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Time-dependent wear process between lubricated soft materials
Wear 225–229 Ž1999. 656–659
Time-dependent wear process between lubricated soft materials K. Ikeuchi ) , J. Kusaka, D. Yamane, S. Fujita Institute for Frontier Medical Sciences, Kyoto UniÕersity, Kyoto, 606-8507, Japan
Abstract At start-up, lubricated compliant solids are subject to wear until complete fluid film is formed due to soft-EHL effect. An apparatus with a half-cylinder sliding on a flat silicon rubber plate was used. According to observation through the transparent half-cylinder, velocity of fluid film formation is close to a half of sliding speed. The wear traces were observed and recorded with a video camera. It is shown that wear is severest at the starting region, while it is slight at the end of the trace. No wear is observed in the middle zone where complete fluid film exists. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Wear; Sliding; Soft-EHL; Fluid film
1. Introduction As fluid film may be easily formed between lubricated cylinders of low elastic materials, wear of compliant surface bearings w1,2x, elastomeric seals w3x or synovial joints w4,5x are kept low although anti-wear property of soft material is generally low. However, during a long rest under load, as the squeeze film diminishes gradually, the mating surfaces contact directly to each other. Then, after restart, direct contact remains until complete fluid film takes place. The authors w6x have reported that full fluid film is not formed until sliding distance exceeds the twice of the initial contact width even if Stribeck number is large enough. Therefore, the coefficient of friction and wear rate of soft material may be high during transient sliding condition. The purpose of this study was to investigate the mechanism of wear for compliant surface bearings and elastometric seals during start-up. The experimental result confirms that high friction and wear arise at the contact region until fluid film is formed.
2. Basic concept [6] Fig. 1 is a schematic illustration of the transient EHL problem for a compliant cylinder sliding on a rigid flat
)
Corresponding author. Tel.: q81-75-751-4139; fax: q81-75-7514144; e-mail: [email protected]
surface. The volume of fluid film per unit depth is given by V s hl
Ž 1.
where l is length of fluid film and h is film thickness. If the Stribeck number defined by Ž viscosity = viscosityrload. is large enough and the surface is compliant enough, shear induced flow ŽQuette flow. predominates pressure induced flow. Thus, the rate of inlet flow is expressed as qs
dV
uh s
dt
2
Ž 2.
where u is sliding velocity. From Eqs. Ž1. and Ž2., the frontal velocity of the fluid film is given as follows. dl
u s
dt
2
Ž 3.
Thus, the speed of film formation is half of sliding speed, the contact region remains and wear is inevitable until the sliding distance exceeds the twice of the initial contact width. Fig. 2 indicates the relative motion and the film formation between a slider and a thruster according to the previous paper w6x. As the coordinate system is fixed to the slider, the thruster moves right while the slider is at rest in this figure. After the start of sliding Ža., the left end of the
0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 8 . 0 0 3 7 6 - 7
K. Ikeuchi et al.r Wear 225–229 (1999) 656–659
657
Fig. 1. Schematic presentation of transient EHL problem for a compliant cylinder sliding on a rigid flat surface. Fig. 3. Experimental apparatus.
contact zone moves rightward from the initial contact region Žarea I. at the same speed with sliding. At the same time, the right end of the contact region moves right at the half of sliding speed. With the movement of the contact region toward transient region Žarea T., the area of the contact region reduces Žb., and finally diminishes Žc., when the sliding distance exceeds the twice of the initial contact width. If complete fluid film is formed Žd., no wear arises in the area S.
3. Apparatus, materials and method Fig. 3 shows the experimental apparatus. A compliant silicon rubber plate Žthickness: 10 mm, width: 74 mm, length: 140 mm, Young’s modulus: 3.0 MPa, Poisson’s ratio: 0.5. is attached on a rigid plate which is driven by an electric linear motor controlled by a computer. The silicon rubber surface is coated with black carbon particles using a white board marker to make wear visible. A cylindrical thruster Žradius: 30 mm, length: 76 mm. of Polymethylmethacrylate ŽPMMA. resin is put on the slider. The thruster and the slider are lubricated with 22 wt.% water solution of sodium alkyl sulfate Žviscosity: 0.31 Pas.. After application of 43 N Ž580 Nrm. vertical load to the thruster, the slider is accelerated for 0.1 s, moves at constant speed and decelerated for 0.1 s. Total sliding distance is about 70 mm. Friction is measured with strain gauges attached to leaf springs. The stiffness of the double spring system is 2 = 10 4 Nrm. After each sliding, the thin
Fig. 2. Film formation at stages a–d with regions I, T and S defined on flat plate.
layer of carbon particles on the slider surface is examined to investigate the wear process.
