Effects of smallscale texturing on ringliner friction

1
Effects of small-scale texturing on ring/liner
friction

• Rosalind Takata
• Dr. Victor Wong
• Nov. 7, 2006
• ICEF2006, Sacramento CA

2
Study overview

• Overall objective
• to reduce ring/liner friction in large natural
  gas ICE
• Objective of current study
• study liner surface texturing and lubricant
  effects for friction reduction
• Texturing parameters studied
• Groove angle, width, depth, area ratio
• Dimple arrangement, diameter, depth, area ratio
• Lubricant effects
• Lubricant viscosity changes may be combined with
  texturing effects to further reduce friction

3
Effects of surface texturing

• Hydrodynamic
• Texturing can increase hydrodynamic load support,
  increasing film thickness
• asperity contact decreases
• shear stress decreases because of decrease in
  shear rate
• Wear particles
• Negative asperities can act as wear particle
  traps, removing them from sliding interface
• Lubricant reservoir
• Negative asperities can act as lubricant
  reservoirs, providing lubricant to sliding
  interface

4
Current uses

• Electromagnetic storage devices
• Face seals (steps, grooves, etc. are the most
  common)
• Texturing being considered for other
  applications, e.g., engine cylinder liners
  (Gehring Gmbh)

5
Recent research

• Several analytical and experimental studies show
  friction reduction when texturing is added
• For most analyses
• full-film assumed between surfaces
• based on Reynolds or EHL analysis
• numerical simulations
• Experimental results
• show increased load support for face seals
  (Etsion)
• show decreased cycle-average friction for
  piston-ring-like applications (Etsion, Sadeghi)

6
Hydrodynamic Effects

• Etsion, et al., 1996-present
• full-film, non-contact analytical Reynolds
  analysis, round dimples
• reduction in friction predicted and observed in
  face seals and piston-ring-like cases
• Experiments show effect of dimples on lubrication
  regime transition

7
Hydrodynamic Effects

• Sadeghi, et. al.
• full-film assumed, asperity contact included.
  Semi-deterministic.
• Low skewness causes less contact near end-strokes
  in reciprocating tester simulation

8
Flow regime effects

• Hu Zhu (2000-2001)
• Elasto-hydrodynamic regime, mixed lubrication
  model
• Topography has only small effect on film
  thickness
• Transverse roughness shows increase in asperity
  contact, and flattening of roughness peaks
• Transverse grooves may have a negative effect
  (more asperity contact) at high load/low speed
  conditions

9
Effects of surface texturing
Asperity contact pressure

• affected directly by surface asperity
  distribution
• affected indirectly, via effects on fluid film
  thickness

Flow resistance

• affects fluid film thickness
• film thickness affects both asperity contact and
  shear stress

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Averaged flow factor method

• Average flow model (Patir and Cheng, 1979) used
  to account for effects of surface roughness
  on lubricant flow and shear stress
• Based on Reynolds relationships
• Combined stochastic deterministic and method

where qx oil flow rate h
nominal film thickness tx shear stress
fiflow factors Rq surface roughness
ffi stress factors
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Flow and stress factors
Pressure factors account for difference in
pressure-driven flow and stresses due to surface
roughness
Shear factors account for difference in flow and
stresses due to movement of rough surface
Geometric factors account for difference between
nominal smooth surface clearance and rough
surface clearance
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Key assumptions

• Limit maximum local shear stress to avoid very
  high shear stress at low film thicknesses

Limiting shear stress of 1 MPa assumed. When
local shear stress exceeds limiting shear stress,
limiting shear stress assumed

• Region between surfaces is filled with fluid
• Texture is relatively smooth

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Limitations

• Size of features analyzed is limited by patch
  size
• features and patterns must be small compared to
  ring width
• Reynolds equation applicability
• For deep or closely spaced features Reynolds
  analysis may not be applicable
• For very small film thicknesses, Reynolds
  analysis breaks down
• Effects are averaged
• Deterministic oil transport and contact phenomena
  not accounted for
• Oil transport within grooves/textures not
  accounted for

14
Factor effects analysis

• The ring simulation was run to evaluate the
  effects of each flow and stress factor
• Effects of factors can indicate physical
  mechanisms for friction reduction (or increase)

15
Factor effects film thickness

• Reducing pressure flow factor increases film
  thickness

decreased pressure flow factor
smooth
16
Factor effects shear stress

• The film thickness increase due to low pressure
  flow factor causes a decrease in shear stress
• Increased shear stress factor causes a small
  increase

smooth
increased shear stress factor
decreased pressure flow factor
17
Factor effects boundary contact

• The film thickness increase due to low pressure
  flow factor causes a decrease in boundary contact

smooth
decreased pressure flow factor
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Factor effects total friction
19
Factor effects results

• The pressure flow factor has the major effect on
  oil film thickness and friction
• Effects of different factors are additive
• Decreased pressure-driven flow leads to thicker
  oil film -gt decreased friction

