Research On Thermoelastic Instability Of Shifting Clutches In The Process Of Engagement

The occurrence of hot spots in clutches is a major problem that limits the performanceand lifetime of clutches. On the basis of thermoelastic instability theory (TEI), the cause,imfluence factors and optimization methods of hot spots in clutches are investigated.Besides, thermoelastic instability problems of friction pairs are researched by theoreticalmodels, finite element methods and bench test experiments. From those researches, weobtain the rule of the critical speed and the dominant mode of thermoelastic instability.Additionanlly, in order to provide references for the clutch optimal design, the effect ofclutch structure parameters and material parameters on thermoelastic instability is alsoinvestigated. Meanwhile, the relation between the fomula of copper-based powdermetallurgy friction materials and the thermoelastic unstable behaviors is also researched.Major scientific works are shown as the followings:①On the basis of the perturbation method, a thermoelastic instability theoreticalmodel of friction pairs is constructed. The growth rate of the perturbation and the criticalspeed are investigated. Besides, in order to evaluate the dominant mode of thermoelasticinstability, the critical speeds under the symmetric mode and the anti-symmetric mode areresearched. Moreover, the expressions of the critical speed under two special cases, i.e.,sufficiently large perturbation frequency or nonconductive friction material, are deduced.②To investigate the effect of lubricating oil on thermoelastic unstable behaviors ofthe friction system, different theoretical models under fluid lubrication and mixturelubrication are built. The result shows that the effects of the lubricating thickness andsurface roughness on the critical speed are considerable. Besides, a relationship betweentheoretical models under dry friction condition and lubrication condition is established withan equivalent friction coefficient, which can simplify the derivation process of thetheoretical model with lubrication.③To simplify the derivation process of the growth rate of the perturbation and thecritical speed, both a two dimension and a three dimension finite element model areestablished and solved by an eigenvalue solution method. Furthmore, a contrast among thecritical speed calculated by a theoretical model, a two dimension finite element model and athree dimension finite element model is made to illustrate the advantages and disadvantagesof different models and the applicable scope of them. Additionally, the effect of intermittentcontact on thermoelastic instability and the symmetric perturbation problem are researchedby a two dimensional finite element model.④On the basis of a three dimensional finite element model, effects of a serious ofclutch structure parameters and material parameters on thermoelastic instability areinvestigated to make references for the clutch optimization design. These parametersinclude the number of friction pairs, the outer radius, the ratio between the inner and outerradius, the thickness of the friction material, the thickness of the mating material, the elstaicmodulus, the thermal conductivity and the thermal expansion coefficient of both frictionand mating material. Additionally, in order to make references for friction material improved design, both the effects of common friction materials on thermoelastic instabilityand the relationship between the fomula of copper-based powder metallurgy frictionmaterials and unstable behaviors of the system are researched.⑤A bench test experiment is designed to test the input and output rotational speedand torque, the temperature field of the mating material during the sliding process. In orderto deduce the critical speed, the perturbation of the temperature field is monitored when theposition of temperature sensors, the sliding speed of the friction pair and the frictionmaterial changes. Hot spot traces on the mating material surface are decected to deduce thenumber of hot spots. Besides, a contrast between the result of the experiment and the finiteelement model is made to prove the unstable behavior in shifting clutches and the accuracyof thermoelastic instability models.