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A basic study of friction noise caused by fretting
Wear 251 (2001) 1492–1503
A basic study of friction noise caused by fretting T. Jibiki a,∗ , M. Shima a , H. Akita b , M. Tamura b a
b
Department of Mechanical Engineering, Tokyo University of Mercantile Marine, Etchujima 2-1-6, Koto-ku, Tokyo 135-0044, Japan Technical Research Centre, Hitachi Construction Machinery Co. Ltd. 650, Kandatsu-machi, Tsuchiura City, Ibaragi Pref. 300-0013, Japan
Abstract Fretting may be accompanied by friction noise and preventing or reducing it can be important for designers and operators. In this study, 0.45% carbon steel (Hv730) and mild steel (Hv240) are used as specimens with the crossed cylinder configuration. Friction force, relative movement between the specimens, friction noise and electrical contact resistance are measured simultaneously during fretting, with normal load 19.6 N, and up to 25,000 cycles in air. Fretting stroke, relative humidity and frequency are varied in the range 25–400 m, 11–78% RH and 1.0–7.3 Hz, respectively. In addition, the wave signals of the friction noise, together with coefficient of friction and relative stroke, are analyzed by FFT analyzer. The main results are as follows: certain cycles of fretting are needed to generate friction noise; there are common features in relation to the occurrence of friction noise; drastic reduction in coefficient of friction µ; self-excited-vibration and negative gradient of µ, sound level of friction increases with increase in fretting stroke and frequency, and is directly related to average sliding velocity; there is a good correlation between the sound level and amount of wear. Based on the results, the mechanism of the friction noise is discussed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Friction noise; Fretting wear; Sound level of friction; Wear debris
1. Introduction Fretting may be accompanied by friction noise and preventing or reducing it can be important for designers and operators. In the past, a lot of studies concerning the friction noises of disk brake, dram brake, tire for automobile and wheel for railway have been reported [1–13]. These friction noises are often called “squeal”. The sound level of such friction noises is generally large and has high frequency. Basic studies concerning the mechanism of friction noises have also been reported [14–33]. Yokoi and Nakai reported that there were two different kinds of friction noise, that is, “rubbing noise” where sound level is low, and “squeal noise” where sound level is high [15]. They also reported that the friction noise tends to occur when the gradient of friction velocity is negative [14]. Recently, Kanehara et al. concluded that friction noise is often caused by stick-slip motion in the case of relatively low friction velocity [34]. Fretting is a phenomenon greatly affected by atmosphere, and wear debris easily accumulates between the contacting surfaces. Therefore, wear debris may play a crucial role in the generation of friction noise. Thus, it is valuable to investigate the relation between friction noise and fretting ∗ Corresponding author. Fax: +81-3-5245-7435. E-mail address: [email protected] (T. Jibiki).
behavior in a wide range of fretting conditions. However, there is no report associated with friction noise under fretting condition. In this study, a system which is able to measure and analyze friction noise under fretting conditions has been developed, and the waveform of friction, friction noise and electrical contact resistance have been measured and analyzed, and also the influence of various parameters such as relative humidity in air, fretting stroke and frequency on friction noise are examined. In addition, observations of fretting surfaces are made by a laser or optical microscope. Based on the results, the mechanism of generating friction noise is discussed.
2. Experimental 2.1. Apparatus Fig. 1 shows a schematic diagram of the fretting rig. A driven specimen is attached to a cantilever, which is horizontally oscillated by a motor through a crank mechanism. A fixed specimen is attached to an upper holder. The test apparatus is covered with a chamber in order to control the relative humidity, which also makes possible the experiment in Argon gas. Friction force is measured by four strain gauges attached on two leaf springs of the upper holder.
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Fig. 1. Schematic diagram of the fretting rig.
Table 1 Specimens
Table 2 Experimental test conditions
Fixed specimen (upper specimen) Driven specimen (lower specimen)
Carbon steel (0.45%) quenched Hv730, Ry 1.0 m Mild steel Hv240, Ry 1.0 m
Relative stroke ∆re between both specimens, which is peak to peak amplitude of tangential movement between the upper and lower specimens, is measured by an eddy current pick up. Friction noise is measured by a microphone and a sound level meter, which is able to measure up to a sound frequency of 8 kHz. Both ac and dc output signals from the sound level meter are used for the analysis. Friction force, relative stroke between the specimens and friction noise are
Fretting stroke Normal load Atmosphere Test duration Frequency Temperature Relative humidity Data sampling frequency Configuration
25–400 m 19.6 N Laboratory air 25,000 cycles 1.0–7.3 Hz 294 ± 2 K 11–78% RH 5 kHz (4096 data) Crossed cylinders
measured simultaneously during fretting. All the data of these signals are fed into a personal computer through A/D converters for the wave analysis.
Fig. 2. Typical example showing waveforms of friction noise (ac output), coefficient of friction µ, and relative stroke ∆re .
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2.2. Specimens Table 1 shows details of the specimens used for the fretting test. Both the fixed and driven specimens are cylindrical, 10 mm in diameter and 30 mm in length. The fixed specimen is a 0.45% carbon steel quenched by high frequency heating (Hv730), and the driven one is a mild steel (Hv240). Both specimens were finished within the range of the surface roughness Ry 1.0 m by grinding process. 2.3. Fretting test Table 2 shows the test conditions. Fretting tests were carried out with normal load 19.6 N, and up to 25,000 cycles in air. Fretting stroke ∆re , relative humidity and frequency varied in the ranges of 25–400 m, 11–78% RH and 1.0–7.3 Hz, respectively. In addition, the wave signals of the friction noise, together with coefficient of friction and relative stroke, are analyzed by FFT analyzer. The data sampling frequency of the A/D converter is 5 kHz, and 4096 data per second can be stored into a memory.
