| In industry, absolutely smooth gear surface doesn't exist. And almost all of the gears transmissions work in partial lubrication state,in which, there are not only lubrication oil film, but also asperities between teeth faces. Undoubtedly, these asperities will have remarkable influence on surface friction and subsurface stress of the contact area . However, the current theory for fatigue strength design is based on Hertz theory which is only applicable to dry contact of smooth surfaces.Obviously, the design results according to it will have assignable errors. Therefore, it is of both academic and practical value to investigate the partial lubrication state of gear transmissions.Taking into account the non-newtonian property of the lubrication fluid, this thesis numerically analyzes the thermal partial EHL problem in line contact, using the multi-grid level method and a sequential column sweeping scheme,respectively,to solve the pressure equations and the temprature equations. On the base of a number of numerical calculations, it is investigated that what influence the rough asperities of teeth faces have on frictionand contact stress. From the calculating results, conclusions can be drawn as follows:1. In a partial lubrication state, the frictional coefficient / decreases sharply when <1.6 and then the change get slower, but the decreasing trend will last until <4.5 However, when > 4.5 with increasing, f also increases instead of decreasing. This shows that a too large value of film thickness ratio (or oil viscosity) will aggravate the wear of teeth faces.2. hr ,the average film thickness in smooth surface contact, is greater than that in rough contact. When A is relatively small ,they have a obvious difference. With A increasing, it decreases sharply, but hr is larger than hs all along. To oursurprise, the asperities result in slightly increase of film thickness instead of decrease. Having been experimentally tested, it can be explained that the transversal rough asperities will have "pump domino effects".3. More often than not, properly raising the viscosity of lubrication oil and velocity of gear will contribute to reducing the contact stress of gear tooth. However, the value of (u) is really not the larger ,the better. In fact, there is a reasonable range. If (u) is too large, the too thick oil will result in a concurrently increase of surface friction and subsurface stress of the contact area, and therefore shorten the fatigue life of gears.4. The film thickness ratio has great effect on the maximumprimary stress rmaxr in rough surface contact. When A < 2.3,with A increasing, rmax r decreases sharply. Nevertheless, but rmax r is larger than it's relevant value in smooth contactrmax,. all along.5. Both the maximum value of the primary sress rnaK and theminimum value of the average film thickness h take place at the inlet point, which is evidently the worst location of the mesh line. For this reason, it is short of scientific basis to design contact fatigue strength of gears on the base of the parameters of the critical point of gears in mesh. Besides,the contact stess of rough tooth faces is about 20~-30% greater than Hertz contact stress.Thus, in strength design of gears, the roughness of tooth faces must be taken into account.This paper has the following deficiencies:l.The non-stabilization and the side leakage effects are neglected in the numerical calculations.2.The results of this paper are in need of being verified by experiments. |
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