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Improving wear resistance of sprocket parts using a fine-blanking process
Wear 271 (2011) 2396–2401
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Short communication
Improving wear resistance of sprocket parts using a fine-blanking process S. Thipprakmas ∗ King Mongkut’s University of Technology Thonburi, 126 Prachautid, Bangmod, Thungkru, Bangkok, Thailand
a r t i c l e
i n f o
Article history: Received 1 September 2010 Received in revised form 22 December 2010 Accepted 22 December 2010
Keywords: Wear Sprocket Fine-blanking Hobbing Hardness
a b s t r a c t A sprocket is a toothed wheel, commonly used in drive systems, to which the strength and wear resistance of the teeth are important. Sprockets are conventionally fashioned by hobbing, followed by heat treatment. However, the fine-blanking process has recently seen increasing use by sprocket manufacturers. The process of fine-blanking has the possibility of reducing the number of process operations, thus reducing production time and cost, as well as improving part quality and process repeatability. Because of the severe plastic deformation in fine-blanking process, the strength, hardness and wear resistance of parts can be improved. In this work, the surface hardness and wear resistance of a fineblanked sprocket are compared with those of a sprocket made using the hobbing process. The source of the wear resistance improvement was identified via examination of the microstructure. The microstructure of the fine-blanked sprocket revealed an increasingly compressed and elongated grain structure, in which grain flow and orientation resulted in pronounced hardening across the tooth width. The wear resistance of the fine-blanked sprocket, as measured by the distance between the teeth and the radius at the tooth bottom, was greater than that of the hobbed and heat-treated sprocket. Based on the results, the material cost of the sprocket could be reduced by using low carbon steel (SS400) instead of medium carbon steel (S50C), and further savings in production time would be realized by eliminating the need for subsequent heat treatment. © 2011 Elsevier B.V. All rights reserved.
1. Introduction A sprocket is a profiled wheel with teeth used in the drive systems of machinery and equipment, such as bicycles, motorcycles, tanks, movie projectors, and printers. Sprockets are mainly used for the transmission of rotary motion between two shafts. Fig. 1 shows an example of a rear sprocket for a motorcycle. The strength and wear resistance of a sprocket are important concerns over its lifetime. Therefore, sprockets are conventionally fabricated using the hobbing process to form the teeth, followed by heat treating to improve the hardness. This sprocket fabrication process is time consuming and results in high production costs. Much research has been done to improve sprocket fabrication efficiency as well as to develop a process for more complicated sprocket shapes. Liu et al. investigated the effect of geometrical parameters, such as cutout length and height, inner diameter and the number of teeth, on the distortion of S45C mid-carbon steel sprockets using numerical simulations during the heat-treatment process [1]. Takagi et al. developed a CNC multilevel compacting press designed to produce complicated shapes and achieve tight dimensional tolerances without a sizing operation for housing sprockets [2]. Fine-blanking
∗ Tel.: +66 2 4709218; fax: +66 2 8729080. E-mail address: [email protected] 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.12.015
technology has been applied to sprocket manufacturing to improve hardness and wear resistance [3,4]. With the advantages of the fine-blanking process, a smooth clean-cut surface across an entire thickness could be achieved, and the production cost and time could be reduced. In addition, the severe plastic deformation that takes place under cold working conditions caused changes in the material properties of the parts, especially hardness properties [4–8]. However, the changes in wear resistance in the fine-blanked parts appear to only have been investigated in commercial studies [9]. Wear resistance is an important system property that is related to sliding forces applied to the surface of parts, as is seen in sprocket use. Therefore, it is important to consider the effects of the fine-blanking process on wear resistance when considering the application of this process to sprocket manufacturing. In order to replace the conventional sprocket manufacturing process with the fine-blanking process, it be must be shown that the wear resistance of fine-blanked sprockets is greater than that of the conventionally manufactured sprockets. Although the fine-blanking process is currently used to fabricate sprockets, the effects on wear resistance and its underlying mechanism have not been clearly investigated or explained. In this study, the wear resistance of the fine-blanked sprocket was compared with that of the commercial sprocket, and the improved wear resistance observed in the fine-blanked sprocket was explained by examination of the microstructure of the material.
S. Thipprakmas / Wear 271 (2011) 2396–2401 Table 1 Chemical compositions of SS400.
Nomenclature Cl FB FC t
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Alloys %
clearance blankholder force counterpunch force material thickness
C 0.066
Si 0.207
Mn 0.360
P 0.008
S 0.003
Mn 0.572
P 0.028
S 0.023
Table 2 Chemical compositions of S50C. Alloys %
Fig. 1. An example of rear sprocket for motorcycles.
