Surface Roughness

1

Surface Roughness

Rajesh SharmaTribology Elective Course Jan

2010

2

Tribology History

• Use of wheels from 3500 B.C.• A chariot with animal fat lubricant in wheel

bearing: Egypt• Use of lubricant during transportation* • Ball thrust bearing used to support a

statue by a sculptor: about 40 BC– Found in a lake Nimi near Rome #

3

Friction

• First postulates by Vinci(1452-1519)– Friction force

– proportional to Load– independent of nominal area of contact

• In 1699, Amonton credited with the laws• Coulomb in 1780 put forth 3rd law of friction

– Friction independent of velocity

4

Early Developments

• 1684 Hooke – Combination of steel shaft and metal bush as preferable

combination instead of wood shod with iron for wheel bearings.

• 1883 Beauchamp Tower– Discovered phenomenon of hydrodynamic lubrication

• 1883 Petrof (Petrov)– Concept of viscous shearing

• 1886 Osborne Reynolds– Theoretical interpretation of hydrodynamic lubrication

5

Tribology in Industry• Energy dissipation or loss

– Resistance to motion and friction • Wear

– Surface changes may be beneficial or disastrous– One half of GNP used to replace components after wear

• Friction and wear not always disadvantageous• Wear effects are more important than frictional

losses• Environmental factors due to use of lubricants

6

Tribological SolutionsLow friction/wear

Low load cases

Reduce intimate contact

Thermal stability important

Low resistance to transverse shear

Graphite or MoS2

Built in press. due to fluid film

Water, oil, air

Transverse displacement small

Polymer with visco-elasticity

Some sort of elastomeric property

Widely employed tribological solution

Load carried without mechanical contact

Electric meters

7

Surface Topography

Mean of Surface

Mean of Asperity Summits

8

Contact of Surfaces

d

Reference PlaneMean of AsperitySummits

Typical Contact

Flat and Rigid Surface

9

Significance• Friction, lubrication, and

wear issues in automobile brake linings, floor surfaces, tires, roller bearings, cam & follower and gears

• Contact resistance in space applications

• Fluid dynamics in pipe flow and flow resistance of ship hulls

• Etc. etc.

10

Categories• Definitions and indications for surface roughness

parameters(for industrial products)are specified. • They are:

– arithmetical mean roughness(Ra)– maximum height(Ry), – ten-point mean roughness(Rz), – mean spacing of profile irregularities(Sm), – mean spacing of local peaks of the profile(S)– profile bearing length ratio(tp).

• Surface roughness is given as the arithmetical mean value for a randomly sampled area.

11

Typical ways for obtaining surface roughness

• Arithmetic Mean Average– A section of standard length is sampled from the

mean line on the roughness chart. The mean line is laid on a Cartesian coordinate system wherein the mean line runs in the direction of the x-axis and magnification is the y-axis.

12

• Maximum peak(Ry)– A section of standard length is sampled from

the mean line on the roughness chart. The distance between the peaks and valleys of the sampled line is measured in the y direction.

13

• Ten-point mean roughness(Rz)– A section of standard length is sampled from

the mean line on the roughness chart. The distance between the peaks and valleys of the sampled line is measured in the y direction. Then, the average peak is obtained among 5 tallest peaks(Yp), as is the average valley between 5 lowest valleys(Yv).

14

15

Mean spacing of profileirregularities

Average of the values of the spacing between irregularities Smi of the profile inside an evaluation length.

16

Mean spacing of local peaks of the profile

• Click to add an outline

17

Roughness Parameters

• Amplitude parameters– Extreme-value

parameters– Average parameters– Height distribution

• Texture parameters

• Rp, Rv

Rt

18

19

Amplitude: Height Distribution

20

Amplitude: Average Parameters

Rq=1.25 Ra

21

Z

22

Typical Contact

Original shape

2a

P

R

Contact area

23

Asperity

Asperities

24

•BAC or Bearing Area Curve is an assessment of the percent of bearing area at a certain distance (P) from the highest peak. It is sometimes used to estimate the percent of contact area available as surface wear occurs.

25

Surface Contacts

• Deformation due to load acting on a point– Elastic

• Simple and reversible– Plastic

• Complex phenominon and irreversible

In most of the cases the deformation may be plastic at the microlevel geometries e.g. Metal working

26

contd.

• Contact of bodies defined by arcs – Wheel on tracks, rolling bearing, gear teeth,

rope drives, etc.• Small protuberances

– Spherical shape at contact• Target : Friction and heat

27

Multi-Asperity Models(Greenwood and Williamson, 1966, Proceedings of the Royal Society

of London, A295, pp. 300-319.)

Assumptions All asperities are spherical and have the same summit

curvature. The asperities have a statistical distribution of heights

(Gaussian).(z)z

28

Click to add title• Click to add an outline

29

Figs

• Click to add an outline

30

172 slaves pulling a large statue weighing about 6x105 N

31

Probably used about A.D. 40 by a sculptor to support a statue.

Found in 1928 in Lake Nimi near Rome

32

Multi-Asperity Models(Greenwood and Williamson, 1966, Proceedings of the Royal Society

of London, A295, pp. 300-319.)

Assumptions (cont’d) Deformation is linear elastic and isotropic. Asperities are uncoupled from each other. Ignore bulk deformation.

(z)z

33

Two Scales of the Contact

Nominal Surface

• Contact Bump (larger, micro-scale)

• Asperities (smaller, nano-scale)

34

Irregularities

35

36

Click to add title