Simulation of Transient Seal Friction in Pneumatic Components

  Sectional View of a Pneumatic Valve

In pneumatic components, the highly non-linear seal friction has a significant influence on the system dynamics. Nowadays, costly experiments are the only possibility for precise system design by generating characteristic curves for all relevant operating points. The aim of this research project is the development of a validated simulation model, which correctly predicts the transient seal friction in pneumatic components on the basis of geometry, lubricant and material parameters.

 
Benefit Procedure
Simulation-supported prediction of pneumatic seal friction Cooperation with the Institute for Machine Elements (IMA) Stuttgart
More accurate system design Physically motivated simulation model
Reduction of the development time Prediction based on geometry, lubricant and material parameters
Avoidance of undesirable operating conditions Generation of characteristic maps for coupling with 1-D simulation environments

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Transient Seal Friction in Pneumatic Components

Seal friction occurs in a variety of fluidic components. While cylinder friction in hydraulics, for example, is often only of minor importance, frictional forces in pneumatics have a significant influence on system behavior due to the significantly lower pressures.

Seal friction is highly non-linear. A measured friction force curve as a function of speed is shown in the figure below as an example for a cylinder. The friction is not only dependent on the speed (Stribeck effect) and the applied pressure p but also on the direction of movement and the current acceleration. In addition, the temperature has a considerable influence on the frictional force, both through its influence on the viscosity of the lubricant and due to the temperature-dependent material properties of the sealing material. Furthermore, the downtime of a pneumatic system also has a considerable influence on the frictional force. Up to now, the friction behavior described can only be inadequately represented by simulation models, which is especially true for the transient behavior.

  Diagram of the Seal Friction of a Pneumatic Cylinder over the Cylinder Speed Seal Friction in a Cylinder, Measured Values

In cooperation with the Institute for Machine Elements (IMA) at the University of Stuttgart, this research project aims to develop a validated simulation model of transient seal friction that correctly predicts seal friction in pneumatic components on the basis of geometry, lubricant and material properties. To validate the model, the seal friction of a valve slide is measured at the IMA and compared with the calculated friction forces.

 

Dynamic Seal Simulation at ifas (ifas-DDS)

Within the context of a Reinhard-Koselleck project of the DFG, a physically motivated model for describing the transient behaviour of translational hydraulic seals has been developed at ifas in cooperation with the Forschungszentrum Jülich in recent years.

Only physical parameters are required as input variables, which are either known or can be determined by standardized experiments. These include, for example, the material data of the sealing material, the density and viscosity of the fluid, system pressures and temperatures or the structure of the surface of the mating body.

The simulation is based on a finite element (FE) model in the ABAQUS software. This model was extended by a detailed description of the solid contact and the fluid film by means of so-called user subroutines. The simulation model uses the following theories and equations:

  • Transient Reynolds equation for modeling the fluid
  • Flow factors according to Patir and Cheng to describe the influences of surface topography
  • Model for contact mechanics and rubber friction according to Persson

The model has already been successfully validated for the friction and wear of translational hydraulic seals. The aim of the current project is to extend the existing simulation model so that the transient frictional force of translational pneumatic seals can also be calculated correctly. The model will be experimentally validated with a test rig, which is currently under development by the project partners at the Institute for Machine Elements (IMA) at the University of Stuttgart.

  Structure of the Dynamic Seal Simulation of ifas (ifas-DDS) Structure of the Dynamic Seal Simulation of ifas (ifas-DDS)
 
 

Modelling of Pneumatic Seal Friction

Seal Passes a Counterface with Diameter Jump

Discontinuous Counterface

In contrast to cylinder rods, for example, the counter surface in conventional pneumatic valves is not continuous. Instead, there are diameter transitions on the counter surface, which have a significant effect on the friction force. The influence of these diameter transitions must therefore be considered in the simulation model.

 

Non-Newtonian Lubricant

While hydraulic oils can be in good approximation to be Newtonian fluids for many operating conditions, the lubricating greases used in pneumatics exhibit highly non-Newtonian behavior. This behavior is first measured and characterized in the project and then implemented into the simulation model.

 
Deficient Lubrication Between Seal and Counterface

Deficient Lubrication

Unlike valves in hydraulics, pneumatic valves are not continuously supplied with new lubricant during operation. Therefore, there may be a local undersupply of lubricant. The theoretical lubricant film height cannot be achieved here, so that the proportion of solid body friction increases, which affects the resulting frictional force. This must be taken into account in the simulation in order to depict this critical system state.

 

Validation and Map Generation

For the validation of the simulation results, the project partners at the IMA are developing a test rig for the measurement of seal friction in pneumatic valves. During validation, an exemplary sealing contact and a reference valve will be measured on the test rig and calculated using the simulation model created during the project.

After the validation, the simulation will be used to generate characteristic maps for various operating conditions, which can be used directly for the correct mapping of the dynamic frictional force in 1D system simulations.

  Project Schedule Project Schedule: From the Physics of the Sealing Contact to the Integration of the Frictional Force in 1D System Simulations
 
 

Acknowledgement

The project is supported by the Forschungskuratorium Maschinenbau e.V. - Fluid Power Research Fund of the VDMA. The ifas would like to thank all project participants.