2. Principle of the fine-blanking process The fine-blanking process is an advanced and precise blanking process by which a cut surface with exact geometry and smooth, crack-free cut surface can be created. Fig. 2 outlines the principle of the fine-blanking process. The fine-blanking principle is based on the application of a hydrostatic pressure on the work piece through the use of a high blankholder force (FB ) and a counterpunch force (FC ). The V-ring indenter is also formed on the blankholder and sometimes on the die in order to firmly tighten the work piece before the blanking operation. In addition, the blanking clearance (Cl) of the fine-blanking process is approximately ten times less than that of the conventional blanking process. With these special features, the plastic deformation generated in the work piece is severely increased within the shearing zone, resulting in changes to the material properties, particularly on the cut surface. 3. Experimental procedure The sprocket investigated in this study was the rear sprocket for a motorcycle (model 428-36T) with 36 teeth and a thickness
C 0.491
Si 0.287
of 7 mm. Low carbon steel SS400 (JIS) was used for the fineblanked sprocket and the hobbed sprocket. A commercial rear sprocket made of medium carbon steel S50C (JIS) with induction heat-treatment applied after the hobbing process was completed was also investigated. The chemical compositions of SS400 and S50C are listed in Tables 1 and 2, respectively. An 800-ton, fineblanking press machine with a blankholder force of 1500 kN and counterpunch force of 1000 kN was used for the experiments. The blanking clearance was set to 0.5% of material thickness (t), and the tool-cutting edge radii were set at 0.01 mm and 0.5 mm on punch and die, respectively. The V-ring indenter height was 0.7 mm and 1.0 mm formed on the blankholder and the die, respectively, with a 90◦ angle, and located 3 mm from the punch side. The SS400 fine-blanked and hobbed sprockets were sectioned and further processed by subsequent mounting, polishing, and etching with 3% nitric acid solution. Optical microscopy was used to observe and capture microstructure images for microscopic examinations. In addition, the surface hardness was measured using the Vickers microhardness test method. In this study, two samples, the fineblanked sprocket SS400 and the commercial sprocket S50C, were investigated. The surface hardness was measured at six observation points on each sprocket and measured across the tooth width every 1 mm at each observation point. The driving test (conditions listed in Table 3) was performed to investigate the wear resistance on both the fine-blanked sprocket SS400 and the commercial sprocket S50C. In this study, no lubrication was applied (dry condition) in order to reduce the testing time. The counterface material surface hardness and roughness were examined as listed in Table 3. Again, the fine-blanked sprocket SS400 and commercial sprocket S50C were tested for wear resistance. The twelve observation points on each sprocket were inspected for wear formation. The wear at the bottom radius and the distance between teeth, as shown in Fig. 3, were inspected. Concerning the sprocket lifetime, due to the wear, formation was not uniform across the tooth width, especially for the fine-blanked sprocket SS400; therefore, the least wear formation over the tooth width was considered. The least wear was calculated by creating a shadow and measuring it with a circle comparator.
Fig. 2. Principle of the fine-blanking process.
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S. Thipprakmas / Wear 271 (2011) 2396–2401
Table 3 Driving test conditions. Wear mode
Sliding mode
Counterface material surface roughness
S50C: 1.04 m SS400: 0.68 m Chain: 0.24 m
Counterface material surface hardness
S50C: 310 HV SS400: 225 HV (face surface), 440 HV (back surface) Chain: 659 HV
Testing time Lubricant
40 h Dry
Driving system
Drive: 17T/1900 rpm × 36T Chain tension: 1.47 kN Chain of links: 116 RE Sliding distance: 342 km Sliding speed: 2.37 m/s Sliding distance per tooth: 9.5 km
4. Experimental results and discussion 4.1. Comparison of SS400 microstructure between hobbed and fine-blanked sprockets Fig. 4 shows the comparison of the microstructure for material SS400 on a tooth from both the hobbed and fine-blanked sprockets. With the hobbing process, the sprocket teeth were formed by cutting and removing the unwanted material. This cutting mechanism did not result in compression or elongation of the microstructure; thus, the microstructure was unchanged across the material thickness, as shown in Fig. 4(a). On the other hand, the special characteristic features of the fine-blanking process resulted in severe plastic deformation within the shearing zone [10–14]. This resulted
Fig. 3. Inspection point of wear on sprocket teeth.
in a more compressed and elongated grain structure along the contributed grain flow, as shown in Fig. 4(b). It was observed that the compressed and elongated grain structure increased in the through-thickness direction. 4.2. Comparison of SS400 surface hardness across the tooth width between hobbed and fine-blanked sprockets To investigate the changes in material properties on the hobbed and fine-blanked sprockets, the surface hardness was measured. The surface hardness value of the initial work piece, SS400, was approximately 130 HV. With the different contributed grain flow, the material properties, especially surface hardness, differed between the hobbed and fine-blanked sprockets. Fig. 5 shows the comparison of surface hardness across the tooth width between the hobbed and fine-blanked sprockets. The 0 corresponds to the counterpunch side (face surface), and the 7 corresponds to the punch side (back surface). In the case of the hobbed sprocket, the grain structure was not compressed or elongated, the surface hardness was approximately constant over the material thickness, and its surface hardness value was unchanged from that of the initial work piece. In contrast, the fine-blanked sprocket had a compressed
Fig. 4. Comparison of SS400 microstructure on the tooth between the hobbed and fine-blanked sprockets.