3. Results 3.1. Measurements and analysis of waveforms Fig. 2 shows a typical example showing waveforms of friction noise (ac output), coefficient of friction and relative stroke. Friction noise always occurs during half cycle of fretting where the lower driven specimen moves to the left side direction in Fig. 1, when the leaf springs supporting the upper specimen are subjected to tension force. This half cycle of fretting is defined as the tension process, the other one as the compression process. During the compression process friction noise never occurs. This behavior will be discussed in Section 4. It is also found that friction noise starts to generate just after the sudden reduction in coefficient
Fig. 4. FFT analysis of Lp , µ, and dl/dt.
of friction at the beginning of the tension process, and is continued until the end of the process. Fig. 3 shows the precise waveforms around the beginning of the tension process together with the velocity of relative motion, dl/dt, which are plotted magnifying the time scale. There is a good correlation between the three waveforms, which have the same period.
Fig. 3. Magnified time axis around the beginning of tension process.
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Fig. 5. Another typical example of waveforms, fretting stroke of 401 m, 30% RH, 1 Hz.
Fig. 4 shows the results of these three waveforms analyzed by FFT analyzer (power spectrum analysis). It is obvious, that they have the same peaks of power spectrum at 1.4 kHz in frequency. This means, the fretting system is in a self-excited-vibration state when the friction noise is generated. This frequency of 1.4 kHz is never changed in the wide range of fretting stroke and driving frequency. Therefore, the frequency is considered to be dependent on the dynamic response of fretting system.
Fig. 5 is another typical example, where the fretting stroke is about two times larger than that above. From these results it is found that there are three common features in relation to generation of friction noise. Firstly, drastic reduction in coefficient of friction occurs at the beginning of the tension process and friction noise starts to begin at the same time. Secondly, the self-excited-vibration state often takes place in the fretting system at a frequency of 1.4 kHz. Finally, friction noise never takes place during compression process.
Fig. 6. Electrical contact resistance Rc and friction noise Lp (dc output) together with µ and ∆re vs. fretting cycles, fretting stroke of 191 m, 46% RH, 7.2 Hz, 25,000 cycles.
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These common features are observed without any exception in the wide range of fretting conditions in air. 3.2. Measurement of electrical contact resistance Rc Electrical contact resistance Rc was measured in order to estimate contact conditions during the occurrence of friction noise. Measuring method is as follows [35]: a 0Ω is set under the condition of metal–metal contact; and ∞Ω is set under the condition of electrical insulation. Fig. 6 shows electrical contact resistance Rc and friction noise Lp (dc output values) together with coefficient of
friction µ and fretting stroke ∆re versus fretting cycles. It is found that certain cycles of fretting are needed to generate friction noise, depending on fretting stroke. The fluctuation of Rc is not so large until intense friction noise occurs, but once it starts to begin Rc fluctuates between 0 and ∞ Ω. Fig. 7(a) and (b) shows the precise Rc and Lp (ac output) together with µ and ∆re before and after the occurrence of friction noise, respectively. For the former, the fluctuation of Rc is small for both tension and compression process, but for the latter very intense fluctuation of Rc periodically takes place during the tension process. The fluctuation period corresponds well to that of Lp . On the other hand, during the
Fig. 7. (a) Precise Rc and Lp (dc output) before the occurrence of friction noise, after 300 cycles; (b) precise Rc and Lp (ac output) after the occurrence of friction noise, after 25,000 cycles.
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compression process, which is not accompanied by friction noise Lp , Rc remains low, which means that at least partial intimate metal–metal contact is taking place between the contacting surfaces. This shows that there is a good correlation between the behavior of Rc and the occurrence of friction noise. 3.3. Observation of wear surfaces Fig. 8(a) and (b) show the appearance of wear surfaces before and after the occurrence of friction noise for the 0.45% carbon steel, and Fig. 8(c) and (d) similarly for the mild steel. In the case of the former, i.e. before the generation of noise, the wear surface of the mild steel is almost covered with gray oxide debris (Fig. 8(c)) which is easily
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removed by wiping. On the contrary, the wear surface of the 0.45% carbon steel mainly consist of bright area which means metallic surface (Fig. 8(a)). On the other hand, in the case of the latter, i.e. after sound generation, both the surfaces of the mild steel and the 0.45% carbon steel consist of partially reddish/gray/black area in which loose oxide debris, glaze oxide layer adhering to the surface and partially bright area may be identified (Fig. 8(b) and (d)). The complicated contacting surfaces may be involved in a lot of real load bearing contact points such as metal–metal, glaze oxide–glaze oxide, glaze oxide–metal, and oxide film–oxide film contacts [36]. In addition, agglomerated loose particles between the contacting surfaces may bear a part of the total load during fretting. This will be discussed later.
Fig. 8. (a) Wear surface observation before the occurrence of friction noise (0.45% carbon steel, fretting stroke of 191 m, 30% RH, 1000 cycles); (b) wear surface observation after the occurrence of friction noise (0.45% carbon steel, fretting stroke of 191 m, 30% RH, 25,000 cycles); (c) wear surface observation before the occurrence of friction noise (mild steel, fretting stroke of 191 m, 30% RH, 1000 cycles) precise Rc and Lp (ac output) together with µ and ∆re before and after the occurrence of friction noise, respectively (for the former the fluctuation of Rc is small); (d) wear surface observation after the occurrence of friction noise (mild steel, fretting stroke of 191 m, 30% RH, 25,000 cycles).
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Fig. 9. Influence of relative humidity on friction noise Lp (dc output), fretting stroke of 191 m,7.2 Hz, 20,000 cycles.
