Improvement of wear resistance and durability of machine elements with sliding friction pairs is the important tribological problems. The wear resistance
has been determined with many configurative parameters, technological parameters, and operational parameters. In this study kinematics of cylindrical joint (CJ), whose motion is reciprocating and rotating, and influence of various parameters on wear resistance of friction pair was investigated

Fujii, Masahiro

Vladimir I., Klochikhin

Yoshida, Akira

Okayama University Digital Information Repository

Improving Methods of Wear Resistance in Heavy Loaded SlidingFriction Pairs

Okayama University Scientific Achievement Repository

Vladimir I. KLOCHIKHIN Russian Academy of Science JSPS Postdoctoral Fellow (1994-1995) Okayama University   Masahiro FUJII* Dept. of Mechanical Engineering Okayama University 3-1-1 Tsushima-naka, Okayama, Japan  Akira YOSHIDA Dept. of Mechanical Engineering Okayama University 3-1-1 Tsushima-naka, Okayama, Japan              Improvement of wear resistance and durability of machine elements with sliding friction pairs is the important tribological problems. The wear resistance has been determined with many configurative parameters, technological parame-ters, and operational parameters. In this study kinematics of cylindrical joint (CJ), whose motion is reciprocating and rotating, and influence of various parameters on wear resistance of friction pair was investigated.    1. INTRODUCTION  Sliding friction joints (SFJ) are widely ap-plied into machine elements such as sliding bearing, cylindrical and spherical joints, and so on. The types of joint are “roller-plate”, “bush-core” and “plate-plate”. Their contact con-ditions are line contact or plane contact with fric-tion. Wear resistance of such contact pairs is de-fined with many configurative, technological and operational parameters. The principal friction parameters are L/D ration (L: bush length, D: shaft diameter), roughnesses of contact surfaces Rz, hardness of contact surface HB, design per-formance DP, clearance 2∆, angle of oscillation α, frequency of oscillation ν, and linear amplitude of oscillation Lα. In addition the important opera-tional factors are contact pressure p, sliding ve-locity VS, and environment temperature TO.  The sliding friction joints (SFJ) are operated in air or in special condition such as in vacuum. For the purpose of reducing friction between contact bodies, lubricants such as solid lubricants, some coatings, self-lubricating materials, and greases are used. The tribological characteristics of greases are improved by anti-frictional addi-tives such as polytetrafluoroethlene, molybdenum disulfide, graphite and so on. Self-lubricating materials are reinforced with fibers and powders. The best performance of improving the wear re-sistance in heavy loaded sliding friction joints (SFJ) were given with solid lubricant coatings on base of fabric layer and polymeric materials [1]. Sliding friction joints (SFJ) were generally oper-ated under heavy loads, low velocity and at high or low temperature, therefore solid lubricating materials are used for lubricant. In order to secure the reliability of sliding friction joints (SFJ), what should be taken ac-count by designers are as follows: (1) required configurative and technological parameters, (2) the numerical values of those parameters, (3) the influence of each configurative, technological, and *E-mail: fujii@mech.okayama-u.ac.jp 1Improving Methods of Wear Resistance in Heavy Loaded SlidingFriction PairsMemoirs of the Faculty of Engineering, Okayama University, Vol.39, pp.1-6, January, 2005(Received November 24, 2004)operational parameter on tribological character-istics.  