S. Thipprakmas / Wear 271 (2011) 2396–2401
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Fig. 5. Comparison of SS400 surface hardness across the tooth width between the hobbed and fine-blanked sprockets.
and elongated grain structure along the contributed grain flow that increased in the through-thickness direction, and the surface hardness increased sharply in the through-thickness direction.
4.3. Comparison of wear resistance between the commercial sprocket S50C and the fine-blanked sprocket SS400 The most important property considered in sprocket fabrication is wear resistance. Commercial sprockets are usually fabricated using medium carbon steel S50C, for which the induction heattreatment can be applied after tooth formation from the hobbing process. To investigate the possibility of replacing conventional sprocket fabrication with the fine-blanking process, the commercial sprocket S50C was compared with the fine-blanked sprocket SS400 in terms of wear resistance as well as surface hardness. The microstructure and surface hardness of the commercial sprocket S50C were examined. The induction heat-treatment to the surface caused increased surface hardness compared with that of the initial surface hardness of S50C. After the induction heat-treatment, a surface hardness of approximately 310 HV was observed. The microstructure was ferrite and pearlite as shown in Fig. 6. Fig. 7 shows the comparison of surface hardness across the tooth width between the commercial sprocket S50C and the fine-blanked sprocket SS400. In the case of the commercial sprocket S50C, the surface hardness was approximately 310 HV. This result could be explained by the hobbing process used to fabricate the commercial sprocket S50C causing no compressed and elongated grain structure along the contributed grain flow. However, the surface hardness across the tooth width was higher compared with the initial S50C, due to the induction heat-treatment process. In the case of the fine-blanked SS400, although there was no induction heat-treatment process, the surface hardness was increased by the effects of the compressed and elongated grain structure along the contributed grain flow. As shown in Fig. 7, the surface hardness of fine-blanked sprocket SS400 was slightly lower than that of com-
Fig. 6. Microstructure of commercial sprocket S50C.
mercial sprocket S50C from the face surface to approximately 40% t depth. However, over the 40% t depth, the surface hardness of the fine-blanked sprocket SS400 was increasingly higher than that of the commercial sprocket S50C. This change in material properties resulted in a non-uniform wear formation across the tooth width. Therefore, the least amount of wear was formed on the back surface side, where the surface hardness was largest. Fig. 8 shows the comparison of wear resistance between the commercial sprocket S50C and the fine-blanked sprocket SS400. Due to the variation in the surface hardness along the tooth width, as shown in Fig. 7, the wear on the back surface side (larger surface hardness side), where the least wear was formed, was inspected. Fig. 8(a) shows the increase in distance between teeth due to wear and Fig. 8(b) shows the increase in bottom radius due to wear. These results showed that the wear increased as the driving test time
Fig. 7. Comparison of surface hardness across the tooth width between the commercial sprocket S50C and the fine-blanked sprocket SS400.
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S. Thipprakmas / Wear 271 (2011) 2396–2401
Wear between teeth (mm)
0.400 0.350
Fine-blanked sprocket SS400
0.300
Hobbed sprocket S50C
0.250 0.200 0.150 0.100 0.050 0.000 0
8
16
24
32
40
48
Testing time (Hrs.) (a) Distance between teeth Fig. 9. Comparison of a single pair of tooth profiles for the commercial sprocket S50C before and after testing.
Wear at bottom radius (mm)
0.140 0.120
Fine-blanked sprocket SS400
0.100
5. Conclusions
Hobbed sprocket S50C
0.080 0.060 0.040 0.020 0.000 0
8
16
24
32
40
48
Testing time (Hrs.) (b) Bottom radius Fig. 8. Comparison of the wear between the commercial sprocket S50C and the fine-blanked SS400.
