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In general, most analyses of radial lip seal behavior assume a static deformation state in the seal lip. Nevertheless, a dynamic excitation of the seal lip always occurs in practice, due to unroundness of the shaft or motions of the shaft center. In this first part of a two part paper the influence of dynamic excitation on the occurrence of clearances in a dry seal-shaft contact is studied, taking into account viscoelasticity and inertia of the seal material. The ultimate goal is to determine whether viscoelastic seal behavior can result in a continuous fluid film or not. First, attention is paid to the mechanical behavior of synthetic rubber under an oscillating load. The seal elastomer shows a glass transition, resulting in a strong increase in stiffness with frequency. Next, a FEM analysis of the seal-shaft contact under dynamic conditions is presented. The steady-state response of the seal is represented by a transfer function approach. It is shown that clearances develop due to viscous and inertial seal material behavior. In practice, these clearances are filled with fluid. Due to entrainment and squeeze effects a fluid film is generated. This phenomenon is designated visco-elastohydrodynamic (VEHD) lubrication, and is subject of Part 2.

It is shown in Part 1 of this work (Stakenborg et al., 1990) that dynamic excitation of a radial lip seal will result in nonuniform clearances, due to viscous and inertial seal material behavior. These clearances are filled with fluid. Due to entrainment effects in a converging part of the clearance, fluid pressures will develop, which are sufficiently high to overcome the radial preload. These fluid pressures are excellently described by short bearing theory. The viscous and inertial effects can lead to a type of full film lubrication which is designated visco-elastohydrodynamic (VEHD) lubrication. VEHD lubrication addresses the (apparent) parallel fluid film lubrication problem in radial lip seals. At present, it is the only macro-hydrodynamic theory that results in calculated fluid film thicknesses, friction torques and leakage rates that are in agreement with experimental data. A novel feature of VEHD lubrication is the increase of frictional torque with decreasing viscosity under conditions of full film lubrication and low viscosity values, hitherto believed to be mixed lubrication.

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