Several characteristics of cylindrical joints (CJ) used in industrial field are shown in Table 1. Table 2 shows the conventional grade of opera-tional conditions of cylindrical joints (CJ). The cylindrical joints (CJ) have been widely operated under the “heavy-loaded” operational condition. In laboratory test the operation under “heavy-loaded” condition are difficult, therefore we have not so much data about the influence of parameters on tribological characteristics of slid-ing friction joints (SFJ). In this study tests have been carried out under “heavy-loaded” opera-tional conditions.  2. EXPERIMENTAL PROCEDURE    The tests were performed with cylindrical joint test machine [2]. The working stand consisted of four working steel blocks. A pair of shaft and bush specimens was fixed in each steel block. The steel blocks were interchangeable. Figure 1 shows the schematic of test machine. The test machine can work in intermittent pressure p=1-100 MPa slid-ing velocity VS=0.0002-0.05 m/s and operational temperature TO=173-532 K. Shaft and bush specimens were made of carbon steel with 0.45% carbon. The bush specimen (2) was fixed in the working steel block (1) by the fixture (5) in which a thermocouple (6) was connected. The shaft specimen (3) was fixed between two journal bearings. The friction pair (bush and shaft) was loaded by a rod of loading device (10) through a self-aligning double-row spherical ball bearing (9). In steel block a heater (4) was installed. The moment of friction is measured with tensometers (8). The signals of tensometers was transmitted to a tenso-intensifier, a oscillograph and a recorder. Tests were performed with solid lubricating coatings of FPF SLC consisting of epoxy resine and polyvinyl alcohol + frictin-polymer-forming Table 1 Technical characteristics of cylindrical joints in industrial fields Operational parameters  Industrial brunches & machines p, MPa VS, m/s TO, K 1 Food, domestic & printing brunches 1-5 0.001-0.1 0.5* 243-313 2 Railway trans-port 5-30 0.001-0.1 233-333 3 Transporters & combine 5-20 40* 0.01-0.05 0.5* 213-333 4 Tractors, trail-ers & tow cars 10-100 140* 0.01-0.05 213-333 5 Cranes, con-veyers & transporters in heavy industry 5-50 100* 0.01-0.1 213-333 6 Ship 20-90 150* 0.001-0.1 233-333 7 Aviation 5-80 100* 0.01-0.1 1* 183-353 393* 8 Space 5-100 140* 0.001-0.1 3* 143-453 573* * : peak value of parameters Table 2 Operational condition of cylindrical  joint (CJ) Operational parameters  Oscillation cycles* p, MPa VS, m/s TO, K Light loaded 4000-5000 p<10 VS<0.1 253-353 Heavy loaded 4000-5000 10<p<100 VS<0.1 193-453 Extreme loaded <5000 100<p 0.1<VS TO<193 453<TO * : Number of oscillation cycles for one opera-tion    Fig.1 Schema of test apparatus 2Vladimir I. KLOCHIKHIN et al. MEM.FAC.ENG.OKA.UNI. Vol.39filler with a coating thickness h=200µm and thin-film coating of VNIINP consisting of MoS2 + epoxy resin with a coating thickness h=20µm.   3. TEST RESULTS AND DISCUSSIONS  3.1 Kinematics and Design Performance of Joint    Kinematics of cylindrical joints (CJ) plays a significant role in wear forming. When the shaft turns against the fixed bush on some deflection angle by certain friction moment, the contact area of the shaft with the bush is transferred from the conventional equilibrium point (point A in Fig.2-a) to friction angle. In the beginning the shaft rotates against the bush without sliding and then the slide between the con-tacting surfaces starts. The angle of oscillation depends on the sliding velocity, friction coeffi-cient, clearance between shaft and bush, and degree of freedom in the friction pair (rigidity of the system). Depending on those single zone contact of sliding and double zone contact of sliding (Figs.2 and 3), wear could be considered. The sliding distance of each element of pair is determined by design performance of the sys-tem. As shown in Fig. 4 there are four types of design performance (DP) of friction pair; the direct and inverse pairs where the shaft oscil-lates against the fixed bush, that is type DP-1, and the direct and inverse pairs where the