increased. It was observed that the wear resistance of the fineblanked sprocket SS400 was better than that of the commercial sprocket S50C. Specifically, the increase in distance between teeth of the fine-blanked sprocket SS400 was approximately half that of the commercial sprocket S50C. The increase in bottom radius of the fine-blanked sprocket SS400 was approximately 3 times smaller than that of the commercial sprocket S50C. In addition, to investigate the effects of variation in surface hardness along the tooth width, the wear formation, with a material thickness of approximately 3.5 mm, was examined. The sprocket was cut using the wire-EDM, and the wear was measured. With the material thickness of 3.5 mm, the increase in distance between teeth and the increase in bottom radius were approximately 0.315 mm and 0.092 mm, respectively. These results confirmed that the wear increased as the surface hardness decreased. After the driving test, the surface hardness was also examined on both sprockets. The results showed that due to work hardening, the surface hardness increased slightly in both cases. Specifically, the surface hardness on the back surface of the commercial sprocket S50C and the fineblanked sprocket SS400 were approximately 332 HV and 468 HV, respectively. Fig. 9 shows an example of a tooth profile comparison for the commercial sprocket S50C before and after testing. As the results show, the special characteristic features in the fineblanking process resulted in an increase in surface hardness. An increase in surface hardness to a level significantly higher than that of the commercial sprocket S50C, leading to a wear resistance for the fine-blanked sprocket SS400 that was better than that of the commercial sprocket S50C. Moreover, based on these results, with application of the fine-blanking process, the low carbon steel SS400 could be used instead of the medium carbon steel S50C, reducing the material cost.
With the advantages of the fine-blanking process, a smooth clean-cut surface across the entire thickness could be produced; this process could be applied by the manufacturer, resulting in a reduction in production costs and time. In this study, the wear resistance of the fine-blanked sprocket was investigated because it is considered to be important for sprocket lifetime. The changes in material properties were also explained as changes in the microstructure of the material. With the special characteristic features of the fine-blanking process, a compressed and elongated grain structure along the contributed grain flow was generated within the shearing zone, resulting in an increase in surface hardness. By contrast, this compressed and elongated grain structure feature was not generated in the sprocket fabricated by the hobbing process; therefore, the additional induction of heat-treatment was necessary to increase the surface hardness. The increase in surface hardness resulted in an improvement in wear resistance. This increase in surface hardness and improved wear resistance for the fine-blanked sprocket SS400 was better than that of the commercial sprocket S50C. Therefore, the application of the fine-blanking process on sprocket fabrication not only reduced production costs and time, but also improved the surface hardness and wear resistance of the sprocket. The material cost was also reduced by using low carbon steel SS400 instead of medium carbon steel S50C. Acknowledgements The presented research is partially supported by a grant from the Thai Research Fund (TRF) under grant No. TRF-MRG5380117, the National University Research Project of Thailand (NRU Project), and the Center of Excellence in Sheet and Bulk Metal Forming Technology (SBMFT). The author would like to express gratitude to Mr. Komdech Vichitjarusgul, Diamond Dimension Co., Ltd., for his support for the experiments. The author also thanks Miss Wasana Thongthing (undergraduate student) and Miss Wiriyakorn Phanitwong (graduate student) for their assistance to this study. References [1] P. Liu, Y.Y. Wang, J. Li, C. Lu, K.P. Quek, G.R. Liu, Parametric study of a sprocket system during heat-treatment process, Finite Elem. Anal. Des. 40 (2003) 25–40. [2] M. Takagi, K. Suganaga, T. Nagata, New PM sprocket meets auto cost, performance concerns, Met. Powder Rep. 64 (2009) 25–29. [3] Y.H Seo, B.K. Kim, H.D. Son, Application of fine-blanking to the manufacture of a sprocket with stainless steel sheet, Key Eng. Mater. 261–263 (2004) 1665–1670. [4] S. Thipprakmas, C. Chanchay, N. Hanwach, W. Wongjan, K. Vichitjarusgul, Investigation on the increasing material hardness on fineblanked sprocket, Adv. Mater. Res. 83–86 (2010) 1099–1106.
S. Thipprakmas / Wear 271 (2011) 2396–2401 [5] S.S.M. Tavares, M.R. Silva, J.M. Pardal, H.F.G. Abreu, A.M. Gomes, Microstructural changes produced by plastic deformation in the UNS S31803 duplex stainless steel, J. Mater. Proc. Technol. 180 (2006) 318–322. [6] F.O. Sonmez, A. Demir, Analytical relations between hardness and strain for cold formed parts, J. Mater. Proc. Technol. 186 (2007) 163–173. [7] W. Zhang, Z. Luo, W. Xia, Y. Li, Effect of plastic deformation on microstructure and hardness of AlSi/Al gradient composites, Trans. Nonferrous Met. Soc. China 17 (2007) 1186–1193. [8] M. Milad, N. Zreiba, F. Elhalouani, C. Baradai, The effect of cold work on structure and properties of AISI 304 stainless steel, J. Mater. Proc. Technol. 203 (2008) 80–85. [9] Feintool Training: Module BT/2—Introduction into the Fineblanking Technology, Feintool Technologie AG Lyss, Switzerland, 2003.
