Dişli çarklarda yenme olayının yağlama bakımından incelenmesi

Abstract

ÖZET Bu çalışmada, dişli çarkların yüksek yük ve hızlar da çalışmalarını engelleyen yenme olayı yağlama bakımın dan teorik ve deneysel olarak incelenmiştir. Buna göre 1. Bölümde yenme ile ilgili günümüzde elastohidrodinamik C EHD) teorisi; sıcaklık teorisi ve yenme yükü teorisi olmak üzere üç teorinin mevcut olduğu gösterilmiş ve tezde esasen bu teorilerden hangisinin geçerli olduğu araştırıl mıştır. Bu amaca uygun olarak 2. Bölümde yenme olayının Fiziksel esası açıklanmış ve literatür taraması yapılmış tır. 3. Bölümde yenmeyi etkileyen profillerin teğetsel hızları, toplam hızları; Kayma hızları ve izafi kayma hız ları gibi kinematik ve Hertz basınçları gibi dinamik fak törleri analizi yapılmıştır. 4. Bölümde dişli çarkların yağlama teorisinin esasını oluşturan yağın f iziko-kimyasal etkisi, EHD teorisi; ani sıcaklık, ortalama sıcaklık ve integral sıcaklık gibi çeşitleri bulunan sıcaklık teorisi, sürtünme katsayısı ve yüzey pürüzlülük gibi olayların ve teorilerin analizi ve sentezi yapılmış ve bu analiz ve sentezden tez amacına uygun sonuçlar elde edilmiştir. S. Bölümde deney çalışmaları hakkında bilgi verilmiş, deney tertibatı tanıtılmış, deneyde kullanılan dişli çarkların geometrik faktörleri ve yağların özellikleri verilmiş, yük kademeleri, hız kademeleri yenmenin tayini ve deney yöntemi hakkında ayrıntılı bilgiler verilmiştir. Aynı bölümde yenme yükü ve çalışma sıcaklığından oluşan deney sonuçları çevresel hıza bağlı olarak tablo ve diyagram halinde verilmiş ve bu sonuçların incelenmesinden pratik bakımından çok önemli neticeler ortaya konulmuştur. 6. Bölümde deney sonuçlarının irdelenmesi yapılmış ve buradan EHD yağlamada kritik yağ tabakasının kalınlığını tayin etmek için en uygun kriter aritmetik ortalamaların `kare kök` ü olduğu pürüzlüğün elastik şekil değiş tirmelerinden dolayı çalışma sırasında kritik yağ taba kasının kalınlığı sabit kalmadığı dolayısıyla teorik o larak h < h. olduğu halde gerçekten h > h, olduğu o kr ^ ^ o kr a sonucuna varıl mıştır. Tez sonuçlarının açıklandığı 7. Bölümde, tezde in celenen tüm konularla ilgili sonuçlar verilmiş ve bilhassa EHD ve sıcaklık teorilerinin hangisi geçerli olduğu sorusuna gelince : h > 1. 5h olduğu durumda tamamen hidrodinamik teorisi 1. S h. > h > O. 6h, olduğu kr o kr 3 durumda EHD ve sıcaklık ; h < O. 6 h, olduğu durumda sıcaklık teorisi geçerli olduğu sonucuna varılmıştır.

SUMMARY A STUDY ON THE SCUFFING OF GEARS WITH RESPECT TO LUBRICATION In this paper, the scuffing cylindirical gears which prevents their use under heavy-duty and high velocity conditions, is theoritically and experimental y investigated with respect to lubrication. Therefore hav ing the differences of various damages such as scuffing Cor scoring) bending -fatigue fracture or surface wear C pil ting) pointed out, the existence of three theories regarding scuffing - the elastohydrodynamic theory, the temperature theory and the scuffing load theory- are shown in the first chapter. The main subject of the thesis is explained after having shown the principles such as h > Sh. of elastohydrodynamic C EHD) ; t < St of temperature theory consisting of contact, average and integral theories : and F <SF, of scuffing load theories. The main concern is which one of the theories mentioned above is correct. Regarding this main topic; the EHD theory, the temperature theory, the scuffing load theory, friction coefficient and surface roughness are theoritically anlyzed and synthesized; then the results of the synthesis are compared to those of the author's experimental values, before conclusion. In the second chapter of the thesis, scuffing is physically investigated and relevant literature has been reviewed. In this respect, scuffing which is a kind of adhesion wear is based on Bowdon and Tabor's microwelding theory. According to this theory, the contact area of metal ic surfaces consist of the contact areas of tiny rough surfaces. Under the effect of load, an extremely high pressure occurs upon these tiny contact areas causing microscopic metal weldings. These weldings are broken during relative movement to cause material loss which leads to adhesion wear. Scuffing is a severe type of adhesion wear. On the other hand, there are two types of scuffing, one being slight the other being heavy scuffing (scoring). After the characteristics of these types having been shown, a short history of scuffing regarding researches and investigation, is given and various present day publishings investigating different aspects of scuffing are scanned. IXIn the first, section of the third chapter named `Analysis of Dynamical and Kinematical Factors Affecting Scuffing`, relative movement between profiles is studied. In the second section of this chapter relations of surface pressure in Hertz t,ype was bualyzed with regard to equivalent curvature radius of profiles. It was concluded that when &=1, the maximum pressure is on points A and E and the ni mumum pressure is in the middle of the pitch line; the load is changing gradually during the working period, that is to say, when &>1 the maximum pressure is on the singular loading points A and E. In the fourth chapter, physico-chemical effect of oil, EHD theory, temperature, friction coefficient and roughness of surface which are basic factors of lubrication theory for