bush oscillates against the fixed shaft, is type DP-2. Taking into account the kinematics of joint, the direct friction pair is the sliding fric-tion joint, in which the harder sliding surface slides against the fixed softer surface. The in-a) Single-area contact  b) Double-area contact  Fig.2 Kinematics of moving cylindrical joint  (CJ)    a) Single-area wear b) Double-area wear  Fig.3 Projection diagram of wear  HB1>HB2         HB1<HB2  Design performance N1 (DP-1)   HB1<HB2         HB1>HB2  Design performance N2 (DP-2)    R : radius of bush   HB : Surface hardness   M : friction moment   Q : force Subscript 1: shaft       Subscript 2: bush  Fig.4 Types of design performance (DP)  3January 2005 Improving Methods of Wear Resistance in Heavy Loaded Sliding Friction Pairsverse friction pair is the sliding friction joint, in which the softer sliding surface slides against the fixed harder surface. The sliding distance of each friction pair is different from the other one. The analyses of the unfolding-diagram of slid-ing distance Sf for one cycle of oscillation angle 2α in the cylindrical joint (CJ) indicates that in most cases the sliding distance of oscillating element is not equal to the sliding distance of non-oscillating element for one cycle of oscilla-tion angle. The sliding distance of non-oscillating element would be larger than that of the oscillating element. Both thermal stress and wear of the sliding surface in the non-oscillating element would be higher than those in the oscillating element. Test results of friction pairs with different design performance indicated that wear of the inverse friction pair was greater than that of the direct pair at both room temperature and high temperature [3]. Note that when coating was employed on the oscillating surface, the term of running-in process decreased by 3-4 times in comparison with the case that the non-oscillating surface was coated. The sliding distance during operation cycle can define the sliding distance of friction pair and the correct line wear intensity of lubricant mate-rial for each design performance. For a case of design performance DP-1, the sliding distance of shaft Sf1 and the sliding distance of bush Sf2 for one cycle of oscillation angle in the cylindrical joint (CJ) are designated as follows:  )4(24:4)2(2:)2(22:20:8)(8242000000001φπαππαφπφπαφφαφφπαφα−<<<<−−<<<<⎪⎪⎩⎪⎪⎨⎧−−=RRRRRcS f  (1) 　RcS f α22 = ,    (2) where  contact. of angle-semi:(CJ)joint  lcylindrica of angle noscillatio:bush of radius:0φαR Sliding distance Sf for overall period of operation is  tcff NcSS *= ,    (3) where  Ntc: number of total operation cycles of friction joint. Parameter of line wear intensity Jh under settled friction region of contacting bodies is defined as:   SLC: MoS2+epoxy resin (h=25µm) p=40MPa, VS=0.02m/s, TO=293K  Fig.5 Relation between durability with solid lubricant coating (SLC) and configura-tive parameter L/D    ● 2∆=0.025mm  ○ 2∆=0.075mm ▲ 2∆=0.112mm  △ 2∆=0.157mm □ 2∆=0.242mm    SLC: MoS2+epoxy resin (h=25µm) p=40MPa, VS=0.005m/s, TO=293K  Fig.6 Variation of friction coefficient f and wear intensity Jh 4Vladimir I. KLOCHIKHIN et al. MEM.FAC.ENG.OKA.UNI. Vol.39flh ShJ /= ,    (4) where hl: wear of coated layer.    If we know these parameters and the thick-ness of anti-frictional material h and the fre-quency of operational cycles ν, we can determine work-time τ of lubricant material as follows: hf JShντ = .    (5)  3.2 Influence of Constructional and Technological Parameters Figure 5 shows the relation between durabil-ity of cylindrical joint (CJ) with solid lubricant coating against configurative parameter L/D. Test conditions were as follows: SLC–VNIINP, p=40 MPa, VS=0.02 m/s, TO=293 K. In L/D=0.5-0.9 the coated joint has high wear resistance. In L/D<0.5 the rigidity of the contact pair increased with in-creasing L/D. Figure 6 shows the variation of friction coefficient f and line wear intensity Jh of solid lubricant coating with oscillation angle α. In order to investigate the influence of the clearance 2∆ on tribological characteristics of cylindrical joint, tests were performed with SLC-VNIINP under p=40 MPa, VS=0.005 m/s, and TO=293 K. The wear intensity parameter has peak value in the oscillation angle α=20-40 degrees. The influ-ence of oscillation frequency ν on wear was stud-ied and great wear of friction pair was observed under the oscillation frequency when the oscilla-tion angle α equal to the contact semi-angle φ0. Judging from the kinematics of cylindrical joint (CJ) when α=φ0 as shown in Figs.2 and 3, all points of sliding surfaces of both bush and shaft take part in friction and the wear particles did not go out from the contact area that led to great wear of coating.  Figure 7 shows the results tested under dif-ferent values of clearance 2∆, sliding velocity VS. At high sliding velocity the minimum tempera-ture was observed in 2∆=60-110 µm and reduction of wear intensity of coating was also observed. That clearance would be suitable for a fitting of H8/d9 in less degree.  The influence of roughness parameter Rz of surfaces was studied with the coating FPF SLC  SLC: MoS2+epoxy resin (h=25µm) p=40MPa, VS=0.005m/s, TO=293K  Fig.7 Influence of clearance 2∆ on temperatureTO, friction coefficient f and wear intensity Jh  p=60MPa   p=100MPa Friction        Temperature   Wear  coefficient                    intensity (×10−8)  SLC: TPF SLC (h=200µm) VS=0.02m/s, TO=293K  Fig.8 Influence of clearance 2∆ on temperatureTO, friction coefficient f and wear intensity Jh HB (HRCc): 20-22 HB (HRCc): 45-50 5January 2005 Improving Methods of Wear Resistance in Heavy Loaded Sliding Friction Pairsunder VS=0.02 m/s, TO=293 K. The results indi-cated that in all cases of pressure, friction coeffi-cient, temperature of pair and line wear intensity increased remarkably in Rz> 1µm. Microcutting of coating material would take place in the cases.  In order to know the influence of surface hardness HB on tribological characteristics of friction joint tests were carried out with FPF SLC. As shown in Fig.8 the high hardness of substrate under the coating did not lead to the improve-ment of tribological characteristics of the friction pair. However, the wear rate of coating decreased with increasing hardness of the counter surface under p>60 MPa. Under low pressure p<40 MPa, the high hardness of the counter surface did not always led the reduction of wear rate of coating.  4. CONCLUSIONS  The following tribological recommendations for designing of sliding friction pairs were ob-tained. 1. Design performance of sliding friction joint (SFJ) should be selected similar to inverse friction pair. That is to say, the coating should be employed on moving surface when the sliding friction joint that the softer surface slides against the harder fixed surface is de-signed.  2. Configurative parameter L/D should be se-lected to L/D=0.5-0.9. 3. The clearance 2∆ should be selected to the fitting H8/d9. 4. Oscillation angle α for reciprocating-rotary motion of joint and linear amplitude Lα for reciprocating motion of joint should be se-lected to be semi-angle contact for recipro-cating- rotary motion of joint and semi-area contact for reciprocating motion of joint. 5. Roughness parameter Rz of surface should be Rz <0.9 µm. 6. In case under p>60 MPa, hardness of coun-terface should be more than 45 HRC.  REFERENCES [1] Y.N.Drozdov, V.I.Klochikhin, F.G.Krymov: Proc. Int. Tribochem. Symp. Lanzhou, CHAS (1989), 223-229. [2] V.I.Klochikhin: Friction and Wear (in Rus-sian), 11-3 (1990), 480-489. [3] V.I.Klochikhin: Thesis of Ph.D., IMASH, RAS, Moscow, (1991) 154.                       6Vladimir I. KLOCHIKHIN et al. MEM.FAC.ENG.OKA.UNI. Vol.39

Improving Methods of Wear Resistance in Heavy Loaded SlidingFriction Pairs

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