20
Parametric studies

• Grooves
• angle
• width
• depth
• area ratio
• Dimples
• arrangement
• width
• depth
• area ratio
• Smooth surface except for features, for flow
  factor calculations

Gaussian profiles
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Results

• Texture depth causes no change in flow factors,
  as function of h/sigma ratio
• Effects of texturing tend to increase with depth
• Groove angle has a major effect on friction
• Deeper/wider/rougher textures tend to have
  lower friction
• Appearance of optimum depths may result from
  model limitations

22
Groove angle pressure flow factor

• Pressure flow factor decreases with decreasing
  groove angle
• Trends agree with previous analysis

From Jocsak, ICEf2004-952, 2004
23
Groove angle

• Decrease in friction with groove angle agrees
  with previous studies and practical observation
  in industry

24
Groove area ratio
Groove angle30 deg.
25
Groove width

• Small effect of groove widths tested may be due
  to limitation of sizes useable in model

angle30 deg.
26
Dimple area ratio

• Friction decreases with area ratio for both
  dimples and grooves

27
Dimple diameter

• Friction is lowest for a mid-range dimple
  diameter
• Optimum effect agrees with some previous studies

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Conclusions parametric study

• Surface texturing may reduce friction
  substantially
• by reducing boundary contact via film thickness
  increase
• by reducing shear stress, via film thickness
  increase
• Effect on pressure-driven flow may be major
  contributor to friction changes
• Want textures that impede pressure-driven flow
• Grooves that are more transverse
• Deeper textures (for the tested conditions)
• Larger area ratio/larger width/larger diameter
• Dimple diameter may have an optimum value for
  given conditions
• Grooves may be more effective than dimples at
  higher film thicknesses
• Limitations of the model must be considered

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Combination of texture and lubricant effects

• Reduction in lubricant viscosity reduces
  friction, but decreases film thickness
• wear increases because of increase in asperity
  contact
• Adding texturing increases film thickness
• May increase oil consumption
• Combine viscosity reduction and surface texturing
  
• no net increase in wear
• friction-reduction effects of surface texturing
  and viscosity reduction can be combined

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Combination effects

• The additional friction reduction from viscosity
  reduction is comparable to that due to texturing

31
Combination effects

• Combining surface texturing and viscosity
  reduction can cause a decrease in friction
  without increasing wear
• Effects of reduced lubricant viscosity on other
  engine components must be considered

32
Conclusions

• Designed surface texturing can reduce ring/liner
  friction
• Physical mechanism may be the effect of texturing
  on pressure-driven flow
• Parameters such as groove angle and depth, and
  dimple size and depth, affect how much reduction
  occurs
• Combining surface texturing with reduced
  lubricant viscosity may provide extra benefits
• More analysis is required to characterize the
  behavior of larger-scale texturing and
  non-Reynolds behavior

33
Thank you
34
Hydrodynamic Effects

• Most results report cycle-average coefficient of
  friction
• Some correlations with texture geometric
  parameters have been derived

Sadeghi (2006) Ddiameter,ddepth,Lspacing
Etsion (2006) aspect ratiodepth/diameter
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One physical interpretation

• When the liner surface is textured, fluid caught
  in the crevices is essentially stagnant (liner is
  not moving)
• effective area for lubricant flow decreases
• both shear and pressure-driven flows are impeded
• the effective oil film is thinner
• resulting higher shear rate causes higher stress
  due to shear
• but stress due to pressure-driven flow decreases

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For rough liner

• A reduction in pressure flow factor (from 1)
• increase in hydrodynamic resistance
• increase in load support
• increase in fluid film thickness
• An decrease in shear flow factor (from 0)
• increase in fluid film thickness
• A reduction in pressure stress factor (from 1)
• decrease in shear stress
• An increase in shear stress factor (from 0)
• increase in shear stress

37
Groove depth

• Pressure flow factor independent of groove depth,
  as a function of h/sigma
• In application, deeper grooves will have a larger
  effect for a given film thickness

Results for 15 degree angle (30 degree
cross-hatch)
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Groove width

• Pressure flow factor only affected at low h/sigma
  values
• Wider grooves have slightly larger effect at a
  given film thickness

Results for 15 degree angle (30 degree
cross-hatch)
39
Groove area ratio

• Pressure flow factor decreases with increasing
  area ratio

Results for 15 degree angle (30 degree
cross-hatch)
40
Dimples - depth

• As for grooves, there is no change in pressure
  flow factor with depth as a function of h/sigma

Aratio.28, diam19 micron
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Dimples - diameter

• There is an ideal diameter for low pressure flow
  factor at a given film thickness a mid-range
  diameter gives the lowest factor

Aratio.28, d3 micron
42
Dimples area ratio

• Area ratio has only a small effect between
  10-22, with a slight decrease in pressure flow
  factor with increasing area ratio

diam19 micron, d3 micron
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Grooves vs. dimples

• For grooves, PFF decreases more gradually as film
  thickness (or h/sigma) decreases
• Grooves may be more effective for thicker films

30 degree CH, Arat.24,w20 micron
Arat.28, diam19 micron