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[10] S. Thipprakmas, M. Jin, M. Murakawa, An investigation of material flow analysis in fineblanking process, J. Mater. Proc. Technol. 192–193 (2007) 237–242. [11] S. Thipprakmas, M. Jin, K. Tomokazu, Y. Katsuhiro, M. Murakawa, Prediction of fineblanked surface characteristics using the finite element method (FEM), J. Mater. Proc. Technol. 198 (2008) 391–398. [12] S. Thipprakmas, Finite-element analysis of V-ring indenter mechanism in fineblanking process, J. Mater. Des. 30 (2009) 526–531. [13] S. Thipprakmas, M. Jin, Investigation mechanism of V-ring indenter geometry in fine-blanking process, Key Eng. Mater. 410–411 (2009) 305–312. [14] S. Yiemchaiyaphum, M. Jin, S. Thipprakmas, Die design in fine-piercing process by chamfering cutting edge, Key Eng. Mater. 443 (2010) 219–224.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Short communication
Improving wear resistance of sprocket parts using a fine-blanking process S. Thipprakmas ∗ King Mongkut’s University of Technology Thonburi, 126 Prachautid, Bangmod, Thungkru, Bangkok, Thailand
a r t i c l e
i n f o
Article history: Received 1 September 2010 Received in revised form 22 December 2010 Accepted 22 December 2010
Keywords: Wear Sprocket Fine-blanking Hobbing Hardness
a b s t r a c t A sprocket is a toothed wheel, commonly used in drive systems, to which the strength and wear resistance of the teeth are important. Sprockets are conventionally fashioned by hobbing, followed by heat treatment. However, the fine-blanking process has recently seen increasing use by sprocket manufacturers. The process of fine-blanking has the possibility of reducing the number of process operations, thus reducing production time and cost, as well as improving part quality and process repeatability. Because of the severe plastic deformation in fine-blanking process, the strength, hardness and wear resistance of parts can be improved. In this work, the surface hardness and wear resistance of a fineblanked sprocket are compared with those of a sprocket made using the hobbing process. The source of the wear resistance improvement was identified via examination of the microstructure. The microstructure of the fine-blanked sprocket revealed an increasingly compressed and elongated grain structure, in which grain flow and orientation resulted in pronounced hardening across the tooth width. The wear resistance of the fine-blanked sprocket, as measured by the distance between the teeth and the radius at the tooth bottom, was greater than that of the hobbed and heat-treated sprocket. Based on the results, the material cost of the sprocket could be reduced by using low carbon steel (SS400) instead of medium carbon steel (S50C), and further savings in production time would be realized by eliminating the need for subsequent heat treatment. © 2011 Elsevier B.V. All rights reserved.
1. Introduction A sprocket is a profiled wheel with teeth used in the drive systems of machinery and equipment, such as bicycles, motorcycles, tanks, movie projectors, and printers. Sprockets are mainly used for the transmission of rotary motion between two shafts. Fig. 1 shows an example of a rear sprocket for a motorcycle. The strength and wear resistance of a sprocket are important concerns over its lifetime. Therefore, sprockets are conventionally fabricated using the hobbing process to form the teeth, followed by heat treating to improve the hardness. This sprocket fabrication process is time consuming and results in high production costs. Much research has been done to improve sprocket fabrication efficiency as well as to develop a process for more complicated sprocket shapes. Liu et al. investigated the effect of geometrical parameters, such as cutout length and height, inner diameter and the number of teeth, on the distortion of S45C mid-carbon steel sprockets using numerical simulations during the heat-treatment process [1]. Takagi et al. developed a CNC multilevel compacting press designed to produce complicated shapes and achieve tight dimensional tolerances without a sizing operation for housing sprockets [2]. Fine-blanking
∗ Tel.: +66 2 4709218; fax: +66 2 8729080. E-mail address: [email protected] 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.12.015
technology has been applied to sprocket manufacturing to improve hardness and wear resistance [3,4]. With the advantages of the fine-blanking process, a smooth clean-cut surface across an entire thickness could be achieved, and the production cost and time could be reduced. In addition, the severe plastic deformation that takes place under cold working conditions caused changes in the material properties of the parts, especially hardness properties [4–8]. However, the changes in wear resistance in the fine-blanked parts appear to only have been investigated in commercial studies [9]. Wear resistance is an important system property that is related to sliding forces applied to the surface of parts, as is seen in sprocket use. Therefore, it is important to consider the effects of the fine-blanking process on wear resistance when considering the application of this process to sprocket manufacturing. In order to replace the conventional sprocket manufacturing process with the fine-blanking process, it be must be shown that the wear resistance of fine-blanked sprockets is greater than that of the conventionally manufactured sprockets. Although the fine-blanking process is currently used to fabricate sprockets, the effects on wear resistance and its underlying mechanism have not been clearly investigated or explained. In this study, the wear resistance of the fine-blanked sprocket was compared with that of the commercial sprocket, and the improved wear resistance observed in the fine-blanked sprocket was explained by examination of the microstructure of the material.
