502 菱錐式無級(jí)變速器

502 菱錐式無級(jí)變速器,菱錐式,無級(jí),變速器 LETTERImproving the fatigue strength of the elements of a steel beltfor CVT by cavitation shotless peeningHitoshi Soyama ? Masanori Shimizu ?Yuji Hattori ? Yuji NagasawaReceived: 9 May 2008 / Accepted: 19 May 2008 / Published online: 6 June 2008C211 Springer Science+Business Media, LLC 2008The elements of steel belts used for continuously variabletransmission (CVT) are subjected to a bending load duringoperation. The weakest portion of the elements is at theroot of the ‘‘neck’’ which works into metallic rings. Inorder to reduce the stress concentration, the root of theneck is rounded and the shape of element is optimized.Nevertheless, if the fatigue strength of the elements can beimproved, the steel belt can be applied to larger engines.Although conventional shot peening is one way ofenhancing the fatigue strength, it is very difficult for shot toreach into deep and narrow regions.Recently, a peening method using the impact producedas cavitation bubbles collapse has been developed [1–9].This method is called ‘‘cavitation shotless peening (CSP)’’,as shot are not required [3–6, 8]. CSP can peen the surfaceeven through deep narrow cavities, as the bubbles canreach these parts and collapse where peening is required.In the present article, improvement of the fatiguestrength of the elements of a CVT metallic belt by CSP wasdemonstrated experimentally. Elements were treated withdifferent processing times and evaluated by a fatigue test tofind the optimum processing time. In order to evaluate thepeening effect by CSP, the residual stress was measured.Note that this is the first report published on theimprovement made in the fatigue strength of a part withregions that cannot be hit directly by shot.Cavitation shotless peening was applied to the elementusing cavitating jet apparatus, the details of which can befound in references [3–6, 8]. The jet was injected into theneck region through grooves in the elements, which werestacked and held together, and scanned perpendicularlyover the elements, as shown in Fig. 1. The processing timeper unit length, tp, is defined by the number of scans n andthe scanning speed v;tp?nve1TThe cavitation number,r, a key parameter for cavitatingjets, is defined by the injection pressure, p1, the tankpressure, p2, and the saturated vapor pressure, pv,asfollows;r ?p2C0 pvp1C0 p2?p2p1e2Tr can be simplified as indicated in Eq. 2 becausep1C29 p2C29 pv. Absolute pressure values were used todetermine the cavitation number. Considering the resultsfrom previous work [3–6, 8], the CSP conditions shown inTable 1 were selected.The shape of the element tested was identical to actualelements used in steel belts for CVT. The element wasmade of Japanese Industrial Standards JIS SK5 and washeat treated in the same way as actual elements.In order to examine the improvements made in thefatigue strength, the residual stress of the elements atposition A in Fig. 2 was measured using X-ray diffractionwith a two-dimensional position sensitive proportionalcounter (2D PSPC) using the 2D method [10]. After CSP,part of the element was cut off and put into the X-rayH. Soyama (&)Tohoku University, 6-6-01 Aoba, Aramaki, Aoba-ku,Sendai 980-8579, Japane-mail: soyama@mm.mech.tohoku.ac.jpM. Shimizu C1 Y. HattoriToyota Motor Corporation, 1200 Mishuku, Susono 410-1193,JapanY. NagasawaToyota Central R&D Labs. Inc, 41-1 Yokomichi,Nagakute 480-1192, Japan123J Mater Sci (2008) 43:5028–5030DOI 10.1007/s10853-008-2743-6apparatus to detect diffractive X-rays, as shown in Fig. 2.A Cr tube operated at 35 kV and 40 mA was used. Thediameter of the collimator was 0.1 mm. X-rays werecounted for 20 min for each frame. The diffractive planewas the (211) plane of a–Fe, and the diffractive angle, 2h,was about 156 degree. The values used for Young’smodulus and the Poisson ratio were 210 GPa and 0.28,respectively. The residual stress in the longitudinal direc-tion of the element was obtained from 13 frames using the2D method.In order to evaluate the fatigue strength of the element, abending fatigue test was carried out on the element, asshown in Fig. 3. As shown in the figure, the element wasfixed and a load F was applied perpendicularly.Figure 4 illustrates the relationship between the numberof cycles to failure, N, and the normalized amplitude of thebending force,C22F, used in the fatigue test, for various pro-cessing times per unit length, tp. The amplitude of thebending force