gears was analyzed. In regarding to physico-chemical effect of oil; it sticks to metal ic surfaces by means of adsorption or chemical reaction, and forms a smooth film of oil when a lubricant substance is placed between these surfaces. This characteristics called oilness is related to the effects of oil and metal ic surfaces. This layer of stuck oil which prevents direct contact of metal ic surfaces might break under pressure to lead to metal ic contact which results in adhesion wear or scuffing. For this reason, fracture resistance and sliding resistance of the stuck oil layer is important. As the fracture resistance increases, the oil layers capability of carrying load also increases; and as the sliding resistance decreases friction also decreases lowering temperature. The stuck oil layer is torn up and loses it protecting property at a temperature called the critical temper at ur e and this leads to scuffing. The principles of EHD theory are studied and equations are put forward in the second section of the fourth chapter where EHD theory is analyzed. The equations to be solved here are that of lubrication surface deformation and viscosity pressure of oil. As the lubrication equation, Reynolds * unidimensional equation (equation 4.2) ; as the deformation equation Hertz deformation equation (equation 4. IS) ; and as the viscosity pressure equation, Bams' equation (equation 4. 9) have been chosen. The isothermal and thermal solutions of these equations have been handled, analythical and numerical solutions analyzed mathematically and as a result, Dowson and Higginson's [233 relation which is most commonly used in our day, is worked on (equation 4.20 and 4.34). In the third section of the fourth chapter where temperature in gears is analyzed, `Blitz Temperature`, which was suggested by Block as the cause of scuffing, is studied; and e>cistance of average temperature and integral temperature in addition in our day, is also shown. The principles ofthese theories has been studied and their equations analyzed to be rearranged to apply on gears more easily. In the fourth section of the fourth chapter, friction coefficient which takes place in temperature equations as the basic cause of high temperature on contacting profiles, has been widely analyzed. Here, friction coefficient was handled under Newtonien, Non-Newtonien fluid conditions, and experimental relations. The relations present in this field are systematized (Table 4.1!) and analyzed; and it was concluded that, the ( 4. 65) relation is the most convenient for practical appl icat ions. In the final section of the fourth chapter, surfa ce roughness is anal yzed-surf ace roughness is used as an exact criterion to define critical oil film in EHD theory; but in `Blitz` heat theory, it is shown as the cause of `Blitz` heat. However, surface roughness is expressed with some criteria such as maximum roughness, peak-to-valley height and arithmetic avarege. The first matter here is which criterion to use in defining critical film thickness ( h. ) and the second matter is kr how to add up the roughness of two surfaces in contact. Relating to the last one, there are such methods as, mathematical addition (relations 4. 71,..., 4.74), square roots of squares (relation 4.75) and simple average (relation 4.77). The matter is solved in the section where experiment results are investigated essentially, as one of the aims of the thesis. Information about experimental studies is given in the fifth chapter. The experimenting installations being a closed system are described (fig 5.3), and it is shown that there are three experimenting methods regarding this system, which are Ryden (U.S.A.), IAE (Britain) and FZG (Germany) methods. Among these, FZG has been chosen for the experiments carried out. During the experiment, the type of gears called A in FZG test is used, and the geometrical dimensions of them is given (Table 5.1) in addition to their kinematical factors (Table 5.4). Later information about the oils used during the experiments is given, and these oils are shown to be VG15, VG68, VG15, VG229 according to ISO and their. characteristics are given in Table 5.2. Besides, the values of loads stages defined according to FZG rules are given (Table 5.3), and the experiments are planned (fig 5. 5) and shown to have been carried out in four different velocities such as n=1460 rpm (-o=6.18 m/s) ; n0=2190 rpm ('i>=9.285 nvs) n =3460 rpm (-o=14.67 mVs) ; n =4500 3 r * 4 rpm ( -o=19.08 m/s). The most important matter to be defined in the experiments was the matter of defining XIscuffing. For this reason» present- methods has been analysed and it is concluded that scuffing covered 20 % of the surfaces. In the final section of the fifth chapter, the results of experiment such as scuffing loads and oil temperatures during scuffing are given in the form of unit load CF /b) C Table 5.4; fig 5.6; fig S. 7; fig S.3; fig S. 9). Based on the results of experiments, it was concluded that: velocities applied in the experiments caused only slight scuffing Cfig S. 6) the scuffing appeared first on the singular load point of pinion and spread towords the top and the bottom; it occur ed suddenly and spread out fast; as the velocity increased the load of scuffing decreased, the load of scuffing depended on the oil viscosity with all kinds of oil, the load of scuffing remained almost fixed after a certain value of velocity; scuffing occurd at low temperatures with low viscosity oils Cfig 5.8); at the stage of load where scuffing took place, the temperature increased suddenly Cfig 5.9, fig 5.10). In the sixth chapter, the experiment results are investigated. It is concluded that, the minurnum film thickness is to be calculated according to point C; the value of a=0.016 mm*VN is to be chosen, when the literature scanned for the viscosity-pressure coefficient a. taking place in relation of minurnum film thickness- delation 6.1) ; and the operating temperature is to be defined in order to establish the value of r/ taking part in the same relation. Finally a minurnum film thicness relation easy to apply on the gears is established, C relation 6. 