4. Result In this study, the theoretical prediction in the previous paper w6x was examined through the experiment. At start-up, after taking the maximum value, coefficient of friction becomes lower ŽFig. 4.. In the case that steady sliding velocity is 20 mmrs, friction increases again until the end of the sliding stage. In the case that sliding velocity is 80 mmrs, coefficient of friction is constant during steady rotation. Then, friction increases again during deceleration. At steady sliding, a low coefficient of friction results from a high-speed sliding. The surface of silicon rubber is observed after each sliding test. The black region shows that the carbon layer is preserved, while the white region indicates that the layer is removed by wear. For 20 mmrs of steady sliding speed ŽFig. 5., the carbon surface film is completely removed at the region I and the region T. Further, considerable part of the surface film is worn in the region S leaving bands and stripes in the direction of sliding. In the case of sliding at 40 mmrs ŽFig. 6., the surface coating is removed in region I and part of the region T. The surface coating is also removed at the center and near both sides of the region S. At high sliding speed Ž80 mmrs., while region I and part
Fig. 4. Measured coefficient of friction for different speed.
K. Ikeuchi et al.r Wear 225–229 (1999) 656–659
658
Fig. 5. Wear trace for steady sliding speed of 20 mmrs, the black part indicates unworn region and the white part indicates worn region.
of region T is worn, most of the coating is preserved in the region S except in the narrow stripes ŽFig. 7..
5. Discussion Judging from the measured coefficient of friction, full fluid film lubrication condition was not attained during steady sliding up to 80 mmrs. The authors suppose that both ends of the thruster continue to contact directly due to side leakage. This assumption is supported by the fact that the both ends are worn ŽFigs. 5–7.. The measured coefficients of friction and the wear traces show that the both surface contact directly to each other at start-up. As direct contact inevitably remains after start of sliding, the most severe wear arises in the region I. Thereafter, the contact area moves rightward while the fluid film develops. Consequently, the left side of the region B is worn more severely than the right side, because the lubrication condition is improved with an increase of sliding distance. According to the experiment for transient soft-EHL, direct contact is inevitable during start-up until sliding
Fig. 7. Wear trace for steady sliding speed of 80 mmrs.
distance exceeds the twice of the initial contact width. As contact area of a compliant bearing or an elastometric seal is generally wide due to large deformation, sliding with partial contact continues long because the sliding distance accompanied by wear is larger than the twice of the initial contact width. Consequently, amount of wear may be excessively high during start-up after a long rest. According to the present result, the following methods will be effective to reduce wear for compliant surface bearings and seals. 1. Courting the surface with solid lubricant or mixing it with the material. 2. Adding boundary lubricant into the fluid. 3. Cutting grooves or making texture to store fluid between the surfaces.
6. Conclusion For the lubrication between a compliant plate and a cylinder during start-up and steady sliding, the conclusion is as follows. Ž1. At start-up of sliding, direct contact is inevitable until complete fluid film is formed. Ž2. The initial contact region may wear severely at start-up. Ž3. Then, wear moves to the transient sliding region. Wear rate decreases with an increase of sliding distance. Ž4. Both sides of the sliding region is subjected to wear due to side leakage.
References
Fig. 6. Wear trace for steady sliding speed of 40 mmrs.
w1x M.K. Benjamin, V. Castelli, A theoretical investigation of compliant surface journal bearings, J. Lub. Technol., Trans. ASME 93 Ž1971. 191–201. w2x H.D. Conway, H.C. Lee, The analysis of the lubrication of a flexible journal bearing, J. Lub. Technol., Trans. ASME 97 Ž1975. 599–604.
K. Ikeuchi et al.r Wear 225–229 (1999) 656–659 w3x A. Gabelli, G. Poll, Formation of lubricant film in rotary sealing contacts: Pt. 1—Lubricant film modeling, J. Lub. Technol. Trans. ASME 114 Ž1992. 280–289. w4x J.B. Medley, D. Dowson, Lubrication of elastic–isoviscous line contacts subject to cyclic time-varying loads and entrainment velocities, ASLE Trans. 27 Ž3. Ž1984. 243–251.
659
w5x K. Ikeuchi, M. Oka, S. Kubo, The relation between friction and creep deformation in articular cartilage, Dissipative Processes in Tribology, Elsevier, Amsterdam, 1994, pp. 247–252. w6x K. Ikeuchi, S. Fujita, M. Ohashi, Analysis of fluid film formation between contacting compliant solids, Tribology International, Žin press..
