Friction and Lubrication in Mechanical Design Episode 2 Part 4 ppsx

8.1 INTRODUCTION Wear can be defined as the progressive loss of surface material due to normal load and relative motion.. Wear types include elastic wear, plastic wear, delamination wear

normal loads, T = 26°C (78.8"F), steel-steel contact, ground surfaces, S = 0.3 pm

(12pin.), U = 3.2m/sec (216 in./sec), IOW30 oil, R = 0.0234m (0.92 in.)

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f Figure 7.36 Calculated coefficient of friction vs sliding/rolling ratio for different effective radii, T = 26°C (78.8"F), steel-steel contact, ground surfaces, S = 0.3 pm

(12pin.), W = 378,812N/m (2160 lbf/in.), 10W30 oil, U1 = 3.2m/s (126 in./sec)

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U1 = 3.2 m/sec (126 in./sec), R = 0.0234 m (0.92 in.)

Grubin, A N., Book No 30, English Translation DSIR, 1949

Dowson, D., and Whitaker, A V., “A Numerical Procedure for the Solution of the Elastohydrodynamic Problem of Rolling and Sliding Contacts Lubricated

by a Newtonian Fluid,” Proc Inst Mech Engrs, 1965-1966, Vol 180, Part 3b,

Trachman, E G., and Cheng, H S., “Traction in EHD Line Contacts for Two

Synthesized Hydrocarbon Fluids,” ASLE Trans., 1974, Vol 17(4), pp 27 1-279

Hirst, W., and Moore, A J., “Non-newtonian Behavior in EHD Lubrication,“

Proc Roy Soc Lond A., 1974, Vol 337, pp 101-121

Johnson, K L., and Cameron, R., “Shear Behavior of EHD Oil Films at High

Rolling Contact Pressures,” Proc Inst Mech Engrs, 1967-1968, Vol 182, Pt

1 , No 14

Plint, M A., “Some Recent Research on the Perbury Variable-Speed Gear,” Proc Inst Mech Engrs, 1965-1966, Vol 180, Pt 3B

Crook, A W., “The Lubrication of Rollers, Part Ill,” Phil Trans Roy Soc

Lond., Ser A, 1961, Vol 254, p 237

Conry, T F., “Thermal Effects on Traction in EHD Lubrication,” J Lubr

Technol., Oct 1981, pp 533-538

Bair, S., and Winer, W O., “Regimes of Traction in Concentrated Contact

Lubrication,” ASME Trans., Vol 104, July 1982, pp 382-386

Plint, M A., “Traction in Elastohydrodynamic Contacts,” Proc Inst Mech Engrs, 1967-1968, Vol 182, Pt 1, No 1 14, pp 300-306

Dyson, A., “Frictional Traction and Lubricant Rheology in

Elastohydrodynamic Lubrication,” Phil Trans Roy Soc Lond., 1970, Vol

266, No 1170

Sasaki, T., Okamura, K., and Isogal, R., “Fundamental Research on Gear

Lubrication,” Bull JSME, 1961, Vol 4(14)

Drozdov, Y N., and Gavrikov, Y A., “Friction and Scoring under the

Conditions of Simultaneous Rolling and Sliding of Bodies,” Wear, 1968, Vol

1 1

O’Donoghue, J P., and Cameron, A., “Friction and Temperature in Rolling

Sliding Contacts,’’ ASLE Trans., 1966, Vol 9, pp 186-194

Benedict, G H., and Kelley, B W., “Instantaneous Coefficients of Gear Tooth

Friction,” ASLE Trans., 1961, Vol 4, pp 59-70

Misharin, J A., “Influence of the Friction Conditions on the Magnitude of the Friction Coefficient in the Case of Rolling with Sliding,” Int Conf on Gearing Proc., Sept 1958

Ku, P M., Staph, H E., and Carper, H J., “Frictional and Thermal Behavior

of the Sliding-Rolling Concentrated Contacts,” ASME Trans., J Lubr

Technol., Jan 1978, Vol 100

Li Y., “An Investigation on the Effects of the Properties of Coating Materials

on the Tribology Behavior of Sliding/Rolling Contacts,” Ph.D Thesis, Univ of

Wisconsin, 1987

Rashid, M K., and Seireg, A., “Heat Partition and Transient Temperature

Distribution in Layered Concentrated Contacts,” ASME Trans., J Tribol.,

July 1987, Vol 109, pp 49604502

Hsue, E Y., “Temperature and Surface Damage under Lubricated Sliding1 Rolling Contacts,” Ph.D Thesis, University of Wisconsin-Madison, 1984

p 57

25 Wilson, W R D., and Sheu, S., “Effect of Inlet Shear Heating Due to Sliding

and EHD Film Thickness,’’ J Lubr Technol., April 1983, Vol 105

26 Cameron, A., Basic Lubrication Theory, Longman Group, London, England,

1970

27 Juvinall, R C., Fundamentals of Machine Component Design, John Wiley &

Sons, New York, NY, 1983

8.1 INTRODUCTION

Wear can be defined as the progressive loss of surface material due to normal load and relative motion This generally leads to degradation of the surface, loss of component functionality, and in many situations, to catastrophic failure

