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Evaluation of tribological properties of bearing materials for marine diesel engines utilizing acoustic emission technique
Tribology International 46 (2012) 183–189
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Evaluation of tribological properties of bearing materials for marine diesel engines utilizing acoustic emission technique Mari Nagata a,n, Masahito Fujita a, Motohira Yamada b, Tatsumi Kitahara c a
Research and Development Division, Daido Metal Co. Ltd., Japan Department of Application Engineering, Technological Division, Daido Metal Co. Ltd., Japan c Department of Mechanical Engineering Science, Kyushu University, Japan b
a r t i c l e i n f o
abstract
Article history: Received 14 October 2010 Received in revised form 18 May 2011 Accepted 20 May 2011 Available online 1 June 2011
Acoustic Emission (AE) technique, which has detection capability for minute failures, has been tried to monitor the condition of a plain bearing under the laboratory conditions. In this paper, the bearing materials for marine diesel engines – tin alloy as known as ‘‘white metal’’, aluminum alloy of 40% tin mass and aluminum alloy 40% tin mass with resin overlay – were tested using a sleeve-to-plate tribotester. The frictional force and back temperature were measured as well as the AE signals. The possibility of AE technique to monitor the bearing condition was also assessed by evaluating tribological properties under different operating conditions such as start–stop simulating the crankshaft turning during engine assembly and seizure tests. These results indicate that AE is useful for monitoring the lubricated condition of the sliding surfaces and evaluating tribological properties of the bearing. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Acoustic Emission (AE) Plain bearing material Resin overlay Sleeve-to-plate tribo-tester
1. Introduction Acoustic Emission (AE) is defined as radiation of mechanical elastic waves produced by a material due to the dynamic local rearrangement of its internal structure [1]. Therefore, AE technique can be used to detect minute failures. It has been studied for the evaluation of tribological properties such as to observe wear and friction processes [2–4], detection of the ball bearing fatigue [5–7] and monitoring of plain bearings in the laboratory—journal bearings for rotating machinery such as a steam turbine and generator [8] and tilting pad thrust bearings for a hydroelectric water turbine [9] or sleeve bearings for a marine engine. As for marine engines, the monitoring technique for bearing failure has been researched, i.e., measurement of bearing temperature, analysis of oil ferrography and analysis of shaft vibration [10]. However, it is not easy to detect bearing failures in the early stage, hence the studies of using AE to monitor marine engine bearings have been carried out [10–12]. In low speed two-stroke diesel engines, the crankshaft is rotated at low speed with repeated starts and stops when an engine is assembled. Sometimes the lubricant on the bearing surfaces can be lacking during crankshaft turning and the bearing surfaces might be damaged at the moment, resulting in bearing failure in service. As the bearings work in a severe state in service, the required bearing properties are seizure resistance, wear resistance, fatigue
n
Corresponding author. E-mail address: [email protected] (M. Nagata).
0301-679X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2011.05.026
resistance and conformability. A bearing material is selected according to operating conditions, i.e., bearing load and sliding velocity. Therefore, it is important to find the correlation between the tribological properties of the bearing materials and monitoring technique. In this investigation, the bearing materials for marine diesel engines – tin alloy as known as ‘‘white metal’’, aluminum alloy of 40% tin mass and aluminum alloy 40% tin mass with resin overlay – were evaluated. White metal has been commonly used as a bearing alloy, and aluminum alloy of 40% tin mass has been successfully applied to plain bearings for large size two-stroke diesel engines that are subjected to severe conditions such as higher loads [13]. The resin overlay was developed with the aim of preventing scoring and seizure of the bearing alloy at low speed during crankshaft turning when the engine is assembled [14]. The objective of this investigation was to assess the possibility of AE technique to monitor the bearing condition of marine engines with the above different materials, using a sleeveto-plate tribo-tester, and to confirm the correlation between the frictional force, back temperature and the AE signals.
2. Experimental 2.1. Test rig Fig. 1 shows a schematic view of a sleeve-to-plate tribo-tester that can measure AE signals as well as the frictional force and back temperature. A test sample with an inner diameter of 22 mm
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M. Nagata et al. / Tribology International 46 (2012) 183–189
Load AE sensor
Sliding surface Bearing holder Lubricant
Φ20
Test sample
Φ22
Counterpart
Groove
Φ30
Φ27.2 Driving shaft Test sample
Counterpart
Fig. 1. Schematic view of sleeve-to-plate tribo-tester.
Table 1 Test samples.
Table 3 Conditions of seizure tests.
Item
Chemical composition (Mass%)
Item
Condition
Bearing alloy material
A: Sn-6% Sb-4%Cu (White metal) B: Al-40%Sn C: Al-40%Sn þ Resin overlay
Load Sliding velocity Lubricant Oil inlet temperature Oil flow rate Test pattern
3 MPa (step-up) 0.1 m/s SEA30 60 1C 20 ml/min
Table 2 Conditions of start–stop tests. Condition
Load Sliding velocity Number of cycles Lubricant Test pattern
3 MPa 0–0.01 m/s (start–stop) 15 SEA30 or unlubricated
Load (MPa)
10 min Item
30
9 6 3
1 cycle Velocity (m/s)
10 min
3MPa Time
6min 6min 0.01 The counterpart is quenched carbon steel (0.55%C) of a hardness of Hv 550 and roughness of Ra 0.10 mm.
Time
and outer diameter of 27.2 mm is attached to the bearing holder. The sliding surface is divided into four parts by the groove. The counterpart is rotated by the motor. The sliding surface is loaded up to 30 MPa. Temperature is measured at the back of the test sample, and the coefficient of friction and frictional force are determined from the measured torque. A wide range type of AE sensor is placed at the rear of the bearing holder. AE signals are amplified at a gain of 70 dB by a pre-amplifier and an amplifier, and are filtered in the range of 0.5–1.25 MHz. The processed data of the AE RMS (Root-MeanSquare) is used. 2.2. Test materials A plain bearing consists of back steel and bearing alloy layer. Table 1 shows the test sample materials. They are tin alloy known as ‘‘white metal’’, aluminum alloy of 40% tin mass, and aluminum alloy of 40% tin mass with a 20 mm-thick resin overlay containing dispersed PTFE (Polytetrafluoroethylene) in PAI (Polyamide–Imide).
2.3. Test conditions Table 2 shows the conditions of the start–stop tests simulating crankshaft turning at low speed during the engine assembly, determined from the condition of the marine diesel engine. The test mode included starting and stopping, and each cycle was 6 min running and 6 min stationary, and repeated 15 cycles. Although the stop time of the real diesel engine is several hours, it was adjusted to 6 min where the back temperature almost restored to its former state, in order to shorten the test. The specific load was kept constant at 3 MPa and the top sliding velocity was 0.01 m/s (7.8 rpm) with 0.025 ml of lubricant placed on sliding surface or the unlubricated condition. Table 3 shows the conditions of seizure tests. The specific load was increased by 3 MPa every 10 min up to 30 MPa, and the sliding velocity was kept constant at 0.1 m/s (78 rpm). Lubricant at 60 1C was supplied to the groove at 20 ml/min. Seizure was defined as occurring when the frictional force reached 406 N because of the limit of the test machine. The lubricating condition was boundary and/or mixed lubrication regime, considering measured coefficient of friction described later. Each test was repeated twice except durability test, and the trends were noticeable although there was practically some scatter.
M. Nagata et al. / Tribology International 46 (2012) 183–189
3. Results and discussion 3.1. Start–stop tests
Coefficient of friction (-)
Fig. 2a and b show the test results of start–stop tests simulating crankshaft turning with 0.025 ml of lubricant placed on the test sample surface, and Fig. 3a and b show those without lubricant under the specific load of 3 MPa and top sliding velocity of 0.01 m/ s. Average values for the coefficient of friction and AE RMS of each cycle are plotted versus the number of turning cycles for the different test materials. In tests with lubricant, the coefficient of friction and AE RMS of each material were at a steady low level: the coefficient of friction was about 0.13, and AE RMS was about 0.01 V during the tests. In tests without lubricant, the coefficient of friction of test material A(white metal) varied from 0.25 to 0.55, while that of test material B(aluminum alloy) was about 0.35. Conversely, the
0.6
Test material A B C
0.4
0.2
0
AE RMS (V)
0.1 Test material
0.08
A B C
0.06 0.04 0.02
185
AE RMS of test material B was larger than that of test material A. The AE RMS of test material A varied from 0.015 to 0.035 V, and that of test material B from 0.035 to 0.08 V. For test material C(aluminum alloyþresin overlay), the coefficient of friction was in the range from 0.1 to 0.14 and AE RMS was in similar level of 0.009 V to those with lubricant. Fig. 4 shows the appearance of sliding surfaces of test samples with and without lubricant after the tests. It can be seen that the sliding surfaces of test material A and B with lubricant do not have much damage, but those without lubricant have suffered adhesion type damage. For test material C, the condition of the sliding surfaces had hardly any damage and the wear amount was comparable to that with lubricant at about 2.8 mm. It is confirmed that test material C has superior tribological performance regardless of lubricant, from coefficient of friction, AE RMS and wear measurements in the present start–stop tests. For test material A and B, the coefficient of friction in dry condition did not vary so much as shown in Fig. 3a. As far as AE RMS is concerned, the values gradually increased with number of cycles resulted from the increase in the worn area, although the scatter of AE RMS of test material B was greater. Also, AE RMS of test material A was lower than that of test material B, whereas the coefficient of friction was reverse. It is thought that material A(white metal) has lower plastic stress resulting in lower radiation of stored strain energy, which is AE. Fig. 5 shows the durability test of the resin overlay without lubricant from long-term tests. In this test, the thickness of the resin overlay was adjusted to 6 mm to shorten the testing time and still identify how the coefficient of friction, back temperature and AE signal measured for the resin overlay changed in the process of wear-out. This test was carried out under the same test conditions as the above mentioned start–stop tests. Average values for the coefficient of friction, back temperature and AE RMS of each cycle are plotted versus the number of turning cycles. The discontinuity in coefficient of friction seen in the graphs was caused by an interruption of the test in order to observe the sliding surface. The coefficient of friction exhibited an abrupt
0 0
5
10
15 With lubricant
Fig. 2. Test results of the start–stop tests with lubricant: (a) coefficient of friction and (b) AE RMS.
A B C
Without lubricant
2mm
2mm
0.4 Test material B
Coefficient of friction (-)
Test material
0.6
Test material A
Number of turning cycles
0.2 0
0.1
2mm
2mm
2mm
2mm
0.08
A B
0.06
C
0.04 0.02 0 0
5 10 Number of turning cycles
15
Fig. 3. Test results of the start–stop tests without lubricant: (a) coefficient of friction and (b) AE RMS.
Test material C
AE RMS (V)
Test material
Fig. 4. Appearance of surfaces of test samples after start–stop tests.
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oefficient of friction (-)
0.3 0.2 0.1
Temperature (°C)
0
40 30 20 10 0
AE RMS (V)
0.012 0.01 0.008 0.006 0.004
0
20
40
60 80 100 Number of turning cycles
120
140
160
Fig. 5. Durability test of resin overlay without lubricant: (a) coefficient of friction, (b) back temperature and (c) AE RMS.
increase from 160 cycles when the resin overlay was worn out and the bearing alloy was exposed. The back temperature did not change during the test. AE RMS rose gently from about 140 cycles. The test sample wear loss were 4, 6.25, 9.5 and 9.8 mm after 70, 120, 155 and 165 cycles (last cycle of the test), respectively. Fig. 6 shows the appearance of the sliding surfaces of test sample after 70, 120, 155 and 165 cycles (last cycle of the test). It can be seen that the sliding surfaces were completely discolored from original light green to light brown after 120 cycles, blackened in spots after 150 cycles, and continued to progressively blacken after 165 cycles. Fig. 7 shows the micrographs of sliding surfaces of test sample before and after the test. The resin overlay, which had uniformly dispersed PTFE before the test, has been partially worn out, and is exposing the bearing alloy. In the start–stop tests, AE RMS is useful to monitor the surface condition and detect the wear-out progress of the resin overlay more sensitively than measurement of the coefficient of friction and back temperature. 3.2. Seizure test In this test, the specific load is changed to increase until seizure occurred, the frictional force is used to evaluate instead of the coefficient of friction in order to have correlation between AE RMS. Fig. 8 shows the seizure test results for test material A(white metal). Frictional force, back temperature, AE RMS and appearance after test are shown in Fig. 8a, b, c and d, respectively. The frictional force, back temperature and AE RMS rose with the increase in specific load, and those behaviors were similar. In detail, up to 9 MPa, AE RMS detected a running-in process. When the specific load increased, AE RMS momentarily rose a bit and then returned to a low level. The frictional force of the present seizure criterion was not reached, but plastic flow occurred as can be seen at the trailing edge of the sliding surface in Fig. 8d.
Fig. 6. Appearance of surfaces of test sample after: (a) 70 cycles, (b) 120 cycles, (c) 155 cycles and (d) 165 cycles.
For test material B(aluminum alloy) shown in Fig. 9, the frictional force of seizure criterion was reached at 15 MPa. During the test, while the frictional force decreased, the back temperature and AE RMS rose with the increase in specific load up to 12 MPa. At this time, AE RMS indicated a running-in process. Thereby, the frictional force decreased and AE RMS increased because of increase in the contact area. Just before seizure, the frictional force and AE RMS rose rapidly at the same time. The appearance of the surface after test is shown in Fig. 9d. The surface was roughened due to adhesion.
M. Nagata et al. / Tribology International 46 (2012) 183–189
187
40 Frictional force Specific load
400
30
300 20 200
15MPa 10
100 0
Specific load (MPa)
Frictional force (N)
500
0
Fig. 7. Micrographs of test sample (a) before test and (b) after test.
20 200 10
100
0
Temperature (°C)
80
40 Back temperature Specific load
60
30
40
20
20
10
0
0
AE RMS (V)
0.5
40 AE RMS Specific load
0.4
30
0.3 20 0.2 10
0.1 0
0
2000
4000 Time (sec)
6000
0
Specific load (MPa)
0
2mm
Fig. 8. Seizure test results for test material A: (a) frictional force, (b) back temperature, (c) AE RMS and (d) appearance after test.
60
30
40
20
20
10
0
0
0.5
Specific load (MPa)
40 Back temperature Specific load
40 AE RMS Specific load
0.4
30
0.3 20 0.2 10
0.1 0
Specific load (MPa)
30
300
Temperature (V)
400
AE RMS (V)
Frictional force Specific load
Specific load (MPa)
40
500
Specific Load (MPa)
Frictional force (N)
80
0 0
2000
4000 Time (sec)
6000
2mm
Fig. 9. Seizure test results for test material B: (a) frictional force, (b) back temperature, (c) AE RMS and (d) appearance after test.
For test material C, which was test material B covered with a resin overlay of 20 mm in thickness as shown in Fig. 10. During the test, the frictional force and back temperature rose with the increase in the specific load. On the other hand, AE RMS did not show much change at 0.2 V or less from 12 MPa. The frictional force of the present seizure criterion, based on the limitation of the test rig, was reached at 27 MPa. However, the sliding surface after the test had no damage and exfoliation of the resin overlay was not found as shown in Fig. 10d. The seizure resistance of the test material B improved due to the covered resin overlay. This is because adhesion of the contact area of the sliding surfaces is prevented by the resin overlay. Fig. 11 shows the case of the test results for test material C in which exfoliation of the resin overlay was found after test. The appearance of the sliding surface after test is shown in Fig. 11d.
0
0
80
40
60
30
40
20
20
Back temperature
0
30
0.3 20 0.2 10
0.1 0
20 200 10
100
0
80
40
60
30
40
20
20
Back temperature Specific load
10 0
0.5 AE RMS (V)
0.4
30
300
0
40 AE RMS Specific load
400
0
0
0.5 AE RMS (V)
10
Specific load
40 Frictional force Spcific load
40 AE RMS Specific load
0.4
30
0.3 20 0.2 10
0.1
0 0
2000
4000 Time (sec)
6000
0
0
During the test, the frictional force and back temperature did not change much from 18 to 30 MPa. However, AE RMS exhibited an extreme a increase up to about 0.4 V at 21 MPa, and maintained a high level when the exfoliation occurred. The abrupt increase is thought to be the onset of the exfoliation of the resin overlay, and the decreasing of the frictional force, presumably because of the transfer of the PTFE in the resin overlay. It was confirmed that AE technique can more sensitively indicate the change in tribological properties with exfoliation of the resin overlay rather than measurements of the frictional force and back temperature. Fig. 12 shows the correlations between frictional force and AE RMS during the seizure tests. The correlations of test material A and B immediately before the seizure, which have single bearing material layer, are linear. The frictional force of the sliding surface
2000
4000 Time (sec)
6000
0
5mm
5mm Fig. 10. Seizure test results for test material C: (a) frictional force, (b) back temperature, (c) AE RMS and (d) appearance after test.
Specific load (MPa)
10
100
500
Specific load (MPa)
20 200
Frictional force (N)
30
300
Temperature (V)
400
Specific load (MPa)
Frictional force Specific load
Specific load (MPa)
40
500
Specific load (MPa)
M. Nagata et al. / Tribology International 46 (2012) 183–189
Specific load (MPa)
Temperature (V)
Frictional force (N)
188
Fig. 11. Seizure test results for test material C showing exfoliation of resin overlay: (a) frictional force, (b) back temperature, (c) AE RMS and (d) appearance after test.
could be estimated from AE RMS using these correlations. If the frictional force of sliding surfaces is difficult to measure, AE could be an alternative of the friction measurement and seizure detection. Although for test material C with the resin overlay, the AE RMS was 0.2 V or less where there was no significant correlation, the AE RMS exhibited an extreme increase when exfoliation of the resin overlay occurred.
4. Conclusions Bearing materials for marine engines were tested to assess the possibility of AE technique to evaluate tribological properties
M. Nagata et al. / Tribology International 46 (2012) 183–189
189
AE RMS is useful to monitor the bearing surface condition and is recommended to use together with measurement of friction or temperature to evaluate tribological properties of materials.
0.5
AE RMS (V)
0.4 References
0.3
0.2
0.1
0
0
100
200 300 Frictional force (N)
400
500
Fig. 12. Correlations between frictional force and AE RMS during seizure tests.
using a sleeve-to-plate tribo-tester. Three materials – A(white metal), B(aluminum alloy) and C(aluminum alloyþresin overlay) – were tested in start–stop and seizure test modes. The followings can be summarized based on the present experiments. 1. In the start–stop tests, AE RMS is useful to monitor the surface condition and detect the wear-out process of the resin overlay more sensitively than measurements of the coefficient of friction and temperature. 2. In the seizure tests, the correlation between the AE RMS and frictional force is good for single bearing material layer. For the bearing material with resin overlay, the AE RMS can detect exfoliation of the resin overlay in advance of seizure.
[1] Baranov V, Kudryavtsev E, Sarychev G, Schavelin V. Acoustic emission in friction. In: BRISCOE BJ, editor. Tribology and interface engineering series, vol. 53; 2007. p. 10–5. [2] Hase A, Wada M, Mishina H. The relationship between acoustic emissions and wear particles for repeated dry rubbing. Wear 2008;265(5–6):831–9. [3] Hase A, Wada M, Mishina H. Acoustic emission signals and wear phenomena on severe–mild wear transition. Tribology Online 2008;3(5):298–303. [4] Hase A, Mishina H, Wada M. Acoustic emission in elementary processes of friction and wear: in-situ observation of friction surface and AE signals. Journal of Advanced Mechanical Design, Systems and Manufacturing 2009;3(4):333–44. [5] Yoshioka T. Detection of rolling contact sub-surface fatigue cracks using acoustic emission. Lubrication Engineering 1993;49(4):303–8. [6] Rahman MdZiaur, Ohba H, Yoshioka T, Yamamoto T. Evaluation of acoustic emission sources during monitoring of incipient damage detection in rolling contact fatigue. Tribology Online 2008;3(2):105–9. [7] Mano H, Korenaga A, Ando Y, Sasaki S, Usami H. Measurement of acoustic emission caused by repetition of one-point loading and rolling contact fatigue. JSME Annual Meeting 2004;4:85–6. [8] Sato I, Yoneyama T, Inoue T. Journal bearing diagnosis with acoustic emission technique. Journal of Japan Society of Lubrication Engineers 1983;28(12):8–876. [9] Shimizu K, Imaizumi Y, Sueyoshi M, Furue T, Matsuo C, Maeda M, et al. Study on the destructive testing for the seizure of slide bearing. Symposium on Evaluation and Diagnosis 2005;05-43:70–3. [10] Shiihara H, Okamoto K, Kurosawa T. Optimum condition monitoring technologies for plane bearing- monitoring technologies using high frequency vibration, acoustic-emission and ferro-particulate in lubricating oil main bearing of main diesel engine. Journal of the JIME 2009;44-2:199–207. [11] Baran I, Nowak M, Darski W. Application of acoustic emission in monitoring of failure in slide bearings. Journal Acoustic Emission 2007;25:341–7. [12] Burger W, Albers A, Scovino R, Dickerhof M. Acoustic emission analysis for monitoring of tribological systems. In: Proceeding of WTC2005, WTC200563117, p. 849–50. [13] Evans C, Warriner FJ. Bearings and bearing metals. In: Challen B, Baranewscu R, editors. Diesel engine reference book. 2nd ed. Society of Automotive Engineers Inc.; 1999. p. 344–5. [14] Kawakami N, Yamada M, Ono A. Recent trend of diesel engine bearings. Journal of JIME 2009;44(2):72–7.
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Evaluation of tribological properties of bearing materials for marine diesel engines utilizing acoustic emission technique Mari Nagata a,n, Masahito Fujita a, Motohira Yamada b, Tatsumi Kitahara c a
Research and Development Division, Daido Metal Co. Ltd., Japan Department of Application Engineering, Technological Division, Daido Metal Co. Ltd., Japan c Department of Mechanical Engineering Science, Kyushu University, Japan b
a r t i c l e i n f o
abstract
Article history: Received 14 October 2010 Received in revised form 18 May 2011 Accepted 20 May 2011 Available online 1 June 2011
Acoustic Emission (AE) technique, which has detection capability for minute failures, has been tried to monitor the condition of a plain bearing under the laboratory conditions. In this paper, the bearing materials for marine diesel engines – tin alloy as known as ‘‘white metal’’, aluminum alloy of 40% tin mass and aluminum alloy 40% tin mass with resin overlay – were tested using a sleeve-to-plate tribotester. The frictional force and back temperature were measured as well as the AE signals. The possibility of AE technique to monitor the bearing condition was also assessed by evaluating tribological properties under different operating conditions such as start–stop simulating the crankshaft turning during engine assembly and seizure tests. These results indicate that AE is useful for monitoring the lubricated condition of the sliding surfaces and evaluating tribological properties of the bearing. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Acoustic Emission (AE) Plain bearing material Resin overlay Sleeve-to-plate tribo-tester
1. Introduction Acoustic Emission (AE) is defined as radiation of mechanical elastic waves produced by a material due to the dynamic local rearrangement of its internal structure [1]. Therefore, AE technique can be used to detect minute failures. It has been studied for the evaluation of tribological properties such as to observe wear and friction processes [2–4], detection of the ball bearing fatigue [5–7] and monitoring of plain bearings in the laboratory—journal bearings for rotating machinery such as a steam turbine and generator [8] and tilting pad thrust bearings for a hydroelectric water turbine [9] or sleeve bearings for a marine engine. As for marine engines, the monitoring technique for bearing failure has been researched, i.e., measurement of bearing temperature, analysis of oil ferrography and analysis of shaft vibration [10]. However, it is not easy to detect bearing failures in the early stage, hence the studies of using AE to monitor marine engine bearings have been carried out [10–12]. In low speed two-stroke diesel engines, the crankshaft is rotated at low speed with repeated starts and stops when an engine is assembled. Sometimes the lubricant on the bearing surfaces can be lacking during crankshaft turning and the bearing surfaces might be damaged at the moment, resulting in bearing failure in service. As the bearings work in a severe state in service, the required bearing properties are seizure resistance, wear resistance, fatigue
n
Corresponding author. E-mail address: [email protected] (M. Nagata).
0301-679X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2011.05.026
resistance and conformability. A bearing material is selected according to operating conditions, i.e., bearing load and sliding velocity. Therefore, it is important to find the correlation between the tribological properties of the bearing materials and monitoring technique. In this investigation, the bearing materials for marine diesel engines – tin alloy as known as ‘‘white metal’’, aluminum alloy of 40% tin mass and aluminum alloy 40% tin mass with resin overlay – were evaluated. White metal has been commonly used as a bearing alloy, and aluminum alloy of 40% tin mass has been successfully applied to plain bearings for large size two-stroke diesel engines that are subjected to severe conditions such as higher loads [13]. The resin overlay was developed with the aim of preventing scoring and seizure of the bearing alloy at low speed during crankshaft turning when the engine is assembled [14]. The objective of this investigation was to assess the possibility of AE technique to monitor the bearing condition of marine engines with the above different materials, using a sleeveto-plate tribo-tester, and to confirm the correlation between the frictional force, back temperature and the AE signals.
2. Experimental 2.1. Test rig Fig. 1 shows a schematic view of a sleeve-to-plate tribo-tester that can measure AE signals as well as the frictional force and back temperature. A test sample with an inner diameter of 22 mm
184
M. Nagata et al. / Tribology International 46 (2012) 183–189
Load AE sensor
Sliding surface Bearing holder Lubricant
Φ20
Test sample
Φ22
Counterpart
Groove
Φ30
Φ27.2 Driving shaft Test sample
Counterpart
Fig. 1. Schematic view of sleeve-to-plate tribo-tester.
Table 1 Test samples.
Table 3 Conditions of seizure tests.
Item
Chemical composition (Mass%)
Item
Condition
Bearing alloy material
A: Sn-6% Sb-4%Cu (White metal) B: Al-40%Sn C: Al-40%Sn þ Resin overlay
Load Sliding velocity Lubricant Oil inlet temperature Oil flow rate Test pattern
3 MPa (step-up) 0.1 m/s SEA30 60 1C 20 ml/min
Table 2 Conditions of start–stop tests. Condition
Load Sliding velocity Number of cycles Lubricant Test pattern
3 MPa 0–0.01 m/s (start–stop) 15 SEA30 or unlubricated
Load (MPa)
10 min Item
30
9 6 3
1 cycle Velocity (m/s)
10 min
3MPa Time
6min 6min 0.01 The counterpart is quenched carbon steel (0.55%C) of a hardness of Hv 550 and roughness of Ra 0.10 mm.
Time
and outer diameter of 27.2 mm is attached to the bearing holder. The sliding surface is divided into four parts by the groove. The counterpart is rotated by the motor. The sliding surface is loaded up to 30 MPa. Temperature is measured at the back of the test sample, and the coefficient of friction and frictional force are determined from the measured torque. A wide range type of AE sensor is placed at the rear of the bearing holder. AE signals are amplified at a gain of 70 dB by a pre-amplifier and an amplifier, and are filtered in the range of 0.5–1.25 MHz. The processed data of the AE RMS (Root-MeanSquare) is used. 2.2. Test materials A plain bearing consists of back steel and bearing alloy layer. Table 1 shows the test sample materials. They are tin alloy known as ‘‘white metal’’, aluminum alloy of 40% tin mass, and aluminum alloy of 40% tin mass with a 20 mm-thick resin overlay containing dispersed PTFE (Polytetrafluoroethylene) in PAI (Polyamide–Imide).
