Study On New Profile High Speed Timing Chain Sprocket

Roller chain drive is widely used in timing mechanism of gasoline engine, such as motorcar and motorcycle engine. However, the chordal action and the meshing impact effect of roller chain drive damage the synchronization and uniformity of transmission. So it is necessary to develop a new kind of roller chain drive to improve the transmission performances of roller chain mechanism. The chordal action and the meshing impact of roller chain drive generated during chain transmission are analyzed in this thesis. Research on developing new teeth profile sprocket is carried out aiming to decrease the meshing impact and fluctuation of chain under high speed condition.First of all, through study on the process of sprocket meshing with tight strand portion of chain, the sprocket shaping process enveloped by roller is simulated by using secondary developing tool ObjectARX of AutoCAD to control the spatial transformation of roller and sprocket blank. The overlapping area of roller and sprocket blank generated during the relative motion is removed through Boolean operation. A method of enveloping the teeth profile of sprocket by roller is proposed, so that an ideal teeth profile of sprocket is derived when sprocket meshing with chain quasi-conjugately. Simultaneously, the method of how to manipulate the position transformation of solid body in the database of AutoCAD is explained, especially the relationship between coordinate vector and transformation matrix when conducting rotational transformation. The factors influencing the precision of sprocket teeth profile and the method of how to improve it are analysied.Secondly, the forming mechanism of new teeth profile that aiming to reduce chordal action and meshing impact during chain transmission is analysized theoretically. The new profile is derived through modifying involute curve profile. The moving distance of chain and rotating arc lenth of sprocket pitch circle are guaranteed to be equal all the time. The center line of tight portion of chain is tangential to the pitch circle of sprocket. Theoretically, the transverse fluctuation of chain and fluctuation of angular velocity of driven sprocket can be eliminated. Additionally, with new profile sprocket, rollers can mesh with sprocket gradually and the number of rollers in mesh is increased so that meshing impact is reduced. The curve equation of new teeth profile is derived and the mothod of asymmetrical modification to the new sprocket teeth profile is proposed.The multi-body dynamics model of double overhead camshaft timing chain transmission mechanism of engine is established. Dynamic simulation experiment is conducted using multi-body dynamics analysis software. Under the condition of various rotating speed, the tension of chain, friction between chain and other transmission parts, fluctuation of chain, meshing impact between sprocket and roller and the nonuniformity coefficient of driven sprocket angular velocity are comparatively analyzed when using GB1244-76 sprocket, DIN8196T1-87 and new teeth profile sprocket. The simulation results show that the meshing impact between sprocket and roller, tension of chain and friction between chain and transimission parts are efficiently reduced using asymmetrically modified new teeth profile sprockets. The fluctuation of chain and angular velocity of driven sprockets are reduced and the steabilty of chain transmission under high speed situation is improved.