The wear of mechanical components has been estimated to cost the U.S

economy between 6% and 7% of the gross national product Understanding

the wear process and its control is, therefore, of major practical importance The highly complex nature of the wear process has made it difficult to develop generalized procedures for predicting its occurrence and intensity Even wear tests under seemingly controlled conditions, are not always reproducible It is not unusual that repeated tests may give wear rates which differ by orders of magnitude

Surface damage or wear can manifest itself in many forms Among these are the commonly used terminology: pitting, frosting, surface fatigue, sur- face cracking, fretting, blistering, plastic deformation, scoring, etc Wear types include elastic wear, plastic wear, delamination wear, abrasive wear, adhesive wear, corrosive wear, cavitation erosion, etc The occurrence of a particular type of wear depends on many factors, which include the geome- try of the surfaces, the nature of surface roughness, the applied load, the rolling and sliding velocities Other important factors which influence wear are the environmental temperature, moisture, and chemical conditions, as well as the mechanical, thermal, chemical, and metalurgical properties of the

surface layer and bulk material The microstructure of the surface layer, its

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ductility, the microhardness distribution in it, and the existence of vacancies and impurities also play critical parts in the wear process Furthermore, wear is highly influenced by the physical, thermal, and chemical properties

of the lubricant, the regime of lubrication, the mutual overlap between the rubbing surfaces, and the potential for removal of the chemical layers and debris generated in the process

This chapter provides a conceptual evaluation of this extremely complex phenomenon, and presents guidelines for its prediction and control Although the mechanism of wear is not fully understood, designers of machine components have to rely on judgement and empirical experiences

to improve the functional life of their design The success of their judgement depends on their depth of understanding of which factors are relevant to a particular situation, and which are only accessories

It is interesting to note that with all the modern tools of experimenta- tion and computation, generalized wear design procedures that would pro- duce practical results are still beyond our reach We have therefore to rely

on thoughtful interpretation of accumulated data and observations One such poignant observation was documented 2000 years ago by the Roman philosophical poet Titus Caras Lucretius [l]: He said,

A ring is worn thin next to a finger with continual rubbing Dripping water hollows a stone, a curved plow share, iron though it is, dwindles imperceptibly in the furrow We see the cobblestones of the highway worn by the feet of many wayfarers The bronze statues by the city gates show their right hands worn thin by the touch of all travelers who have greeted them in passing We shall see that all these are being diminished since they are worn away But to perceive what particles drop off at any particular time is a power grudged to us by our ungenerous sense of sight

It has not yet been possible to devise a single classification of the different types of wear Some of the mechanisms by which rubbing surfaces are damaged are [2]:

Mechanical destruction of interlocking asperities;

Surface fatigue due to repeated mechanical interaction between asperi- ties or the variation of pressure developed in the lubrication; Failure due to work hardening and increasing brittleness caused by deformation;

Flaking away of oxide films;

312 Chapter 8

Mechanical damage due to atomic or molecular interactions;

Mechanical destruction of the surface due to the high temperatures Adhesion or galling;

Corrosion;

Abrasion due to the presence of loose particles;

Cutting or ploughing of a soft material by a harder rough surface; Erosion produced by impinging fluid or fluids moving with high rate of produced by frictional heating;

shear

The treatment in this chapter attempts to formulate general concepts about the nature of wear, which can be readily associated with practical experience and to provide equations which can be used for design purposes based on these concepts The broad categories to be considered are:

The wear volume per unit sliding distance has been evaluated according

to this concept by several investigations Their results are illustrated in the following

Archard [3, 41, as well as Burwell and Strang [5], proposed wear equa-

tions of the following form:

where

V = wear volume

L = sliding distance

P = applied load

oy = yield stress of the softer material

K = proportionality constant depending on the material combination and test conditions (wear coeficient)

H,,, = microhardness of the softer material

Results obtained by Archard from dry tests where the end of a cylinder 6mm diameter was rubbed against a ring of 24mm diameter under a 400g load at a speed of 1.8m/sec are given in Table 8.1

Rabinowicz [6, 71 gave a similar equation:

Table 8.1 Dry Wear Coefficients for Different Material Pairs

Sliding against hardened tool steel

unless otherwise stated Wear coefficient, K Microhardness, H,,, ( 103 kg/cm2) Mild steel on mild steel

Ferritic stainless steel

Laminated bakelite type 292/16

Moulded bakelite type 11085/1

Tungsten carbide on mild steel

Moulded bakelite type 547/1

1.3 1 0 - ~ 5.5 x 10-’

1.7 1 0 - ~

7.5 10-7

4 x 10-6

3 1 0 - ~ 1.3 1 0 - ~

130

314 Chapter 8

where

Y = wear volume (in.3)

L = sliding distance (in.)