Time-dependent wear process between lubricated soft materials K. Ikeuchi ) , J. Kusaka, D. Yamane, S. Fujita Institute for Frontier Medical Sciences, Kyoto UniÕersity, Kyoto, 606-8507, Japan
Abstract At start-up, lubricated compliant solids are subject to wear until complete fluid film is formed due to soft-EHL effect. An apparatus with a half-cylinder sliding on a flat silicon rubber plate was used. According to observation through the transparent half-cylinder, velocity of fluid film formation is close to a half of sliding speed. The wear traces were observed and recorded with a video camera. It is shown that wear is severest at the starting region, while it is slight at the end of the trace. No wear is observed in the middle zone where complete fluid film exists. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Wear; Sliding; Soft-EHL; Fluid film
1. Introduction As fluid film may be easily formed between lubricated cylinders of low elastic materials, wear of compliant surface bearings w1,2x, elastomeric seals w3x or synovial joints w4,5x are kept low although anti-wear property of soft material is generally low. However, during a long rest under load, as the squeeze film diminishes gradually, the mating surfaces contact directly to each other. Then, after restart, direct contact remains until complete fluid film takes place. The authors w6x have reported that full fluid film is not formed until sliding distance exceeds the twice of the initial contact width even if Stribeck number is large enough. Therefore, the coefficient of friction and wear rate of soft material may be high during transient sliding condition. The purpose of this study was to investigate the mechanism of wear for compliant surface bearings and elastometric seals during start-up. The experimental result confirms that high friction and wear arise at the contact region until fluid film is formed.
2. Basic concept [6] Fig. 1 is a schematic illustration of the transient EHL problem for a compliant cylinder sliding on a rigid flat
)
Corresponding author. Tel.: q81-75-751-4139; fax: q81-75-7514144; e-mail: [email protected]
surface. The volume of fluid film per unit depth is given by V s hl
Ž 1.
where l is length of fluid film and h is film thickness. If the Stribeck number defined by Ž viscosity = viscosityrload. is large enough and the surface is compliant enough, shear induced flow ŽQuette flow. predominates pressure induced flow. Thus, the rate of inlet flow is expressed as qs
dV
uh s
dt
2
Ž 2.
where u is sliding velocity. From Eqs. Ž1. and Ž2., the frontal velocity of the fluid film is given as follows. dl
u s
dt
2
Ž 3.
Thus, the speed of film formation is half of sliding speed, the contact region remains and wear is inevitable until the sliding distance exceeds the twice of the initial contact width. Fig. 2 indicates the relative motion and the film formation between a slider and a thruster according to the previous paper w6x. As the coordinate system is fixed to the slider, the thruster moves right while the slider is at rest in this figure. After the start of sliding Ža., the left end of the
0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 8 . 0 0 3 7 6 - 7
K. Ikeuchi et al.r Wear 225–229 (1999) 656–659
657
Fig. 1. Schematic presentation of transient EHL problem for a compliant cylinder sliding on a rigid flat surface. Fig. 3. Experimental apparatus.
contact zone moves rightward from the initial contact region Žarea I. at the same speed with sliding. At the same time, the right end of the contact region moves right at the half of sliding speed. With the movement of the contact region toward transient region Žarea T., the area of the contact region reduces Žb., and finally diminishes Žc., when the sliding distance exceeds the twice of the initial contact width. If complete fluid film is formed Žd., no wear arises in the area S.
3. Apparatus, materials and method Fig. 3 shows the experimental apparatus. A compliant silicon rubber plate Žthickness: 10 mm, width: 74 mm, length: 140 mm, Young’s modulus: 3.0 MPa, Poisson’s ratio: 0.5. is attached on a rigid plate which is driven by an electric linear motor controlled by a computer. The silicon rubber surface is coated with black carbon particles using a white board marker to make wear visible. A cylindrical thruster Žradius: 30 mm, length: 76 mm. of Polymethylmethacrylate ŽPMMA. resin is put on the slider. The thruster and the slider are lubricated with 22 wt.% water solution of sodium alkyl sulfate Žviscosity: 0.31 Pas.. After application of 43 N Ž580 Nrm. vertical load to the thruster, the slider is accelerated for 0.1 s, moves at constant speed and decelerated for 0.1 s. Total sliding distance is about 70 mm. Friction is measured with strain gauges attached to leaf springs. The stiffness of the double spring system is 2 = 10 4 Nrm. After each sliding, the thin
Fig. 2. Film formation at stages a–d with regions I, T and S defined on flat plate.
layer of carbon particles on the slider surface is examined to investigate the wear process.