3.4. Influences of various parameters on friction noise Influence of various parameters on friction noise such as relative humidity, fretting stroke and frequency were investigated. Fig. 9 shows the influence of relative humidity on friction noise Lp (dc output). Coefficient of friction µ (peak–peak value divided by 2 from waveform measured) and total wear volume Vt are also plotted in this figure. In this study, Vt was calculated by Eq. (1) [37,38] as follows:
Fig. 10. Influence of fretting stroke on Lp , µ, and Vt at the humidity of 30% RH, 7.2 Hz, 25,000 cycles.
Vt =
π a4 4r
(1)
where a is the mean value of damaged half width in the fretting direction and that perpendicular to the direction, r the radius of the specimen. Sound level of friction is a maximum at 50% RH. On the contrary, wear volume is gently decreased with the increasing RH. Opposite to the tendency of sound level of friction, the coefficient of friction µ is a minimum at 50% RH. Thus, the sound level of friction is not directly related to the coefficient of friction.
Fig. 11. Influence of fretting frequency and fretting stroke on Lp , 30% RH, 25,000 cycles.
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Fig. 10 shows the influence of fretting stroke on Lp , µ and Vt at a humidity of 30% RH. The sound level of friction as well as wear volume increases with increasing fretting stroke. In this case, µ increases with increasing fretting stroke and reaches around 0.6, after which it levels off as the fretting stroke is increased. Fig. 11 shows the influence of fretting frequency and stroke on Lp . It is clearly seen that the sound level tends to increase with increasing frequency and stroke.
4. Discussion 4.1. Difference between tension and compression processes on friction noise From Section 3.1, it was found that friction noise always occurred during the tension process, and the waveform was quite different during the tension and compression processes. In order to examine the unexpected behavior additional experiments were conducted. First of all, the spring stiffness of the fretting apparatus was examined under the two loading conditions shown in Fig. 12(a) and (b), distinguishing the stiffness in tension process from that in compression process. Fig. 13 shows the spring system of the fretting apparatus schematically. The relations between given displacement s and followed tangential force of the spring system F are shown in Fig. 14(a) and (b). It is clearly seen that the stiffness in tension process is the exactly same as that in compression process, 2.2–2.3 N/m, under the no gross slip condition for both loading types. On the contrary, some reduction in the stiffness takes place during the tension process under
Fig. 12. Two loading conditions.
Fig. 13. Spring system of the fretting apparatus.
the gross slip condition for the loading type (A), while such phenomenon never occurs for the loading type (B). This phenomenon may be attributed to the change in tangential stiffness of contacting surfaces, which can be caused by slight leaning of the contacting surfaces due to the bending of the upper specimen holder. All the experiments to measure the friction noise were made using loading type (A) in this study, so it is assumed that friction noise tends to easily occur in the tension process where the spring stiffness is lowered. In addition, the slight leaning of the contacting surfaces itself may be involved in generation of friction noise through expulsion process of wear debris between the contacting surfaces. It is well-known that wear debris can be easily expelled from the contacting surfaces under lubricated condition [39]. So, fretting experiments were carried out in distilled water and in grease lubricant to examine the effect of wear debris on tangential stiffness of contacting surfaces. As shown in Fig. 15(a) and (b), both waveforms of µ and ∆re are symmetric for the tension and compression processes, while no friction noise is generated. These results suggest that the asymmetry of tension and compression processes may be responsible for wear debris between the contacting surfaces. Finally, electrical contact resistance Rc was measured, as described in Section 3.2. As shown in Fig. 7 there is a good correlation between the behavior of Rc and friction
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becomes more predominant with increasing Vave . Thus, the sound level of friction increases linearly with the increase in the Vave . 4.3. Relationship between wear volume and sound level of friction
Fig. 14. Relations between displacement s and tangential force F .
noise, and the intense fluctuation of Rc takes place only during the tension process. This result, also suggests that wear debris accumulated between the contacting surfaces may play an important role in generation of friction noise as well as tangential stiffness of contacting surfaces. 4.2. Influence of fretting stroke and frequency In the Section 3.4, it was found that the sound level of friction increases with the increase in both fretting stroke and frequency (Figs. 10 and 11). These parameters are directly related to relative sliding velocity between the contacting surfaces. So the average sliding velocity Vave was calculated, and the sound level of friction Lp (dB value) was plotted against Vave . The result is shown in Fig. 16. It is obvious that there is a good linearity between Lp and the log of Vave . The result indicates that “squeal noise” which is of high sound level tends to occur and to generate a self-excited vibration state when the sliding velocity is relatively high, and that “rubbing noise” and/or “stick-slip” easily occur when the sliding velocity is relatively low. In fretting, sliding velocity is not constant but varies sinusoidally. Therefore, “squeal noise” and “rubbing noise” exists simultaneously in each cycle. The ratio of two kinds of friction noise is considered to depend on Vave strongly, and “squeal noise”
In the Section 3.4, the influence of relative humidity, fretting stroke and frequency on Vt together with Lp , µ were examined (Figs. 9 and 10). Using those results, the relation between specific wear rate Vs and sound level Lp was plotted (Fig. 17). Vs was calculated from Vt , load W , and sliding length L. The result shows that there is a clear linearity between Vs and Lp , when the relative humidity is kept constant (30% RH). On the other hand, the data slightly deviate from the line if the relative humidity or fretted cycles is different. However, the tendency that Vs increases with increasing Lp is clearly recognized, that is, the higher the sound level the larger the wear volume, and vice versa. These results mean that sound level is strictly connected with fretting wear. Thus, the behavior of wear debris between the contacting surfaces is thought to play an important role in generating friction noise. Previously, one of the authors examined the debris formed between steel surfaces, focusing on the particle size and shape of the debris, and found that great many of oxidized debris below 1 m as well as relatively large metallic particles covered by oxide film above 10 m, which are often plate-like, exist between the surfaces [40]. Compacted glaze oxide was also found on the surfaces. It seems that the number of loose wear particles between the surfaces are dependent on fretting stroke ∆re and relative humidity, and the larger the ∆re the more the particles and the higher the relative humidity the fewer the particles. The generation of friction noise is thought to be related to the behavior of such particles directly or indirectly. Especially, fluctuation of the rate of load carried by wear particles to the whole load may be related to the generation of friction noise. 4.4. Effect of restricting oxygen on friction noise In order to investigate the influence of oxide debris on the generating of friction noise, an investigation was made in dry and wet Ar gas, in which oxide debris does not tend to occur. The Ar gas used is 99.999% in purity. Though complete exclusion of oxygen from the chamber is impossible, limited oxide free experiment can be made. As shown in Fig. 9, it is found that values of Lp in Ar gas are lower than those in air. This phenomenon, may be attributed to restricting effect of oxide debris. The penetration of oxygen into the contacting surfaces is also limited in distilled water or grease lubricant. In addition, wear debris generated are easily washed away from the contacting surfaces. No friction noise during fretting may be attributed to those effects.