S. Thipprakmas / Wear 271 (2011) 2396–2401 Table 1 Chemical compositions of SS400.
Nomenclature Cl FB FC t
2397
Alloys %
clearance blankholder force counterpunch force material thickness
C 0.066
Si 0.207
Mn 0.360
P 0.008
S 0.003
Mn 0.572
P 0.028
S 0.023
Table 2 Chemical compositions of S50C. Alloys %
Fig. 1. An example of rear sprocket for motorcycles.
2. Principle of the fine-blanking process The fine-blanking process is an advanced and precise blanking process by which a cut surface with exact geometry and smooth, crack-free cut surface can be created. Fig. 2 outlines the principle of the fine-blanking process. The fine-blanking principle is based on the application of a hydrostatic pressure on the work piece through the use of a high blankholder force (FB ) and a counterpunch force (FC ). The V-ring indenter is also formed on the blankholder and sometimes on the die in order to firmly tighten the work piece before the blanking operation. In addition, the blanking clearance (Cl) of the fine-blanking process is approximately ten times less than that of the conventional blanking process. With these special features, the plastic deformation generated in the work piece is severely increased within the shearing zone, resulting in changes to the material properties, particularly on the cut surface. 3. Experimental procedure The sprocket investigated in this study was the rear sprocket for a motorcycle (model 428-36T) with 36 teeth and a thickness
C 0.491
Si 0.287
of 7 mm. Low carbon steel SS400 (JIS) was used for the fineblanked sprocket and the hobbed sprocket. A commercial rear sprocket made of medium carbon steel S50C (JIS) with induction heat-treatment applied after the hobbing process was completed was also investigated. The chemical compositions of SS400 and S50C are listed in Tables 1 and 2, respectively. An 800-ton, fineblanking press machine with a blankholder force of 1500 kN and counterpunch force of 1000 kN was used for the experiments. The blanking clearance was set to 0.5% of material thickness (t), and the tool-cutting edge radii were set at 0.01 mm and 0.5 mm on punch and die, respectively. The V-ring indenter height was 0.7 mm and 1.0 mm formed on the blankholder and the die, respectively, with a 90◦ angle, and located 3 mm from the punch side. The SS400 fine-blanked and hobbed sprockets were sectioned and further processed by subsequent mounting, polishing, and etching with 3% nitric acid solution. Optical microscopy was used to observe and capture microstructure images for microscopic examinations. In addition, the surface hardness was measured using the Vickers microhardness test method. In this study, two samples, the fineblanked sprocket SS400 and the commercial sprocket S50C, were investigated. The surface hardness was measured at six observation points on each sprocket and measured across the tooth width every 1 mm at each observation point. The driving test (conditions listed in Table 3) was performed to investigate the wear resistance on both the fine-blanked sprocket SS400 and the commercial sprocket S50C. In this study, no lubrication was applied (dry condition) in order to reduce the testing time. The counterface material surface hardness and roughness were examined as listed in Table 3. Again, the fine-blanked sprocket SS400 and commercial sprocket S50C were tested for wear resistance. The twelve observation points on each sprocket were inspected for wear formation. The wear at the bottom radius and the distance between teeth, as shown in Fig. 3, were inspected. Concerning the sprocket lifetime, due to the wear, formation was not uniform across the tooth width, especially for the fine-blanked sprocket SS400; therefore, the least wear formation over the tooth width was considered. The least wear was calculated by creating a shadow and measuring it with a circle comparator.
Fig. 2. Principle of the fine-blanking process.
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S. Thipprakmas / Wear 271 (2011) 2396–2401
Table 3 Driving test conditions. Wear mode
Sliding mode
Counterface material surface roughness
S50C: 1.04 m SS400: 0.68 m Chain: 0.24 m
Counterface material surface hardness
S50C: 310 HV SS400: 225 HV (face surface), 440 HV (back surface) Chain: 659 HV
Testing time Lubricant
40 h Dry
Driving system
Drive: 17T/1900 rpm × 36T Chain tension: 1.47 kN Chain of links: 116 RE Sliding distance: 342 km Sliding speed: 2.37 m/s Sliding distance per tooth: 9.5 km
4. Experimental results and discussion 4.1. Comparison of SS400 microstructure between hobbed and fine-blanked sprockets Fig. 4 shows the comparison of the microstructure for material SS400 on a tooth from both the hobbed and fine-blanked sprockets. With the hobbing process, the sprocket teeth were formed by cutting and removing the unwanted material. This cutting mechanism did not result in compression or elongation of the microstructure; thus, the microstructure was unchanged across the material thickness, as shown in Fig. 4(a). On the other hand, the special characteristic features of the fine-blanking process resulted in severe plastic deformation within the shearing zone [10–14]. This resulted
Fig. 3. Inspection point of wear on sprocket teeth.
