

a=

PO =

V=

ap =

K=

P=

C=

E=

B=

heat flux caused by friction = pPOVap

coefficient of friction

coefficient of thermal expansion

pressure on the real area of contact

sliding velocity

m

m+%PGG coefficient of heat partition =

thermal conductivity

density

thermal capacity

modulus of elasticity 7 & thermal diffusivity =

A combination of mechanically induced stresses and thermal stresses in the

nominal contact region, or in the real area of contact, generate surface or

near surface cracks, which can propogate with repeated asperity action to

generate delamination of the surface layer [17] or wear debris from shallow

pits. The influence of the thermal effect becomes more significant at high

loads, high coefficient of friction, and high sliding speeds.

As illustrated by the parametric analysis in Chapter 5, the physical,

chemical, and thermal properties of the lubricant can have significant influ￾ence on the maximum surface temperature. These properties control the

amount of separation between rubbing surfaces and the thermal properties

of the chemical layers generated on them.