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Fig. 15. (a) Experimental result fretted in distilled water, fretting stroke of 191 m, after 20,000 cycles; (b) experimental result fretted in grease lubricant, fretting stroke of 191 m, after 10,00,000 cycles.
Fig. 16. Lp (dB value) plotted against Vave ; 30% RH, 25,000 cycles.
Fig. 17. Specific wear rate Vs and sound level Lp ; fretting stroke of 191 m, 7.2 Hz, 25,000 cycles.
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Fig. 18. Type of contact point.
Table 3 Effect of types of contact point on µ and Rc Type of contact point
µ
Rc
Oxide film/oxide film Metal/metal Loose particles Glaze oxide/glaze oxide Glaze oxide/metal
0.33 1.09 0.69 0.83 0.83
Low Very low High Low Low
4.5. Possible mechanism of friction noise The observation of fretted surfaces and measurement of electrical contact resistance suggest that many types of real contact points can be formed during fretting in air without lubricant, as illustrated in Fig. 18. Since, these real contact points have different values of coefficient of friction with each other, as shown in Table 3, average coefficient of friction over the whole apparent contact area may fluctuate due to dominant types of real contact points. If drastic reduction in average coefficient of friction occurs during half cycles of fretting resulting from changes in dominant types of real contact points, it can act as a trigger to release an elastic strain energy stored in the fretting system, and to generate friction noise. On the other hand, the fretting in distilled water or in grease lubricant does not change the types of real contact points drastically, so friction noise tends not to be generated.
5. Conclusion The carbon steel (0.45%) was fretted against the mild steel with the crossed cylinder configuration in air without lubrication, and the occurrence of friction noise was investigated in a wide range of fretting conditions. The following conclusions were obtained:
1. Certain cycles of fretting are needed to generate friction noise. 2. Friction noise always occurs during the half cycle of fretting (tension process), but never during the other half (compression process). 3. There are common features in relation to the occurrence of friction noise: drastic reduction in coefficient of friction µ, self-excited-vibration and negative gradient of µ. 4. Sound level of friction increases with increase in fretting stroke and frequency, and is directly related to average sliding velocity. 5. There is a good correlation between the sound level and amount of wear.
Acknowledgements We would like to express our appreciation to Profs. R.B. Waterhouse and I.R. MacColl of Nottingham University for many helpful suggestions and discussions. References [1] M. Inoue, J. Jpn. Soc. Mech. Eng. C 51 (466) (1985) 1433–1439. [2] K. Ohta, K. Kagawa, T. Eto, S. Nishikawa, J. Jpn. Soc. Mech. Eng. C 50 (457) (1984) 1585–1593. [3] H. Okamura, M. Nishiwaki, J. Jpn. Soc. Mech. Eng. C 54 (497) (1988) 166–174. [4] T. Senda, M. Nakai, M. Yokoi, Y. Chiba, J. Jpn. Soc. Mech. Eng. C 50 (449) (1984) 125–133. [5] M. Nakai, Y. Chiba, M. Yokoi, J. Jpn. Soc. Mech. Eng. C 47 (423) (1981) 1466–1475. [6] R.A.C. Fosberry, Z. Holubechi, MIRA Report, 1957/3. [7] A.M. Lnag, H. Smales, I. Mech. E. C 37/83 (1983) 223. [8] A. Felske, et al., Oscillations in Squealing Disk Brake — Analysis of Vibration Modes by Holographic Interaerometry, SAE paper 780333, 1978. [9] S.W.E. Eales, A Mechanism of Disk-Brake Squeal, SAE paper 770181, 1977.
T. Jibiki et al. / Wear 251 (2001) 1492–1503 [10] N. Millner, An Analysis of Disk Brake Squeal, SAE paper 780332, 1978. [11] N. Miller, A Theory of Drum Brake Squeal, Braking of Road Vehicles, Loughborouugh, Paper C 39/76, London, 1977. [12] H. Lamb, Flexure and the Vibration of a Curved Bar, London Mathematical Society, 1888. [13] H. Stappenbeck, VDI-Z 96 (6) (1954) 171. [14] M. Nakai, M. Yokoi, J. Jpn. Soc. Tribol. 35 (5) (1990) 295–300. [15] M. Yokoi, M. Nakai, J. Jpn. Soc. Mech. Eng. C 45 (391) (1979) 346–355. [16] M. Yokoi, M. Nakai, J. Jpn. Soc. Mech. Eng. C 46 (404) (1980) 383–391. [17] M. Yokoi, M. Nakai, J. Jpn. Soc. Mech. Eng. C 46 (412) (1980) 1523–1530. [18] M. Yokoi, M. Nakai, J. Jpn. Soc. Mech. Eng. C 46 (412) (1980) 1531–1538. [19] M.J. Rudo, J. Sound Vibr. 46 (2) (1976) 381. [20] N. Millner, I. Mech. E, 1976. [21] N. Millner, SAE 780332. [22] R.A.C. Fosberry, MIRA Report, No. 1957/3, 1957. [23] M.J. Rudd, J. Sound Vibr. 46 (3) (1976) 381. [24] R.F. Miller, J.G. Slaby, Deutschekautschuk-Gesellschaft, 1960.