A = surface area (in.2)

P = applied load (lb)

U,, = yield strength of the softer material (psi)

h = depth of wear of the softer material (in.)

k = wear coefficient

Values of k for different material combinations are given in Table 8.2

The depth of wear of the harder material hh, can be calculated from:

2

$=(&) (8.3)

For conditions where the load and or the surface temperature are high enough to cause plastic deformation, the wear rate as calculated from Eqs (8.1) and (8.2) can be several orders of magnitude higher (in the order of

Table 8.2 Wear Coefficients, k, for Metal Combinations

Metal combination k x 10-4 Metal combination k x 10-4

36.5

38.5 59.5 77.5 81.0 126.0 286.0

1000 times) This is generally known as “plastic wear” and often leads to

very rapid rate of material removal

Krushchov and coworkers [8, 91 developed a similar linear relationship

between wear resistance and hardness for commercially pure and annealed

materials This relationship is given in Fig 8.1 A particularly interesting result was obtained by them for heat-treated alloy steels As shown in Fig

8.2, the wear resistance for the steels in the annealed condition increased linearly with hardness However, increasing the hardness of a particular alloy by heat treating produced a smaller rate of increase of the relative wear resistance This clearly suggests that the relative wear resistance of a material does not only depend on its hardness but is also influenced by the

H (kglmm‘) Figure 8.1

commercially pure metals (From Ref 8.) Relationship between relative wear resistance and hardness for some

316 Chapter 8

H (kg/mm2) Figure 8.2

treated steels (From Ref 8.)

Relationship between relative wear resistance and hardness for heat

presence of microscopic and submicroscopic inhomogeneities in the lattice structure by distortions of the lattice It was also found by them that increas- ing the hardness further, by work hardening, did not improve the relative wear resistance and, in some cases, even reduced it

Frictional surface damage can also occur as a result of the interpenetra- tion of asperities, which produce tensile stress in the surface layer due to the bulge formed ahead of the indentor (refer to Fig 8.3) Cracks can form

perpendicular to the surface at imperfections such as lattice vacancies, grain boundaries and metalurgical defects including pores, gas bubbles, slag inclusions, and marked disparity in grain size

Figure 8.3 Cracks at surface imperfections due to repeated asperity action

8.4.1 Contact Fatigue

The most common example of the type of surface damage is what is gen- erally known as “pitting” or contact fatigue It often exists in rolling element bearings and gears and is attributed to the propogation of fatigue cracks originating on or below the surface when the Hertzian pressure exceeds an

allowable value As one element rolls many times over the other element, a

subsurface region undergoes cycles of shear ranging from zero to maximum This situation would be expected to promote fatigue damage when the maximum shearing stress is higher than the fatigue limit for the material

in this region Subsurface cracks may occur and these cracks will propogate

to the surface under repeated loading and consequently forming a pit or a spall The equations for calculating the maximum subsurface shear stress and its location can be written as follows

For cylindrical contacts:

qo = maximum contact stress = 0.418 {E:, -

tmax = maximum subsurface shear stress - = o.304q0

For spherical contacts:

The number of cycles to pitting failure, N , generally follows the follow-

ing fatigue equation:

N"n~max = C

where

rmax = maximum shear stress

C and n are constants for each material

Accordingly, the life ratio depends on maximum shear stress:

The value of n varies between 6 and 18 for most materials

generally used It can be expressed as: For cumulative fatigue under different stress cycles, the Miner theory is

where

Ni = number of cycles at any stress level

Nir = number of cycles to failure at that stress level

8.4.2 The IBM Zero Wear Concept

Because of the stringent requirements on the minimization of wear in elec- tronic equipment, IBM conducted extensive wear experiments in order to allow reliable prediction of their useful life [lO-12] The criterion for zero wear is that the depth of the wear scar does not exceed one half of the peak- to-peak value of the surface roughness This may be a severe requirement for most mechanical equipment, which can tolerate considerably larger amounts of wear without loss of functionality

The empirical equation developed by IBM is given as follows, based on

2000 cycles as the reference number in their tests:

N = number of passes one element undergoes in the relative motion

(or number of contact cycles)

Y = yield point in shear (psi) which is a function of the microhardness of the surface as given in Fig 8.4) and Table 8.3

G = empirical factor determined from the tests Surprisingly, it was found to take one of the following two values depending on the material pair

and the lubrication condition

G = 1.0 for full film lubrication

G = 0.54 for quasihydrodynamic lubrication

For unlubricated or boundary lubrication conditions, G , takes one of only

two possible values:

G = 0.54 for systems with low susceptibility for transfer

G = 0.20 for systems with high susceptibility for transfer

Table 8.4 gives the values of G for different material combinations tested by IBM

IBM used the concept of mutual overlap in defining the number of

passes The coefficient of mutual overlap (KmUl) can be defined as: