surface roughness
TRANSCRIPT
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Surface Roughness
Rajesh SharmaTribology Elective Course Jan
2010
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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 #
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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
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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
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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
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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
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Surface Topography
Mean of Surface
Mean of Asperity Summits
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Contact of Surfaces
d
Reference PlaneMean of AsperitySummits
Typical Contact
Flat and Rigid Surface
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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.
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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.
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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.
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• 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.
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• 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).
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Mean spacing of profileirregularities
Average of the values of the spacing between irregularities Smi of the profile inside an evaluation length.
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Mean spacing of local peaks of the profile
• Click to add an outline
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Roughness Parameters
• Amplitude parameters– Extreme-value
parameters– Average parameters– Height distribution
• Texture parameters
• Rp, Rv
Rt
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Amplitude: Height Distribution
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Amplitude: Average Parameters
Rq=1.25 Ra
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Z
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Typical Contact
Original shape
2a
P
R
Contact area
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Asperity
Asperities
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•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.
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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
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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
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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
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Click to add title• Click to add an outline
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Figs
• Click to add an outline
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172 slaves pulling a large statue weighing about 6x105 N
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Probably used about A.D. 40 by a sculptor to support a statue.
Found in 1928 in Lake Nimi near Rome
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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
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Two Scales of the Contact
Nominal Surface
• Contact Bump (larger, micro-scale)
• Asperities (smaller, nano-scale)
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Irregularities
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Click to add title