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[25] R.P. Jarvis, B. Mills, Roc. Inst. Mech. Eng. Pt. 1 178-32 (1963-64) 847. [26] H.J. Klepp, Angew. Math. U. Phys. 20 (1969) 537. [27] R. Baglin, R. Courtel, Lett. Appl. Eng. Sci. 1–6 (1975) 571. [28] R.M. Carson, K.L. Johnson, Wear 17 (1971) 59. [29] P.R. Nayak, J. Sound Vibr. 22 (3) (1972) 297. [30] J.J. Stoker, Nonlinear Vibrations, Interscience, New York, 1950, p. 125. [31] D.E. Gilsinn, A hight order generazed member of averagings, Sim. J. Appl. Math. 42 (1) 1985. [32] W.J. Cunningham, Introduction to Nonlinear Analysis, McGraw-Hill, New York, 1958, p. 32. [33] W.S. Krogdahl, Z. Angew, Math. Phys. 11 (1) (1960) 15. [34] E. Kanehara, A. Asai, M. Muraki, in: Proceedings of JAST Tribology Conference Tokyo, May 1999, D17 (1999) 263–264. [35] T. Jibiki, M. Shima, J. Sato, Jpn. J. Tribol. 36 (4) (1991) 282–288. [36] M. Shima, Y. Kimura, Proc. ITC Yokohama, 1995, p. 265. [37] T. Sasada, J. Jpn. Soc. Lubr. Eng. 34 (5) (1989) 364. [38] T. Kayaba, A. Iwabuchi, J. Jpn. Soc. Lubr. Eng. 24 (9) (1979) 598. [39] R.B. Waterhouse, Fretting Corrosion, Pergamon Press, Oxford, 1975, p. 127. [40] M. Shima, J. Sato, Jpn. J. Tribol. 30 (3) (1985) 201.
A basic study of friction noise caused by fretting T. Jibiki a,∗ , M. Shima a , H. Akita b , M. Tamura b a
b
Department of Mechanical Engineering, Tokyo University of Mercantile Marine, Etchujima 2-1-6, Koto-ku, Tokyo 135-0044, Japan Technical Research Centre, Hitachi Construction Machinery Co. Ltd. 650, Kandatsu-machi, Tsuchiura City, Ibaragi Pref. 300-0013, Japan
Abstract Fretting may be accompanied by friction noise and preventing or reducing it can be important for designers and operators. In this study, 0.45% carbon steel (Hv730) and mild steel (Hv240) are used as specimens with the crossed cylinder configuration. Friction force, relative movement between the specimens, friction noise and electrical contact resistance are measured simultaneously during fretting, with normal load 19.6 N, and up to 25,000 cycles in air. Fretting stroke, relative humidity and frequency are varied in the range 25–400 m, 11–78% RH and 1.0–7.3 Hz, respectively. In addition, the wave signals of the friction noise, together with coefficient of friction and relative stroke, are analyzed by FFT analyzer. The main results are as follows: certain cycles of fretting are needed to generate friction noise; there are common features in relation to the occurrence of friction noise; drastic reduction in coefficient of friction µ; self-excited-vibration and negative gradient of µ, sound level of friction increases with increase in fretting stroke and frequency, and is directly related to average sliding velocity; there is a good correlation between the sound level and amount of wear. Based on the results, the mechanism of the friction noise is discussed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Friction noise; Fretting wear; Sound level of friction; Wear debris
1. Introduction Fretting may be accompanied by friction noise and preventing or reducing it can be important for designers and operators. In the past, a lot of studies concerning the friction noises of disk brake, dram brake, tire for automobile and wheel for railway have been reported [1–13]. These friction noises are often called “squeal”. The sound level of such friction noises is generally large and has high frequency. Basic studies concerning the mechanism of friction noises have also been reported [14–33]. Yokoi and Nakai reported that there were two different kinds of friction noise, that is, “rubbing noise” where sound level is low, and “squeal noise” where sound level is high [15]. They also reported that the friction noise tends to occur when the gradient of friction velocity is negative [14]. Recently, Kanehara et al. concluded that friction noise is often caused by stick-slip motion in the case of relatively low friction velocity [34]. Fretting is a phenomenon greatly affected by atmosphere, and wear debris easily accumulates between the contacting surfaces. Therefore, wear debris may play a crucial role in the generation of friction noise. Thus, it is valuable to investigate the relation between friction noise and fretting ∗ Corresponding author. Fax: +81-3-5245-7435. E-mail address: [email protected] (T. Jibiki).
behavior in a wide range of fretting conditions. However, there is no report associated with friction noise under fretting condition. In this study, a system which is able to measure and analyze friction noise under fretting conditions has been developed, and the waveform of friction, friction noise and electrical contact resistance have been measured and analyzed, and also the influence of various parameters such as relative humidity in air, fretting stroke and frequency on friction noise are examined. In addition, observations of fretting surfaces are made by a laser or optical microscope. Based on the results, the mechanism of generating friction noise is discussed.
2. Experimental 2.1. Apparatus Fig. 1 shows a schematic diagram of the fretting rig. A driven specimen is attached to a cantilever, which is horizontally oscillated by a motor through a crank mechanism. A fixed specimen is attached to an upper holder. The test apparatus is covered with a chamber in order to control the relative humidity, which also makes possible the experiment in Argon gas. Friction force is measured by four strain gauges attached on two leaf springs of the upper holder.
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Fig. 1. Schematic diagram of the fretting rig.
Table 1 Specimens
Table 2 Experimental test conditions
Fixed specimen (upper specimen) Driven specimen (lower specimen)
Carbon steel (0.45%) quenched Hv730, Ry 1.0 m Mild steel Hv240, Ry 1.0 m
Relative stroke ∆re between both specimens, which is peak to peak amplitude of tangential movement between the upper and lower specimens, is measured by an eddy current pick up. Friction noise is measured by a microphone and a sound level meter, which is able to measure up to a sound frequency of 8 kHz. Both ac and dc output signals from the sound level meter are used for the analysis. Friction force, relative stroke between the specimens and friction noise are
Fretting stroke Normal load Atmosphere Test duration Frequency Temperature Relative humidity Data sampling frequency Configuration
25–400 m 19.6 N Laboratory air 25,000 cycles 1.0–7.3 Hz 294 ± 2 K 11–78% RH 5 kHz (4096 data) Crossed cylinders
measured simultaneously during fretting. All the data of these signals are fed into a personal computer through A/D converters for the wave analysis.
Fig. 2. Typical example showing waveforms of friction noise (ac output), coefficient of friction µ, and relative stroke ∆re .
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2.2. Specimens Table 1 shows details of the specimens used for the fretting test. Both the fixed and driven specimens are cylindrical, 10 mm in diameter and 30 mm in length. The fixed specimen is a 0.45% carbon steel quenched by high frequency heating (Hv730), and the driven one is a mild steel (Hv240). Both specimens were finished within the range of the surface roughness Ry 1.0 m by grinding process. 2.3. Fretting test Table 2 shows the test conditions. Fretting tests were carried out with normal load 19.6 N, and up to 25,000 cycles in air. Fretting stroke ∆re , relative humidity and frequency varied in the ranges of 25–400 m, 11–78% RH and 1.0–7.3 Hz, respectively. In addition, the wave signals of the friction noise, together with coefficient of friction and relative stroke, are analyzed by FFT analyzer. The data sampling frequency of the A/D converter is 5 kHz, and 4096 data per second can be stored into a memory.
3. Results 3.1. Measurements and analysis of waveforms Fig. 2 shows a typical example showing waveforms of friction noise (ac output), coefficient of friction and relative stroke. Friction noise always occurs during half cycle of fretting where the lower driven specimen moves to the left side direction in Fig. 1, when the leaf springs supporting the upper specimen are subjected to tension force. This half cycle of fretting is defined as the tension process, the other one as the compression process. During the compression process friction noise never occurs. This behavior will be discussed in Section 4. It is also found that friction noise starts to generate just after the sudden reduction in coefficient
Fig. 4. FFT analysis of Lp , µ, and dl/dt.
of friction at the beginning of the tension process, and is continued until the end of the process. Fig. 3 shows the precise waveforms around the beginning of the tension process together with the velocity of relative motion, dl/dt, which are plotted magnifying the time scale. There is a good correlation between the three waveforms, which have the same period.
Fig. 3. Magnified time axis around the beginning of tension process.
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Fig. 5. Another typical example of waveforms, fretting stroke of 401 m, 30% RH, 1 Hz.
Fig. 4 shows the results of these three waveforms analyzed by FFT analyzer (power spectrum analysis). It is obvious, that they have the same peaks of power spectrum at 1.4 kHz in frequency. This means, the fretting system is in a self-excited-vibration state when the friction noise is generated. This frequency of 1.4 kHz is never changed in the wide range of fretting stroke and driving frequency. Therefore, the frequency is considered to be dependent on the dynamic response of fretting system.
Fig. 5 is another typical example, where the fretting stroke is about two times larger than that above. From these results it is found that there are three common features in relation to generation of friction noise. Firstly, drastic reduction in coefficient of friction occurs at the beginning of the tension process and friction noise starts to begin at the same time. Secondly, the self-excited-vibration state often takes place in the fretting system at a frequency of 1.4 kHz. Finally, friction noise never takes place during compression process.
Fig. 6. Electrical contact resistance Rc and friction noise Lp (dc output) together with µ and ∆re vs. fretting cycles, fretting stroke of 191 m, 46% RH, 7.2 Hz, 25,000 cycles.
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These common features are observed without any exception in the wide range of fretting conditions in air. 3.2. Measurement of electrical contact resistance Rc Electrical contact resistance Rc was measured in order to estimate contact conditions during the occurrence of friction noise. Measuring method is as follows [35]: a 0Ω is set under the condition of metal–metal contact; and ∞Ω is set under the condition of electrical insulation. Fig. 6 shows electrical contact resistance Rc and friction noise Lp (dc output values) together with coefficient of
friction µ and fretting stroke ∆re versus fretting cycles. It is found that certain cycles of fretting are needed to generate friction noise, depending on fretting stroke. The fluctuation of Rc is not so large until intense friction noise occurs, but once it starts to begin Rc fluctuates between 0 and ∞ Ω. Fig. 7(a) and (b) shows the precise Rc and Lp (ac output) together with µ and ∆re before and after the occurrence of friction noise, respectively. For the former, the fluctuation of Rc is small for both tension and compression process, but for the latter very intense fluctuation of Rc periodically takes place during the tension process. The fluctuation period corresponds well to that of Lp . On the other hand, during the
Fig. 7. (a) Precise Rc and Lp (dc output) before the occurrence of friction noise, after 300 cycles; (b) precise Rc and Lp (ac output) after the occurrence of friction noise, after 25,000 cycles.
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compression process, which is not accompanied by friction noise Lp , Rc remains low, which means that at least partial intimate metal–metal contact is taking place between the contacting surfaces. This shows that there is a good correlation between the behavior of Rc and the occurrence of friction noise. 3.3. Observation of wear surfaces Fig. 8(a) and (b) show the appearance of wear surfaces before and after the occurrence of friction noise for the 0.45% carbon steel, and Fig. 8(c) and (d) similarly for the mild steel. In the case of the former, i.e. before the generation of noise, the wear surface of the mild steel is almost covered with gray oxide debris (Fig. 8(c)) which is easily
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removed by wiping. On the contrary, the wear surface of the 0.45% carbon steel mainly consist of bright area which means metallic surface (Fig. 8(a)). On the other hand, in the case of the latter, i.e. after sound generation, both the surfaces of the mild steel and the 0.45% carbon steel consist of partially reddish/gray/black area in which loose oxide debris, glaze oxide layer adhering to the surface and partially bright area may be identified (Fig. 8(b) and (d)). The complicated contacting surfaces may be involved in a lot of real load bearing contact points such as metal–metal, glaze oxide–glaze oxide, glaze oxide–metal, and oxide film–oxide film contacts [36]. In addition, agglomerated loose particles between the contacting surfaces may bear a part of the total load during fretting. This will be discussed later.
Fig. 8. (a) Wear surface observation before the occurrence of friction noise (0.45% carbon steel, fretting stroke of 191 m, 30% RH, 1000 cycles); (b) wear surface observation after the occurrence of friction noise (0.45% carbon steel, fretting stroke of 191 m, 30% RH, 25,000 cycles); (c) wear surface observation before the occurrence of friction noise (mild steel, fretting stroke of 191 m, 30% RH, 1000 cycles) precise Rc and Lp (ac output) together with µ and ∆re before and after the occurrence of friction noise, respectively (for the former the fluctuation of Rc is small); (d) wear surface observation after the occurrence of friction noise (mild steel, fretting stroke of 191 m, 30% RH, 25,000 cycles).
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Fig. 9. Influence of relative humidity on friction noise Lp (dc output), fretting stroke of 191 m,7.2 Hz, 20,000 cycles.
3.4. Influences of various parameters on friction noise Influence of various parameters on friction noise such as relative humidity, fretting stroke and frequency were investigated. Fig. 9 shows the influence of relative humidity on friction noise Lp (dc output). Coefficient of friction µ (peak–peak value divided by 2 from waveform measured) and total wear volume Vt are also plotted in this figure. In this study, Vt was calculated by Eq. (1) [37,38] as follows:
Fig. 10. Influence of fretting stroke on Lp , µ, and Vt at the humidity of 30% RH, 7.2 Hz, 25,000 cycles.
Vt =
π a4 4r
(1)
where a is the mean value of damaged half width in the fretting direction and that perpendicular to the direction, r the radius of the specimen. Sound level of friction is a maximum at 50% RH. On the contrary, wear volume is gently decreased with the increasing RH. Opposite to the tendency of sound level of friction, the coefficient of friction µ is a minimum at 50% RH. Thus, the sound level of friction is not directly related to the coefficient of friction.
Fig. 11. Influence of fretting frequency and fretting stroke on Lp , 30% RH, 25,000 cycles.
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Fig. 10 shows the influence of fretting stroke on Lp , µ and Vt at a humidity of 30% RH. The sound level of friction as well as wear volume increases with increasing fretting stroke. In this case, µ increases with increasing fretting stroke and reaches around 0.6, after which it levels off as the fretting stroke is increased. Fig. 11 shows the influence of fretting frequency and stroke on Lp . It is clearly seen that the sound level tends to increase with increasing frequency and stroke.
4. Discussion 4.1. Difference between tension and compression processes on friction noise From Section 3.1, it was found that friction noise always occurred during the tension process, and the waveform was quite different during the tension and compression processes. In order to examine the unexpected behavior additional experiments were conducted. First of all, the spring stiffness of the fretting apparatus was examined under the two loading conditions shown in Fig. 12(a) and (b), distinguishing the stiffness in tension process from that in compression process. Fig. 13 shows the spring system of the fretting apparatus schematically. The relations between given displacement s and followed tangential force of the spring system F are shown in Fig. 14(a) and (b). It is clearly seen that the stiffness in tension process is the exactly same as that in compression process, 2.2–2.3 N/m, under the no gross slip condition for both loading types. On the contrary, some reduction in the stiffness takes place during the tension process under
Fig. 12. Two loading conditions.
Fig. 13. Spring system of the fretting apparatus.
the gross slip condition for the loading type (A), while such phenomenon never occurs for the loading type (B). This phenomenon may be attributed to the change in tangential stiffness of contacting surfaces, which can be caused by slight leaning of the contacting surfaces due to the bending of the upper specimen holder. All the experiments to measure the friction noise were made using loading type (A) in this study, so it is assumed that friction noise tends to easily occur in the tension process where the spring stiffness is lowered. In addition, the slight leaning of the contacting surfaces itself may be involved in generation of friction noise through expulsion process of wear debris between the contacting surfaces. It is well-known that wear debris can be easily expelled from the contacting surfaces under lubricated condition [39]. So, fretting experiments were carried out in distilled water and in grease lubricant to examine the effect of wear debris on tangential stiffness of contacting surfaces. As shown in Fig. 15(a) and (b), both waveforms of µ and ∆re are symmetric for the tension and compression processes, while no friction noise is generated. These results suggest that the asymmetry of tension and compression processes may be responsible for wear debris between the contacting surfaces. Finally, electrical contact resistance Rc was measured, as described in Section 3.2. As shown in Fig. 7 there is a good correlation between the behavior of Rc and friction
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becomes more predominant with increasing Vave . Thus, the sound level of friction increases linearly with the increase in the Vave . 4.3. Relationship between wear volume and sound level of friction
Fig. 14. Relations between displacement s and tangential force F .
noise, and the intense fluctuation of Rc takes place only during the tension process. This result, also suggests that wear debris accumulated between the contacting surfaces may play an important role in generation of friction noise as well as tangential stiffness of contacting surfaces. 4.2. Influence of fretting stroke and frequency In the Section 3.4, it was found that the sound level of friction increases with the increase in both fretting stroke and frequency (Figs. 10 and 11). These parameters are directly related to relative sliding velocity between the contacting surfaces. So the average sliding velocity Vave was calculated, and the sound level of friction Lp (dB value) was plotted against Vave . The result is shown in Fig. 16. It is obvious that there is a good linearity between Lp and the log of Vave . The result indicates that “squeal noise” which is of high sound level tends to occur and to generate a self-excited vibration state when the sliding velocity is relatively high, and that “rubbing noise” and/or “stick-slip” easily occur when the sliding velocity is relatively low. In fretting, sliding velocity is not constant but varies sinusoidally. Therefore, “squeal noise” and “rubbing noise” exists simultaneously in each cycle. The ratio of two kinds of friction noise is considered to depend on Vave strongly, and “squeal noise”
In the Section 3.4, the influence of relative humidity, fretting stroke and frequency on Vt together with Lp , µ were examined (Figs. 9 and 10). Using those results, the relation between specific wear rate Vs and sound level Lp was plotted (Fig. 17). Vs was calculated from Vt , load W , and sliding length L. The result shows that there is a clear linearity between Vs and Lp , when the relative humidity is kept constant (30% RH). On the other hand, the data slightly deviate from the line if the relative humidity or fretted cycles is different. However, the tendency that Vs increases with increasing Lp is clearly recognized, that is, the higher the sound level the larger the wear volume, and vice versa. These results mean that sound level is strictly connected with fretting wear. Thus, the behavior of wear debris between the contacting surfaces is thought to play an important role in generating friction noise. Previously, one of the authors examined the debris formed between steel surfaces, focusing on the particle size and shape of the debris, and found that great many of oxidized debris below 1 m as well as relatively large metallic particles covered by oxide film above 10 m, which are often plate-like, exist between the surfaces [40]. Compacted glaze oxide was also found on the surfaces. It seems that the number of loose wear particles between the surfaces are dependent on fretting stroke ∆re and relative humidity, and the larger the ∆re the more the particles and the higher the relative humidity the fewer the particles. The generation of friction noise is thought to be related to the behavior of such particles directly or indirectly. Especially, fluctuation of the rate of load carried by wear particles to the whole load may be related to the generation of friction noise. 4.4. Effect of restricting oxygen on friction noise In order to investigate the influence of oxide debris on the generating of friction noise, an investigation was made in dry and wet Ar gas, in which oxide debris does not tend to occur. The Ar gas used is 99.999% in purity. Though complete exclusion of oxygen from the chamber is impossible, limited oxide free experiment can be made. As shown in Fig. 9, it is found that values of Lp in Ar gas are lower than those in air. This phenomenon, may be attributed to restricting effect of oxide debris. The penetration of oxygen into the contacting surfaces is also limited in distilled water or grease lubricant. In addition, wear debris generated are easily washed away from the contacting surfaces. No friction noise during fretting may be attributed to those effects.
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Fig. 15. (a) Experimental result fretted in distilled water, fretting stroke of 191 m, after 20,000 cycles; (b) experimental result fretted in grease lubricant, fretting stroke of 191 m, after 10,00,000 cycles.
Fig. 16. Lp (dB value) plotted against Vave ; 30% RH, 25,000 cycles.
Fig. 17. Specific wear rate Vs and sound level Lp ; fretting stroke of 191 m, 7.2 Hz, 25,000 cycles.
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Fig. 18. Type of contact point.
Table 3 Effect of types of contact point on µ and Rc Type of contact point
µ
Rc
Oxide film/oxide film Metal/metal Loose particles Glaze oxide/glaze oxide Glaze oxide/metal
0.33 1.09 0.69 0.83 0.83
Low Very low High Low Low
4.5. Possible mechanism of friction noise The observation of fretted surfaces and measurement of electrical contact resistance suggest that many types of real contact points can be formed during fretting in air without lubricant, as illustrated in Fig. 18. Since, these real contact points have different values of coefficient of friction with each other, as shown in Table 3, average coefficient of friction over the whole apparent contact area may fluctuate due to dominant types of real contact points. If drastic reduction in average coefficient of friction occurs during half cycles of fretting resulting from changes in dominant types of real contact points, it can act as a trigger to release an elastic strain energy stored in the fretting system, and to generate friction noise. On the other hand, the fretting in distilled water or in grease lubricant does not change the types of real contact points drastically, so friction noise tends not to be generated.
5. Conclusion The carbon steel (0.45%) was fretted against the mild steel with the crossed cylinder configuration in air without lubrication, and the occurrence of friction noise was investigated in a wide range of fretting conditions. The following conclusions were obtained:
1. Certain cycles of fretting are needed to generate friction noise. 2. Friction noise always occurs during the half cycle of fretting (tension process), but never during the other half (compression process). 3. There are common features in relation to the occurrence of friction noise: drastic reduction in coefficient of friction µ, self-excited-vibration and negative gradient of µ. 4. Sound level of friction increases with increase in fretting stroke and frequency, and is directly related to average sliding velocity. 5. There is a good correlation between the sound level and amount of wear.
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