| INTRODUCTION The mechanism that image is transferred from the plate to paper in a sheet-fed offset press is an important issue all the time because of the complexity of printing press. ‘Gear streak’ is a king of typical printing trouble，which is a transversal strips on print。resulting from driving gears．When ‘streak’ occurs in a print it is difficult problem for all operators． Lithographic process involves the passage of paper ink through the nip between impression cylinder．and the rubber cylinder, and the passage of ink through the between rubber cylinder and plate cylinder．In the printing unit of offset presses，surfaces of impression and plate cylinders are rigid，but the surface of rubber cylinder is soft，which the rubber cylinder consists of a metal core of cylinder, cushion and pack wrapping the surface of cylinder．The ink is observed to transfer from the lower to the faster surface．Transfer was found to depend on the radio of surface speeds and adhering to the slower surface．the relative motion between a pair of involute spur gear teeth is one combined rolling and sliding except at the pitch point，where the sliding velocity is zero and changes from one direction to the other． Famous printing machinery manufactures in the world，such as Heidelberg Company，Man Roland company in Germany and Mitsubishi Company, are devoting to the effect the stability of transmission on printing quality．Sheet fed offset printing presses have a continuous gear train to drive all cylinders synchronously．Buck analyzed nonlinear vibration phenomena arise resulting from changes in the teeth contact in the gear train，taking into consideration of necessary measures in simulation and measurement with rather simple model．Yoshiaki Okubo developed a vibration control system for container cranes．In the developed system．50%of the maximum amplitude of girder vibration has been decreased to half． The process that ink is transferred through the nip of rollers is still a difficult problem．Johnson studied the process a tape or sheet of deformable material passing through the nip between hard cylindrical rollers．Usually the rolls are driven at the same peripheral speed，but small differences in speed，often referred to as 'creep’，can occur if one of the rolls is externally driven and the other is driven by the friction in the contact．The result shows that the tangential velocity of points on the flank of a tooth relative to the point of contact is always less than that of the mating point on the tip of the adjacent tooth．A printing unit comprises three cylinders contacting between plate cylinder and rubber cylinders between rubber cylinder and impression cylinder．of different diameter or identical diameter, which rotate in opposite direcfions at the angular speeds of the gears commonly referred to as cylinder gears． ‘Gear streak’ is related with the variation of angular velocity in gear rolling process．Uematsu dealed with the effect of variation of angular velocity in gear rolling on the tooth profile error．It is considered that the fluctuation of the center distance induces the variation of angular velocitV．It is assumed that the fluctuation of center distance induces the variation of angular velocity．The effect of this variation of the tooth profile error was theoretically revealed the variation of angular velocity in printing unit result to micro sliding pitch between contact teeth． All the above work studied relation between the material passing through the nip between rollers and the transmission system separately．To the best of our knowledge，no researchers revealed the mechanism of ‘gear streak’，with the way connecting the printing quality and the gear transmission．In this paper, we will derive the governing equations of relative velocity between paper and blanket in printing area．And establish the model micro sliding because the variation of centre distance of printing cylinders．Then we give a numerical example to analyze the effect of the transmission system on the ‘gear streak’． 1 THE REQUIREMENTS OF PRINTING TECHNOLOGY FOR GEAR TRANSMISSION No backlash exists between the profiler of wheel teeth on any gear transmission in theory, except for press．The printing pressure between the cylinders of the offset press，which has an effect on the printing quality, is an important technological parameter．If the printing pressure is too high，it will bring on formation of the halftone dot．If the printing pressure is too low，the dot will not be sharp and the graphic context on the printed material will be unsharp， The right printing pressure must be adjusted according to the thickness of the selected paper when the printing production carries through．It is carried out currently through the pressure adjustment device， which can change the center distance between the printing cylinders(gears)．The centre distance change between the gears will certainly affect the transmission characteristics of gears，it also influence on the velocity of the surface of blanket cylinder and impression cylinder． When printing on the lightest(thinnest) pape．the gears driving the cylinders(taking the impression cylinder with out backlash． In order to ensure the image is accurately devolved，the cylinders of the press are commonly driven by involute helical gears．Backlash of the teeth can cause the change of the speed ratio during the gear drive．It comes into being impact、vibration and noise． 2 MECHANISM CAUSING ‘GEAR STREAK’ ON PRINT 2.1 Physically squeezing course in printing units In order to determine the position that gear streak’ happens．it is necessary to be analyzed that the physical course squeezing between blanket and paper．It is assumed that no clearance between the surface of the shaft and the sleeve，and air buffer blanket is selected，and Paper is uncompressible．Radius of impression cylinder is denoted with R1，which is equal to the radius of reference circle being denoted with r．Paper is attached on the surface of impression cylinder by gripper, and the thickness of sheet is denoted with δ． In impressing area，there exists relative sliding between paper sticking to surface of impression cylinder and blanket．So friction force exists in impressing area．Blanket is deformed under the friction，and the amount of deforming lies on the distribution of friction force and its value．It is assumed that air buffer blanket is only deformed along the radial direction of impression at the contact point．Since distribution of friction force is unsymmetrical，there exists a blanket bulge at the entering position． According to the changing rule of relative velocity，the position of the point without sliding onthe blanket can be found．Furthermore，the ‘gear streak’ on the print is diagnosed． 2.2 Solution of circumferential backlash Circumferential backlash of pnnting cylinder gear influences the printing quality, since the printing pressure is adjustable through changing the center distance between cylinders．The transmission ratio of printing cylinders. 2.3 Threshold value that center distance change is allowed According to the identification ability of human eyes and print’s requirement，threshold value that center distance change is allowed call be determined． 3 CONCLUSIONS Micro slippage between blanket and paper is the essential cause inducing ‘gear streak’ in printing．Two reasons bring micro slippage．One is the blanket deforming under pressure；the other is backlash since the center distance is adjusted． According to the physical procedure，the mathematical model about sliding between the blanket and paper in impressing area is established，and the model about slippage due to the center distance change between printing cylinders is derived．It is concluded as follows： (1)In the case of assemble with standard center distance，only the center point on the line linking two cylinders’ centers．Any point except the center pint exists sliding．‘Gear streak’ is prone to occur at the exit point since the relative velocity between blanket and paper is high． (2)Center distance change results in backlash increasing，which pricking the slippage between blanket and paper．In this case，point with no sliding moves to the exist pint．When the sliding velocity reaches a criticalvalue，‘gear streak’ appears． WangYiming Zhao Jibin Wu Shuqin，(Dept．of Mechatronics Engineering，Beijing Institute of Graphic communication，China) |
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