Dynamic Behaviors In Orbital Grinding Of Crankshaft Pin And Their Influence On Precision And Surface Integrity

Orbital grinding,or oscillation grinding,which was originally developed for grinding of non–circular parts,has been adopted for crankshaft grinding,because the main journals and pins can be ground in a single clamping of the crankshaft.However,the technical challenge to precision and surface integrity still remains since the circumferential feeding rate varies periodically along the pin contour in orbital grinding of crankshaft pin resulting in unsteady tangential and normal grinding force.This thesis is devoted to a systematic investigation of orbital grinding of crankshaft pin on the kinematics of the grinding point,the unsteadiness of grinding force and grinding temperature as well as their influence on structure deformation and surface integrity.Orbital grinding force model is developed based on kinematics behavior investigation of grinding point in pin chasing and experiment measurement.The grinding velocity is firstly examined on its undulation due to the unsteady oscillating of the wheelhead.Followed,the variation of the feeding speed along the pin circumstance is investigated because of the planetary movement of the grinding point.The speed ratio between the feeding speed and grinding speed is reviewed to indicate the unsteadiness.By considering the interactions between grit and pin as well as the material removal mechanisms during grinding process,the models of grinding force are proposed in terms of the speed ratio and grinding depth.In order to validate the force model,a new method to measure the grinding force on site in orbital grinding is developed.A base plane is defined including pin center,journal centers,and the crank arms.The grinding force acting on the pin contour is divided into two components with one point to the journal center along the arm and the other vertical to the base plane.A rotational dynamometer is mounted to the main journal,and the relative position between the base plane and the built-in coordinate system of measured force components is determined by calibration with forces applied to the pin center.Hence,the pin grinding force components in orbital grinding can be reflected directly with the indicated force components by means of the above transformation.The effects of grinding parameters including grinding speed of wheel,the turning speed of the crankshaft and the grinding depth on force ratio of the tangential over normal components as well as specific energy are reviewed.Nonuniform deformation and its influence on pin precision are analyzed due to action of orbital grinding force and clamping force as well as the asymmetric structure.A typical unit of crankshaft including one pin and two journals together with two arms is abstracted as the physical model.The forces acting on the unit are classified into two groups,one is defined as the axial force acting along the journal such as clamping force applied to the centers at the two side of the crankshaft,another is named as the transverse loads including grinding force,gravity,centrifugal force.The transverse loads are further divided into forces in base plane and forces vertical to the base plane.The clamping load acting on the journal center results in symmetrical deformation along the pin in the base plane,which remain steady when the crankshaft rotates.The deformation of the pin due to transverse loads acting in the base plane will also be systematic but changes with rotation of the crankshaft because of the unsteady grinding force.The deformation of the pin resulted from the transverse loads vertical to the base plane will be asymmetric along the pin.The three kinds of deformations are integrated to establish the final positions of the pin cross so as to evaluate the influence of processing forces on the dimension and form errors of the pin during orbital grinding.In order to assess the improvement of auto-rest supporting system which supply constrain on main journal in orbital grinding of crankshaft pin,FEM analysis is conducted to indicate that the the asymmetric constrain and structure will influence the asymmetric deformation of the pin.The thermodynamical model is developed based on time-varied heat flux during the orbital grinding to access the characteristic of temperature along the pin circumstance.In order to obtain the heat flux into the pin,the real length of contact zone and energy partition ratio at the grain level are analyzed firstly considering the deformation of the grinding point and characteristic of the grinding wheel.Followed,the analytical model of temperature distribution under the heat flux load at specific grinding contact zone concerning the original temperature condition is developed with the uniform moving heat source assumption instead of triangular moving heat source because of the tinny grinding depth,so as to validate the prediction model of heat flux during the orbital grinding of crankshaft pin.Since the temperature distribution over the pin changes with the variation of the grinding point,which affects the temperature status as the heat flux is exerted and the thermal property of material,a two-dimensional FE model is presented with a uniform moving heat source to reveal the continuous effection of the entire orbital grinding process along the pin circumstance on the temperature distribution over the whole pin.The effects of grinding parameters including numbers of grinding cycle,the turning speed of the crankshaft and the grinding depth on grinding temperature as well as variation of the temperature along the circumstance of pin are investigated.The research shows that the difference of temperature along the pin circumstance decrease with the increasing number of grinding cycle and turning speed of crankshaft.A new test rig is developed to validate the pin temperature distribution during orbital grinding with embedded thermocouples as pin section in a reconfigurable mode.Surface integrity in orbital grinding of crankshaft pin is investigated in terms of surface roughness and residual stresses as well as their distribution along the circumstance of pin experimentally.Surface roughness of crankshaft pins are revealed to improve with grinding speed and deteriorate with the grinding depth and feeding rotation.In order to enhance the uniformity of surface finish over the pin circumstance,the spark-out grinding process is introduced to orbital grinding as the final cycle by means of the sliding and ploughing effection of grit on pin surface,which turn out to be effective in lowering both the magnitude and the divergence of the surface roughness.The residual stresses remained under surface of orbital ground pin with CBN wheel are investigated with induction heating quenching and the integral heating quenching respectively.It is reviewed that compressive residual stresses can be achieved in both conditions,while induction surface hardening offers higher compressive residual stress.With the aim to improve the uniformity of grinding quality over the pin circumstance,the research in the thesis start from the modeling and measuring of pin grinding force and temperature during orbital grinding,with the investigation of kinematic behavior of the grinding point over the whole pin circumstance.The deformation of crankshaft pins is investigated along the axis and the circumstance based upon the FEM model with experiment validation,which reveals the influencing factors including process forces and displacement constrain and their significance on both the dimension and the cylintricity of the pins.The surface integrity in terms of surface roughness and residual stress is studied to indicate the variation with the orbital grinding parameters and the crankshaft rotation,and the effective strategy is presented.The research results and conclusions will support the orbital grinding of crankshaft to achieve high quality products.