Tribology Analysis And Design Method Of Line Contact Higher Pair Mechanisms

Tribology research involving friction control, wear reduction, improvement of lubrication property, reasonable play of materials potential and so on has important significance in engineering. Due to small contact area, big contact stress etc. in the line-contact higher pair mechanisms, the issues related to its friction, wear and lubrication have been always paid high attention by scientific researchers. Many research achievements have been reached in relative field nationally and internationally. However, the tribological research on line-contact higher pair mechanisms is very unbalanced. Currently, the studies on tribology engineering application mainly focus on the wheel-rail contact, rolling bearing and gear contact. However, the tribological researches on the line-contact higher pair mechanisms such as cam are relatively few. Moreover, the shakedown analysis on the mechanical frictional contact pair is much less.Based on the basic tribology theory, combining the operating principle and the contact model of common line-contact higher pair mechanisms such as cam, gear etc. and using the basic principles and methods for contact fatigue, elastohydrodynamic lubrication and wear prediction, the tribological design and analysis methods for gear and cam in line-contact higher pair mechanisms are deeply discussed in this paper. Meanwhile the plastic deformation failure criteria of gear are discussed based on the shakedown limit analysis model of plastoelasticity mechanics. The main results and conclusions obtained are as follows:1. There is relative sliding between the tooth surfaces during the gear engagement. Sliding friction between the tooth surfaces has an affect on the calculation of tooth surface contact fatigue strength. The calculation model for the tooth surface contact fatigue strength, which considers the sliding friction, can be obtained by introducing the friction effect factor. The calculated results show that the influence of the sliding friction between gears on the tooth surface contact fatigue strength varies with the change of lubrication conditions and the average value is about 6%. To ensure reliable tooth surface contact fatigue strength in gear design, the allowable safety coefficient for the tooth surface contact fatigue can be appropriately increased or it can be directly calculated with the checking model by text derivation. 2. Straight moving roller disc cam is prone to fatigue pitting and wear failure, so the contact stress of the cam must be checked. The pressure angle and the actual contour curvature radius of the cam are the function of cam rotation angle. The calculation of pressure angle and contour curvature radius of the cam should be performed by deriving the equation of motion rule repeatedly. With the provision of the calculation model for cam contact stress and the corresponding calculation program based on MATLAB, the requirements for calculating the cam contact stress can be met in engineering practically.3. According to the contact strength requirement of cam and in combination with the cam transmission condition, the relationship between contact stress, allowable pressure angle, base circle radius and eccentricity, etc. is discussed and the design model of straight moving roller cam meeting the contact strength requirement is provided.4. Through calculating the minimal oil film thickness and the ratio of roughness depth to film thickness of the straight moving roller disc cam, the lubrication status of cam mechanism can be estimated, thus the possibility of cam wear can be judged. Cam lubrication relates to the roughness of contact surface, the properties of lubrication oil and cam materials, the movement rule of cam driving system, the cam parameters and so on. All factors aforementioned constantly change during the operation of cam. The problem of dynamic lubrication on the cam contour is converted into quasi-steady problem through the method of point-by-point distribution calculation. With the provision of calculation program for the minimal cam oil film thickness based on MATLAB, the variation of the minimal cam oil film thickness with the change of cam rotation angle can be known. Therefore, the lubrication status of various points on the cam contour can be learnt.5. Based on the calculation model for the minimal oil film thickness of flat-bottomed disc cam, the relationship between the minimum cam contour curvature radius, the base circle radius, the motion rule of followers and the minimal oil film thickness is discussed. The design model of straight moving flat-bottomed disc cam is provided based on the theory of elasto-hydrodynamic lubrication, which can determine the optimal base circle radius of cam according to the given cam lubrication status. Calculation examples show that the lubrication effect of cam can be improved by reasonably selecting the best cam base circle radius and contour shape, or choosing reasonable lubrication oil and additives, or improving the surface quality of the cam and followers in most economical processing methods.6. Since the adhesion wear is one of the major failure modes of the straight moving flat-bottomed follower disc cam, considering the affect of lubrication conditions on the cam wear, the simplified relationship model between the film thickness ratio and the adhesion wear coefficient is established based on the film thickness ratio of cam lubrication derived from the elasto-hydrodynamic lubrication theory. Thus the coupling calculation model for the adhesion wear of flat-bottomed follower disc cam and the elasto-hydrodynamic lubrication is provided. Calculation examples show that this model can approximately estimate the wear conditions of various contour points on the flat-bottomed follower disc cam.7. Tiny plastic deformation will occur on the tooth surface in initial gear engagement stage. The corresponding residual stress is produced inside the gear teeth under the action of the contact stress, which is helpful to improve the resistance of the gear to the contact stress. Considering the influence of the residual stress, the contact shakedown limit of gear material is derived based on lower bound shakedown theorems. It is indicated that the utilization ratio of gear material can be improved by about 50% in case this shakedown limit is used as the allowable value of tooth surface plastic deformation failure rather than elastic limit.