5). As a second matter during the investigation of experiment results, the fact of defining the operating temperature to settle 77 viscosity, is handled. For this purpose the contact temperature (relation 6.6), the average temperature C relation 6.7), integral temperature (relation 6.8), Lehner's [40] amperical relation (relation 6. IO) and experimental data are compared. It is concluded that the temperatures measured during experiments could be taken as the operating temperatures of the gears. In addition the critical film thickness corresponding to various roughness criteria is calculated (relations 6.17...6.21) depending on the real roughness value of gear profiles used in the experiments (on an average R s^J. 5fjm>. Regarding to matters mentioned above, a computer program has been designed to calculate the minurnum film the flow chart of which is given in fig 8.13 (see appendix 1), and the film thickness depending on loads of scuffing and circumferential velocity is calculated by means of this program. These values are given, for VG15 oil in fig 6.15; VG88 oil in fig 6. 16; VGl500il in fig XII6.17; VG22Û oil in fig 6-18- In this diagrams» each film thickness of corresponding to each load of scuffing is represented with a special sign C%5. Besides, critical film calculated according to various criteria are given together with minumum film thickness in fig 6.19. Upon the analysis this last figure, it is concluded that the most conveni eni er i ter ion for El astohydr odynami c lubrication is the söuare root of the sum of squar ed ar i thmet i cal aver ages of surface roughness. This value indicated by h, is represented in diagrams of fig 6.15, 6.16, 6.17 and 6.18. The following conclusion is drawn upon the study of these diagrams. 1- Depending on the load and velocity, scuffing for the same pair of oil and metals occurs or fails under conditions of h >h, and h <h, o kr o kr In this respect scuffing occurs as; w=14. 67 rn/s and -o=19.08 m/s, that is at high velocities h s£ 1 -1. 3) h. : ro=G. 18 m/'s and -o=3. 285 m/s, that is at lov; (_i k r velocities hs*C 0. 6-0. 8) h,. kr As a result, for EHD lubrication, on expression like h >h. ; S=h /h, >1 o kr o kr is not valid to prevent scuffing as it is for lubrication, applied on journal bearings; in this case S is safety coefficent. 2- This characteristic of EHD lubrication can be explained with respect to two facts. One of them is that, during running elastic and plastic deformations take place on surface roughness, as a result, the established critical film thickness which is assumed to remain constant during operation decreases, and actual y h >h, whereas it is r J o kr h <h, theoretically as not to cause scuffing. And the o kr J ^ second fact is that occurence of scuffing when h < 3 o h, depend on the critical temperature. For this reason, if concepts such as first critical temper atur e C t,. ) and ma i n critical temperature C t ) were put Nil K. I /-r forward scuffing would not occur when i<t, 1 ; scuffing would occur when t>t, t but t<t, _; scorring would occur krl krO* =` when t->t. r. These explanations are estimated by the author as a contribution to the scuffing theory. In the seventh chapter with the title of `Thesis Conclusions and Future Studies`, all the results of the matters studied within the thesis are given and especially original results regarding thesis studies are emphasised. In this respect, the most important XIIIresults can be summarised as follows: 1 - As for the question `which one of the EHD and temperature theories is valid regarding scuffing`, both of these theories are concluded to be valid. Therefore when h >1. 5h. o kr is true, the hydrodynamic theory is totally valid; when l.Sh, >h >0.6h, k r o k r is true temperature theory is valid. 2- Due to the facts mentioned above, scuffing a) does not when h >I. 5h, ; o k r ' b) when l.Sh, >h >0. Sh, does not occur if t>t, kr o kr kr and occurs if t, >t>t. _ krl krO c) when h <0. 6h, does not occur if t<t. t ; o kr krl ' occurs if t, <t<t,,_ krl krO and scoring would occur if t>t, ^ kr O 3- Elastohydrodynarnic lubrication is different from hydrodynamic lubrication in quality and quantity. Here under the effect of high pressures, roughness shows an elastic and plastic deformation altering the thickness of critical film thickness as a result. Another point to be taken into consideration in this event is that; changes take place in the substance of oil under the influence of high pressures, and it becomes semi solid to prevent direct contact of surfaces even on the occassion when h < h. kr XIV