2.3. Test conditions Table 2 shows the conditions of the start–stop tests simulating crankshaft turning at low speed during the engine assembly, determined from the condition of the marine diesel engine. The test mode included starting and stopping, and each cycle was 6 min running and 6 min stationary, and repeated 15 cycles. Although the stop time of the real diesel engine is several hours, it was adjusted to 6 min where the back temperature almost restored to its former state, in order to shorten the test. The specific load was kept constant at 3 MPa and the top sliding velocity was 0.01 m/s (7.8 rpm) with 0.025 ml of lubricant placed on sliding surface or the unlubricated condition. Table 3 shows the conditions of seizure tests. The specific load was increased by 3 MPa every 10 min up to 30 MPa, and the sliding velocity was kept constant at 0.1 m/s (78 rpm). Lubricant at 60 1C was supplied to the groove at 20 ml/min. Seizure was defined as occurring when the frictional force reached 406 N because of the limit of the test machine. The lubricating condition was boundary and/or mixed lubrication regime, considering measured coefficient of friction described later. Each test was repeated twice except durability test, and the trends were noticeable although there was practically some scatter.
M. Nagata et al. / Tribology International 46 (2012) 183–189
3. Results and discussion 3.1. Start–stop tests
Coefficient of friction (-)
Fig. 2a and b show the test results of start–stop tests simulating crankshaft turning with 0.025 ml of lubricant placed on the test sample surface, and Fig. 3a and b show those without lubricant under the specific load of 3 MPa and top sliding velocity of 0.01 m/ s. Average values for the coefficient of friction and AE RMS of each cycle are plotted versus the number of turning cycles for the different test materials. In tests with lubricant, the coefficient of friction and AE RMS of each material were at a steady low level: the coefficient of friction was about 0.13, and AE RMS was about 0.01 V during the tests. In tests without lubricant, the coefficient of friction of test material A(white metal) varied from 0.25 to 0.55, while that of test material B(aluminum alloy) was about 0.35. Conversely, the
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AE RMS of test material B was larger than that of test material A. The AE RMS of test material A varied from 0.015 to 0.035 V, and that of test material B from 0.035 to 0.08 V. For test material C(aluminum alloyþresin overlay), the coefficient of friction was in the range from 0.1 to 0.14 and AE RMS was in similar level of 0.009 V to those with lubricant. Fig. 4 shows the appearance of sliding surfaces of test samples with and without lubricant after the tests. It can be seen that the sliding surfaces of test material A and B with lubricant do not have much damage, but those without lubricant have suffered adhesion type damage. For test material C, the condition of the sliding surfaces had hardly any damage and the wear amount was comparable to that with lubricant at about 2.8 mm. It is confirmed that test material C has superior tribological performance regardless of lubricant, from coefficient of friction, AE RMS and wear measurements in the present start–stop tests. For test material A and B, the coefficient of friction in dry condition did not vary so much as shown in Fig. 3a. As far as AE RMS is concerned, the values gradually increased with number of cycles resulted from the increase in the worn area, although the scatter of AE RMS of test material B was greater. Also, AE RMS of test material A was lower than that of test material B, whereas the coefficient of friction was reverse. It is thought that material A(white metal) has lower plastic stress resulting in lower radiation of stored strain energy, which is AE. Fig. 5 shows the durability test of the resin overlay without lubricant from long-term tests. In this test, the thickness of the resin overlay was adjusted to 6 mm to shorten the testing time and still identify how the coefficient of friction, back temperature and AE signal measured for the resin overlay changed in the process of wear-out. This test was carried out under the same test conditions as the above mentioned start–stop tests. Average values for the coefficient of friction, back temperature and AE RMS of each cycle are plotted versus the number of turning cycles. The discontinuity in coefficient of friction seen in the graphs was caused by an interruption of the test in order to observe the sliding surface. The coefficient of friction exhibited an abrupt
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Fig. 2. Test results of the start–stop tests with lubricant: (a) coefficient of friction and (b) AE RMS.
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Fig. 4. Appearance of surfaces of test samples after start–stop tests.
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Fig. 5. Durability test of resin overlay without lubricant: (a) coefficient of friction, (b) back temperature and (c) AE RMS.