was normalized by the fatigue strength of thenon-peened specimen, which was obtained by Little’smethod [11]. The fatigue tests were terminated at N = 106,as it was confirmed that specimens which survived 106cycles also survived 107cycles. From the figure, it is clearthat CSP can extend the lifetime of specimens compared tonon-peened specimens. The normalized fatigue strength,C22FFS, of specimens treated by CSP is 1.22 at tp= 2.5 s/mm,1.38 at tp= 5 s/mm, 1.48 at tp= 10 s/mm, 1.32 attp= 20 s/mm, and 1.28 at tp= 40 s/mm, respectively. Attp= 10 s/mm, the fatigue strength of the element has beenimproved by 48% compared with that of the non-peenedelement.Figure 5 shows the normalized fatigue strengthC22FFSas afunction of CSP processing time per unit length, tp.C22FFSincreases with tpuntil tp= 10 s/mm and then decreasesTable 1 CSP conditionsInjection pressure p1MPa 30Tank pressure p2Mpa 0.42Cavitation number r 0.014Nozzle diameter d mm 2Standoff distance s mm 80Fig. 2 Measurement position of the residual stress using X-raydiffractionFig. 3 Schematic diagram of the bending fatigue test of the elementFig. 4 Improvement of the fatigue strength of the element by CSPFig. 1 Setup of the elements treated by CSPJ Mater Sci (2008) 43:5028–5030 5029123slightly. This shows that, as with shot peening, there is anoptimum processing time, and that too long processingtimes cause the fatigue strength to decrease. For the con-ditions applied here, the optimum CSP processing time perunit length was 10 s/mm.Figure 6 shows the variation in the residual stress of theelement at position A in Fig. 2 with processing time perunit length, tp. In order to evaluate the reproducibility, theresidual stress of two elements was measured for eachvalue of tpusing the 2D X-ray diffraction method. Standarddeviations for each measurement are shown in Fig. 6.Without CSP, the residual stress was -140 ± 50 MPa andafter CSP this was greater than -600 MPa. Thus, CSP canintroduce compressive residual stress into the surface evenwhere there are deep and narrow cavities. The impactinduced by collapsing cavitation bubbles can introducecompressive residual stress into surfaces that cannot be hitdirectly by shot (see Fig. 1). The residual stress on thesurface increased to between -800 MPa and -1,000 MPafor short processing times, tp= 2.5 s/mm, then decreasedslightly saturating at about -800 MPa, as shown in Fig. 6.According to a previous report [5], the compressiveresidual stress of the sub-surface in materials increasesafter the residual stress on the surface has saturated. Thusthe compressive residual stress of the sub-surface wouldincrease for tpC 2.5 s/mm. This is one of the reasons whythe optimum processing time for the present conditions wastp= 10 s/mm, even though the compressive residual stresshad reached its maximum at tp= 2.5 s/mm.In order to increase the fatigue strength of the elementsof a steel belt for CVT, the elements were treated by CSP.The fatigue strength of the element was evaluated and theresidual stress was measured by X-ray diffraction using a2D method with a 2D PSPC. It was revealed that thefatigue strength of the element could be improved by 48%by CSP. It was also shown that CSP can introduce com-pressive residual stress even into the surface of deep andnarrow cavities.This work was partly supported by Japan Society for thePromotion of Science under Grant-in-Aid for ScientificResearch (A) 20246030.References1. Soyama H, Park JD, Saka M (2000) Trans ASME J Manuf SciEng 122:83. doi:10.1115/1.5389112. Soyama H, Kusaka T, Saka M (2001) J Mater Sci Lett 20:1263.doi:10.1023/A:10109475283583. Soyama H, Saito K, Saka M (2002) Trans ASME J Eng MaterTechnol 124:135. doi:10.1115/1.14479264. Odhiambo D, Soyama H (2003) Inter J Fatigue 25:1217. doi:10.1016/S0142-1123(03)00121-X5. Soyama H, Sasaki K, Odhiambo D, Saka M (2003) JSME Int J46A:398. doi:10.1299/jsmea.46.3986. Soyama H, Macodiyo DO, Mall S (2004) Tribol Lett 17:501. doi:10.1023/B:TRIL.0000044497.45014.f27. Soyama H (2004) Trans ASME J Eng Mater Technol 126:123.doi:10.1115/1.16314348. Soyama H, Macodiyo DO (2005) Tribol Lett 18:181. doi:10.1007/s11249-004-1774-79. Soyama H (2007) J Mater Sci 42:6638. doi:10.1007/s10853-007-1535-810. He BB (2003) Powder Diffr 18:71. doi:10.1154/1.157735511. Little RE (1972) ASTM STP 511:29Fig. 5 Optimum CSP processing time per unit lengthFig. 6 Introduction of compressive residual stress into the elementby CSP5030 J Mater Sci (2008) 43:5028–5030123