Bevel gears are one of the key elements of the power transmitting system with intersecting axes. Similar to other types of gears, study on the tribology aspects of bevel gears is important to have a perfect design and acceptable protection against wear and surface fatigue. Parameters such as lubricant film thickness and friction coefficient play an important role in bevel gear's life time. Geometrical complexity of bevel gears, however, makes it difficult to obtain distribution of these parameters for points in contact. The lubrication regime for gears is mainly mixed-elastohydrodynamic lubrication (mixed-EHL) to which lots of researches has been devoted so far. Gears, like other mechanical elements, have rough surfaces and thus it is important to consider the surface roughness in gear-contact modeling. The purpose of this study is to model bevel gear contact considering surface roughness. Each pair of straight bevel gear teeth is replaced with multiple pairs of spur gear teeth using Tredgold approximation. The transmitted load and radii of curvature are evaluated for substituted spur gears. The summation of spur gears width is equal to bevel gear face width. At each point, contact of bevel gear teeth is replaced with two cylinders. The film thickness and friction coefficient are calculated under mixed-lubrication regime in straight bevel gears. These two parameters are important in surface protection of bevel gears. Then, using the Johnson's load-sharing concept and considering elastic, elasto-plastic and plastic deformation for asperities, the load carried by asperities and lubricant film are predicted. Two components for friction coefficient (which are the asperities and the fluid film components) are considered. It is known that under mixed-lubrication regime, the friction coefficient due to the contact of asperities is higher than the friction coefficient due to the shear of the lubricant film. Thus, establishing a full EHL lubrication regime makes a considerable decease in friction coefficient. The parametric study has been conducted to investigate the effect of load, speed, material hardness and roughness on the film thickness and friction coefficient. Increase in applied torque result in a decrease in the lubricant film thickness. It has been shown that increasing the rolling speed results in an increase in the thickness of lubricant film and thus, decreases in friction coefficient. Increasing the velocity results in an increase in the lubricant film thickness and consequently the friction coefficient will decrease. Increase in surface roughness results in more asperity- to- asperity contact and thus increase the load carried by asperities and naturally increase in the friction coefficient. Material's hardness acts as spring's stiffness. Increase in the hardness of the gears results in an increase in the load carried by asperities. The contact simulation of bevel gear is conducted using Abaqus software the results are compared to the simulation results. The effect of misalignment on the contact stress has been studied. According to the results obtained, the Hertzian stress value is constant along face width while the load acting on teeth changing along the face width. Keyword : straight bevel gear, equivalent spur gear, load shearing concept, friction coefficient, film thickness
