Unsteady lubrication in metal forming processes

Processes in the transient state, intrinsically time dependent processes, and the structural vibration of machines, result in lubrication systems which are not in the steady state, and friction is thus time dependent. A thorough understanding of unsteady lubrication phenomena can help to build a more realistic friction model for intrinsically unsteady processes, or to devise an operating strategy to avoid vibrations.; A flat-rolling process with vibration is chosen as the case for analysis. An isothermal, full film unsteady lubrication model is developed. The influences of time dependent variables such as back tension stress, roll and strip speeds, and inlet angle variation on lubrication are investigated. By utilizing this model, the inlet film thickness variation, pressures, and friction stresses in the work zone, and roll force and torque variation with respect to time can be successfully predicted. The model is extended to the mixed lubrication regime by taking the surface roughness interactions between tooling and workpiece surfaces into account. Chatter vibration in cold rolling due to unsteady lubrication phenomena is discussed. The current unsteady lubrication theory indicates that, when the nondimensional frequency of back tension stress variation is in a specific range, the roll force and torque variations can be significantly "amplified".; Thermohydrodynamic lubrication models for wedge, squeeze and combined actions are developed. Film thickness reductions due to thermal effects under these actions can be estimated. For the combined action with linear tool geometry, a lubrication model utilizing a coordinate transformation technique is developed. As an application, an unsteady thermohydrodynamic lubrication model for an axisymmetric stretch forming process is developed.