in a more compressed and elongated grain structure along the contributed grain flow, as shown in Fig. 4(b). It was observed that the compressed and elongated grain structure increased in the through-thickness direction. 4.2. Comparison of SS400 surface hardness across the tooth width between hobbed and fine-blanked sprockets To investigate the changes in material properties on the hobbed and fine-blanked sprockets, the surface hardness was measured. The surface hardness value of the initial work piece, SS400, was approximately 130 HV. With the different contributed grain flow, the material properties, especially surface hardness, differed between the hobbed and fine-blanked sprockets. Fig. 5 shows the comparison of surface hardness across the tooth width between the hobbed and fine-blanked sprockets. The 0 corresponds to the counterpunch side (face surface), and the 7 corresponds to the punch side (back surface). In the case of the hobbed sprocket, the grain structure was not compressed or elongated, the surface hardness was approximately constant over the material thickness, and its surface hardness value was unchanged from that of the initial work piece. In contrast, the fine-blanked sprocket had a compressed
Fig. 4. Comparison of SS400 microstructure on the tooth between the hobbed and fine-blanked sprockets.
S. Thipprakmas / Wear 271 (2011) 2396–2401
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Fig. 5. Comparison of SS400 surface hardness across the tooth width between the hobbed and fine-blanked sprockets.
and elongated grain structure along the contributed grain flow that increased in the through-thickness direction, and the surface hardness increased sharply in the through-thickness direction.
4.3. Comparison of wear resistance between the commercial sprocket S50C and the fine-blanked sprocket SS400 The most important property considered in sprocket fabrication is wear resistance. Commercial sprockets are usually fabricated using medium carbon steel S50C, for which the induction heattreatment can be applied after tooth formation from the hobbing process. To investigate the possibility of replacing conventional sprocket fabrication with the fine-blanking process, the commercial sprocket S50C was compared with the fine-blanked sprocket SS400 in terms of wear resistance as well as surface hardness. The microstructure and surface hardness of the commercial sprocket S50C were examined. The induction heat-treatment to the surface caused increased surface hardness compared with that of the initial surface hardness of S50C. After the induction heat-treatment, a surface hardness of approximately 310 HV was observed. The microstructure was ferrite and pearlite as shown in Fig. 6. Fig. 7 shows the comparison of surface hardness across the tooth width between the commercial sprocket S50C and the fine-blanked sprocket SS400. In the case of the commercial sprocket S50C, the surface hardness was approximately 310 HV. This result could be explained by the hobbing process used to fabricate the commercial sprocket S50C causing no compressed and elongated grain structure along the contributed grain flow. However, the surface hardness across the tooth width was higher compared with the initial S50C, due to the induction heat-treatment process. In the case of the fine-blanked SS400, although there was no induction heat-treatment process, the surface hardness was increased by the effects of the compressed and elongated grain structure along the contributed grain flow. As shown in Fig. 7, the surface hardness of fine-blanked sprocket SS400 was slightly lower than that of com-
Fig. 6. Microstructure of commercial sprocket S50C.
mercial sprocket S50C from the face surface to approximately 40% t depth. However, over the 40% t depth, the surface hardness of the fine-blanked sprocket SS400 was increasingly higher than that of the commercial sprocket S50C. This change in material properties resulted in a non-uniform wear formation across the tooth width. Therefore, the least amount of wear was formed on the back surface side, where the surface hardness was largest. Fig. 8 shows the comparison of wear resistance between the commercial sprocket S50C and the fine-blanked sprocket SS400. Due to the variation in the surface hardness along the tooth width, as shown in Fig. 7, the wear on the back surface side (larger surface hardness side), where the least wear was formed, was inspected. Fig. 8(a) shows the increase in distance between teeth due to wear and Fig. 8(b) shows the increase in bottom radius due to wear. These results showed that the wear increased as the driving test time
Fig. 7. Comparison of surface hardness across the tooth width between the commercial sprocket S50C and the fine-blanked sprocket SS400.