increase from 160 cycles when the resin overlay was worn out and the bearing alloy was exposed. The back temperature did not change during the test. AE RMS rose gently from about 140 cycles. The test sample wear loss were 4, 6.25, 9.5 and 9.8 mm after 70, 120, 155 and 165 cycles (last cycle of the test), respectively. Fig. 6 shows the appearance of the sliding surfaces of test sample after 70, 120, 155 and 165 cycles (last cycle of the test). It can be seen that the sliding surfaces were completely discolored from original light green to light brown after 120 cycles, blackened in spots after 150 cycles, and continued to progressively blacken after 165 cycles. Fig. 7 shows the micrographs of sliding surfaces of test sample before and after the test. The resin overlay, which had uniformly dispersed PTFE before the test, has been partially worn out, and is exposing the bearing alloy. In the start–stop tests, AE RMS is useful to monitor the surface condition and detect the wear-out progress of the resin overlay more sensitively than measurement of the coefficient of friction and back temperature. 3.2. Seizure test In this test, the specific load is changed to increase until seizure occurred, the frictional force is used to evaluate instead of the coefficient of friction in order to have correlation between AE RMS. Fig. 8 shows the seizure test results for test material A(white metal). Frictional force, back temperature, AE RMS and appearance after test are shown in Fig. 8a, b, c and d, respectively. The frictional force, back temperature and AE RMS rose with the increase in specific load, and those behaviors were similar. In detail, up to 9 MPa, AE RMS detected a running-in process. When the specific load increased, AE RMS momentarily rose a bit and then returned to a low level. The frictional force of the present seizure criterion was not reached, but plastic flow occurred as can be seen at the trailing edge of the sliding surface in Fig. 8d.
Fig. 6. Appearance of surfaces of test sample after: (a) 70 cycles, (b) 120 cycles, (c) 155 cycles and (d) 165 cycles.
For test material B(aluminum alloy) shown in Fig. 9, the frictional force of seizure criterion was reached at 15 MPa. During the test, while the frictional force decreased, the back temperature and AE RMS rose with the increase in specific load up to 12 MPa. At this time, AE RMS indicated a running-in process. Thereby, the frictional force decreased and AE RMS increased because of increase in the contact area. Just before seizure, the frictional force and AE RMS rose rapidly at the same time. The appearance of the surface after test is shown in Fig. 9d. The surface was roughened due to adhesion.
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Fig. 9. Seizure test results for test material B: (a) frictional force, (b) back temperature, (c) AE RMS and (d) appearance after test.
For test material C, which was test material B covered with a resin overlay of 20 mm in thickness as shown in Fig. 10. During the test, the frictional force and back temperature rose with the increase in the specific load. On the other hand, AE RMS did not show much change at 0.2 V or less from 12 MPa. The frictional force of the present seizure criterion, based on the limitation of the test rig, was reached at 27 MPa. However, the sliding surface after the test had no damage and exfoliation of the resin overlay was not found as shown in Fig. 10d. The seizure resistance of the test material B improved due to the covered resin overlay. This is because adhesion of the contact area of the sliding surfaces is prevented by the resin overlay. Fig. 11 shows the case of the test results for test material C in which exfoliation of the resin overlay was found after test. The appearance of the sliding surface after test is shown in Fig. 11d.
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During the test, the frictional force and back temperature did not change much from 18 to 30 MPa. However, AE RMS exhibited an extreme a increase up to about 0.4 V at 21 MPa, and maintained a high level when the exfoliation occurred. The abrupt increase is thought to be the onset of the exfoliation of the resin overlay, and the decreasing of the frictional force, presumably because of the transfer of the PTFE in the resin overlay. It was confirmed that AE technique can more sensitively indicate the change in tribological properties with exfoliation of the resin overlay rather than measurements of the frictional force and back temperature. Fig. 12 shows the correlations between frictional force and AE RMS during the seizure tests. The correlations of test material A and B immediately before the seizure, which have single bearing material layer, are linear. The frictional force of the sliding surface
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Fig. 11. Seizure test results for test material C showing exfoliation of resin overlay: (a) frictional force, (b) back temperature, (c) AE RMS and (d) appearance after test.
could be estimated from AE RMS using these correlations. If the frictional force of sliding surfaces is difficult to measure, AE could be an alternative of the friction measurement and seizure detection. Although for test material C with the resin overlay, the AE RMS was 0.2 V or less where there was no significant correlation, the AE RMS exhibited an extreme increase when exfoliation of the resin overlay occurred.
4. Conclusions Bearing materials for marine engines were tested to assess the possibility of AE technique to evaluate tribological properties
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AE RMS is useful to monitor the bearing surface condition and is recommended to use together with measurement of friction or temperature to evaluate tribological properties of materials.
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Fig. 12. Correlations between frictional force and AE RMS during seizure tests.
using a sleeve-to-plate tribo-tester. Three materials – A(white metal), B(aluminum alloy) and C(aluminum alloyþresin overlay) – were tested in start–stop and seizure test modes. The followings can be summarized based on the present experiments. 1. In the start–stop tests, AE RMS is useful to monitor the surface condition and detect the wear-out process of the resin overlay more sensitively than measurements of the coefficient of friction and temperature. 2. In the seizure tests, the correlation between the AE RMS and frictional force is good for single bearing material layer. For the bearing material with resin overlay, the AE RMS can detect exfoliation of the resin overlay in advance of seizure.
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