Study On Transmission Characteristics Of Dual-cones Traction Drive

The continuously variable transmission (CVT) is able to ensure the best matchbetween the vehicle running resistance and the engine output torque so that the enginecan always operate in the high efficiency region with more sufficient combustion,reducing pollution emissions and the noise level by adjusting the speed ratiocontinuously. Therefore, it has a great potential in improving the fuel economy of thevehicle and making the vehicle work at the nearly ideal dynamic performance,implying a wide range of applications. Currently, the metal belt CVT has beensuccessfully applied to the low-power automotive field. Compared with the metal belttype CVT, the traction type CVT delivers power by the shearing force of the elasticfluid film between the two rolling components, avoiding the direct metal to metalcontact, so it is easier to transfer bigger power with a higher efficiency.Dual-cones traction CVT can change the radius of both the inner and outer conessimultaneously, which produces many advantages such as fast response, compactstructure, larger power density and higher efficiency. In addition, the power transferprocess can be controlled according to actual working conditions by changing the oilpressure in hydraulic chamber, which is able to not only ensure the high efficiency,but also to control the film thickness to avoid the direct contact between the twosurfaces which usually can cause wear. This paper focuses on the study of the doublecone traction drive transmission characteristics. The traction coefficient and slip ratiobetween the rolling curves is obtained by the experimental test of oil drag traction onthe double-disc friction machine. First, combining theoretical analysis the rheologicalmodel of traction oil is established and the various rheological parameters areobtained. Second, the disc-traction mechanism existing in the double cones CVT isdetailed investigated and the mathematical EHL contact model for the contact areastudy is also developed. On the premises that the full film lubrication in the contactzone and a given transmission efficiency is guaranteed, the normal loading force inthe whole range of speed ratio is solved according to the delivery power. This researchis supposed to provide the experimental and theoretical guideline for the controlstrategy development for the double-cones traction CVT. The performance of double cone traction drive transmission device dependsstrongly on the rheological properties of the traction oil in the EHL contact zone. Thepaper first gets the traction curves under different conditions by the oil dragcharacteristics test. Based on the Evans-Johnson model, a simple method fordetermining traction oil rheological model parameters is presented and thecharacteristics of the stress, sticky pressure coefficient, ultimate shear stress andelastic modulus coefficient formula of the traction oil is fitted. Therefore an accuraterheological model for the study of the driving features of double cones traction CVTis provided.On the basis of the proposed traction oil rheological model, the disc-tractionmechanism in the double cones traction CVT is discovered. The EHL contact areamathematical model is built up and the numerical method for solution is discussed. Inthe solution process, the film thickness equation is introduced to Reynolds equation,thereby resulting in a uniform format of the differential equations. By using linerelaxation and catching method iterative calculation Reynolds equation and using FFTand discrete convolution theorem for elastic loop deformation simultaneous solutionto obtain double disc traction drives the minimum film thickness at cone-disk contactarea. As a result, the traction coefficient and normal loading force, slip roll ratio, gearratio and entrainment relationship between speed curve is obtained, and therefore thedouble traction disk drives cone-traction mechanism disc is revealed.A hydraulic control method is imposed to adjust the normal loading forcebetween the inner and the outer contact zone exerting on the double cone tractiondrives. The bigger the normal force is loaded, the larger the delivered torque will be.However, if the loading force is so large at the contact area that the oil film becomesvery thin, leading to the direct contact between the two rollers and resulting in wear.For this reason, the control strategy of the loading force is the core task. For a deeperanalysis of the correlation between the normal loading force and transmissionefficiency, the minimum film thickness, considering the comprehensive relation of themechanical properties of the inner and outer EHL contact area and the frictionbetween them, the transmission characteristics model of double cone traction drive isproposed. In order to achieve inner and outer friction contact area is in full film lubrication condition and higher transmission efficiency, the speed ratio is divided intotwo sections in accordance to the critical transmission ratio. By solving the elasticfluid lubrication equations for the inner and outer friction contact area, the normalloading force which should be provided to the double cone traction drive is given.Consequently, the changing law of the normal loading force, the contact area and theminimum film thickness in terms of the drive transmission efficiency ratio ischaracterized, providing an effective method for solving the required loading force ofthe system.