4. Result In this study, the theoretical prediction in the previous paper w6x was examined through the experiment. At start-up, after taking the maximum value, coefficient of friction becomes lower ŽFig. 4.. In the case that steady sliding velocity is 20 mmrs, friction increases again until the end of the sliding stage. In the case that sliding velocity is 80 mmrs, coefficient of friction is constant during steady rotation. Then, friction increases again during deceleration. At steady sliding, a low coefficient of friction results from a high-speed sliding. The surface of silicon rubber is observed after each sliding test. The black region shows that the carbon layer is preserved, while the white region indicates that the layer is removed by wear. For 20 mmrs of steady sliding speed ŽFig. 5., the carbon surface film is completely removed at the region I and the region T. Further, considerable part of the surface film is worn in the region S leaving bands and stripes in the direction of sliding. In the case of sliding at 40 mmrs ŽFig. 6., the surface coating is removed in region I and part of the region T. The surface coating is also removed at the center and near both sides of the region S. At high sliding speed Ž80 mmrs., while region I and part
Fig. 4. Measured coefficient of friction for different speed.
K. Ikeuchi et al.r Wear 225–229 (1999) 656–659
658
Fig. 5. Wear trace for steady sliding speed of 20 mmrs, the black part indicates unworn region and the white part indicates worn region.
of region T is worn, most of the coating is preserved in the region S except in the narrow stripes ŽFig. 7..
5. Discussion Judging from the measured coefficient of friction, full fluid film lubrication condition was not attained during steady sliding up to 80 mmrs. The authors suppose that both ends of the thruster continue to contact directly due to side leakage. This assumption is supported by the fact that the both ends are worn ŽFigs. 5–7.. The measured coefficients of friction and the wear traces show that the both surface contact directly to each other at start-up. As direct contact inevitably remains after start of sliding, the most severe wear arises in the region I. Thereafter, the contact area moves rightward while the fluid film develops. Consequently, the left side of the region B is worn more severely than the right side, because the lubrication condition is improved with an increase of sliding distance. According to the experiment for transient soft-EHL, direct contact is inevitable during start-up until sliding
Fig. 7. Wear trace for steady sliding speed of 80 mmrs.
distance exceeds the twice of the initial contact width. As contact area of a compliant bearing or an elastometric seal is generally wide due to large deformation, sliding with partial contact continues long because the sliding distance accompanied by wear is larger than the twice of the initial contact width. Consequently, amount of wear may be excessively high during start-up after a long rest. According to the present result, the following methods will be effective to reduce wear for compliant surface bearings and seals. 1. Courting the surface with solid lubricant or mixing it with the material. 2. Adding boundary lubricant into the fluid. 3. Cutting grooves or making texture to store fluid between the surfaces.
6. Conclusion For the lubrication between a compliant plate and a cylinder during start-up and steady sliding, the conclusion is as follows. Ž1. At start-up of sliding, direct contact is inevitable until complete fluid film is formed. Ž2. The initial contact region may wear severely at start-up. Ž3. Then, wear moves to the transient sliding region. Wear rate decreases with an increase of sliding distance. Ž4. Both sides of the sliding region is subjected to wear due to side leakage.
References
Fig. 6. Wear trace for steady sliding speed of 40 mmrs.
w1x M.K. Benjamin, V. Castelli, A theoretical investigation of compliant surface journal bearings, J. Lub. Technol., Trans. ASME 93 Ž1971. 191–201. w2x H.D. Conway, H.C. Lee, The analysis of the lubrication of a flexible journal bearing, J. Lub. Technol., Trans. ASME 97 Ž1975. 599–604.
K. Ikeuchi et al.r Wear 225–229 (1999) 656–659 w3x A. Gabelli, G. Poll, Formation of lubricant film in rotary sealing contacts: Pt. 1—Lubricant film modeling, J. Lub. Technol. Trans. ASME 114 Ž1992. 280–289. w4x J.B. Medley, D. Dowson, Lubrication of elastic–isoviscous line contacts subject to cyclic time-varying loads and entrainment velocities, ASLE Trans. 27 Ž3. Ž1984. 243–251.
659
w5x K. Ikeuchi, M. Oka, S. Kubo, The relation between friction and creep deformation in articular cartilage, Dissipative Processes in Tribology, Elsevier, Amsterdam, 1994, pp. 247–252. w6x K. Ikeuchi, S. Fujita, M. Ohashi, Analysis of fluid film formation between contacting compliant solids, Tribology International, Žin press..