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Wear between teeth (mm)
0.400 0.350
Fine-blanked sprocket SS400
0.300
Hobbed sprocket S50C
0.250 0.200 0.150 0.100 0.050 0.000 0
8
16
24
32
40
48
Testing time (Hrs.) (a) Distance between teeth Fig. 9. Comparison of a single pair of tooth profiles for the commercial sprocket S50C before and after testing.
Wear at bottom radius (mm)
0.140 0.120
Fine-blanked sprocket SS400
0.100
5. Conclusions
Hobbed sprocket S50C
0.080 0.060 0.040 0.020 0.000 0
8
16
24
32
40
48
Testing time (Hrs.) (b) Bottom radius Fig. 8. Comparison of the wear between the commercial sprocket S50C and the fine-blanked SS400.
increased. It was observed that the wear resistance of the fineblanked sprocket SS400 was better than that of the commercial sprocket S50C. Specifically, the increase in distance between teeth of the fine-blanked sprocket SS400 was approximately half that of the commercial sprocket S50C. The increase in bottom radius of the fine-blanked sprocket SS400 was approximately 3 times smaller than that of the commercial sprocket S50C. In addition, to investigate the effects of variation in surface hardness along the tooth width, the wear formation, with a material thickness of approximately 3.5 mm, was examined. The sprocket was cut using the wire-EDM, and the wear was measured. With the material thickness of 3.5 mm, the increase in distance between teeth and the increase in bottom radius were approximately 0.315 mm and 0.092 mm, respectively. These results confirmed that the wear increased as the surface hardness decreased. After the driving test, the surface hardness was also examined on both sprockets. The results showed that due to work hardening, the surface hardness increased slightly in both cases. Specifically, the surface hardness on the back surface of the commercial sprocket S50C and the fineblanked sprocket SS400 were approximately 332 HV and 468 HV, respectively. Fig. 9 shows an example of a tooth profile comparison for the commercial sprocket S50C before and after testing. As the results show, the special characteristic features in the fineblanking process resulted in an increase in surface hardness. An increase in surface hardness to a level significantly higher than that of the commercial sprocket S50C, leading to a wear resistance for the fine-blanked sprocket SS400 that was better than that of the commercial sprocket S50C. Moreover, based on these results, with application of the fine-blanking process, the low carbon steel SS400 could be used instead of the medium carbon steel S50C, reducing the material cost.
With the advantages of the fine-blanking process, a smooth clean-cut surface across the entire thickness could be produced; this process could be applied by the manufacturer, resulting in a reduction in production costs and time. In this study, the wear resistance of the fine-blanked sprocket was investigated because it is considered to be important for sprocket lifetime. The changes in material properties were also explained as changes in the microstructure of the material. With the special characteristic features of the fine-blanking process, a compressed and elongated grain structure along the contributed grain flow was generated within the shearing zone, resulting in an increase in surface hardness. By contrast, this compressed and elongated grain structure feature was not generated in the sprocket fabricated by the hobbing process; therefore, the additional induction of heat-treatment was necessary to increase the surface hardness. The increase in surface hardness resulted in an improvement in wear resistance. This increase in surface hardness and improved wear resistance for the fine-blanked sprocket SS400 was better than that of the commercial sprocket S50C. Therefore, the application of the fine-blanking process on sprocket fabrication not only reduced production costs and time, but also improved the surface hardness and wear resistance of the sprocket. The material cost was also reduced by using low carbon steel SS400 instead of medium carbon steel S50C. Acknowledgements The presented research is partially supported by a grant from the Thai Research Fund (TRF) under grant No. TRF-MRG5380117, the National University Research Project of Thailand (NRU Project), and the Center of Excellence in Sheet and Bulk Metal Forming Technology (SBMFT). The author would like to express gratitude to Mr. Komdech Vichitjarusgul, Diamond Dimension Co., Ltd., for his support for the experiments. The author also thanks Miss Wasana Thongthing (undergraduate student) and Miss Wiriyakorn Phanitwong (graduate student) for their assistance to this study. References [1] P. Liu, Y.Y. Wang, J. Li, C. Lu, K.P. Quek, G.R. Liu, Parametric study of a sprocket system during heat-treatment process, Finite Elem. Anal. Des. 40 (2003) 25–40. [2] M. Takagi, K. Suganaga, T. Nagata, New PM sprocket meets auto cost, performance concerns, Met. Powder Rep. 64 (2009) 25–29. [3] Y.H Seo, B.K. Kim, H.D. Son, Application of fine-blanking to the manufacture of a sprocket with stainless steel sheet, Key Eng. Mater. 261–263 (2004) 1665–1670. [4] S. Thipprakmas, C. Chanchay, N. Hanwach, W. Wongjan, K. Vichitjarusgul, Investigation on the increasing material hardness on fineblanked sprocket, Adv. Mater. Res. 83–86 (2010) 1099–1106.
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