Transient Friction and Leakage Behaviour of Translatory Hydraulic Seals

  Design of the Test Bench and Associated Simulation Model

Seals are an indispensable component in hydraulics. They close off volumes and thus enable pressure to build up and prevent leakage. Both the leakage, friction and wear behavior are highly non-linear and currently poorly understood. For this reason, the Reinhart Koselleck project "Transient friction and leakage behavior of translational hydraulic seals" is currently investigating a theoretical, validated model of the translatory moving seal, taking into account transient processes.

 
Benefit Procedure
Detailed understanding of the dynamic sealing behaviour Cooperation with Forschungszentrum Jülich
Determination of influences on the sealing process Systematic investigations on the sealing tribometer
Simulative prediction of friction, leakage and wear Development of a physical motivated EHD simulation
Identification of optimization potentials for seals Validation on the basis of test bench measurements

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Physical Motivated Simulation of Translatory Hydraulic Seals

Seals are an indispensable component in hydraulics. They close off volumes and thus enable pressure to build up and prevent leakage. Both the leakage, friction and wear behavior are highly non-linear and currently poorly understood. The classical method of investigating sealing behaviour is strongly experimentally supported. However, a profound understanding can only be gained from a close interlocking of numerical analytical and experimental considerations. Therefore, in the Reinhart Koselleck project "Transient Friction and Leakage Behavior of Translational Hydraulic Seals", a theoretical, validated model of the translatory moving seal is currently being researched, taking into account transient processes.

 

Experimental Investigation of Dynamic Hydraulic Seals

In order to examine the friction contact of a hydraulic seal in detail, a tribometer was developed, manufactured and put into operation at ifas. In this sealing tribometer an O-ring cord is pressed onto a rotating steel disc (see cover picture).

Unlike the translatory movement of a hydraulic cylinder, the rotatory contact allows not only accelerations but also stationary operating points to be observed over long periods of time. A detailed description of the test bench can be found in [1].

 

EHD simulation of the sealing contact

Elements of the Simulation Model Elements of the Simulation Model

FE-Simulation in ABAQUS

Using an FE simulation, the seal preload, the influence of fluctuating fluid pressures on the deformation of the seal and the influence of the friction force can be dynamically mapped. Non-linear, visco-elastic material behavior of the seal is taken into account. The modeling described in the following is based on the commercial program ABAQUS.

Transient Reynolds Equation for Lubricant Film Calculation

The calculation of the fluid behaviour in the dynamic sealing gap is based on the transient Reynolds equation including the flow factors according to Patir and Cheng. The fluid was implemented in ABAQUS via the subroutine user element (UEL). The pressure build-up in the lubrication gap is calculated from the current node positions and velocities.

Physically Based Solid State Contact Model

The description of the solid contact (normal contact and frictional force) is based on the model developed at PGI. For example, based on the measured spectral power density C of a surface and the known material data of the elastomer, a relationship between gap height h and contact pressure p' can be determined. With the help of the subroutine User Interaction (UINTER) the implementation of the physically based solid state contact model is performed.

 
 

Validation of the Simulated Friction Force

left: Simulated Fluid Pressures and Gap Heights; right: Simulated and Measured Friction Force left: Simulated Fluid Pressures and Gap Heights; right: Simulated and Measured Friction Force

In the adjacent graphic, the simulated fluid pressures and gap heights for three different surface topographies are shown on the left. A surface ground orthogonally to the direction of movement leads to a higher fluid pressure, to a larger gap height and, as also observed in the experiment, to a lower friction force. The right diagram shows the simulated and measured friction forces for an isotropic surface. A good agreement between measurement and simulation was found. The influences of different surface topographies, normal forces and relative velocities were shown correctly.

 
 

Simulation of Wear on Seals

Comparison of the Wear Profile of Simulated and Measured Seals Comparison of the Wear Profile of Simulated and Measured Seals

In addition to the lubrication condition and the resulting frictional force, the dynamic sealing model can also be used to simulate the wear of seals. For this purpose the wear model according to Archard was implemented. In Abaqus the subroutine UMESHMOTION serves as an interface. The figure shows a comparison of the wear profile of simulated and measured samples from the seal tribometer.

 
 

Acknowledgement

This research is carried out within a Reinhart-Koselleck project (MU 1225/36-1) of the Deutsche Forschungsgemeinschaft (DFG). We would like to thank the DFG for its financial support.

 

Publications

[1] Angerhausen, et al.: Influence of anisotropic surfaces on the friction behaviour of hydraulic seals, BATH/ASME 2016 Symposium on Fluid Power and Motion Control, FPMC2016, 2016, Bath, UK

[2] Angerhausen, et al.: Influence of Anisotropic Surfaces on the Friction Behaviour in Hard/Soft Line Contacts, 19th International Sealing Conference, 9. ISC, 2016, Stuttgart, Germany

[3] Scaraggi, et al.: Elastohydrodynamics for Soft Solids with Surface Roughness:Transient Effects, Tribology letters, Band: 65, Springer International Publishing, 2017, DOI: 10.1007/s11249-017-0878-9

[4]Angerhausen, et al.: The Influence of Temperature and Surface Structure on the Friction of Dynamic Hydraulic Seals: Numerical and Experimental Investigations, The 10th JFPS International Symposium on Fluid Power, JFPS, 2017, Fukuoka, Japan

[5] Scaraggi, et al.: Influence of anisotropic surface roughness on lubricated rubber friction : Extended theory and an application to hydraulic seals, Wear, Band 410/411, Elsevier Science, Amsterdam, 2017, DOI: 10.1016/j.wear.2018.02.023

[6] Angerhausen, et al.: Influence of transient effects on the behaviour of translational hydraulic seals, Fluid power networks : proceedings: 11th International Fluid Power Conference, 2018, Aachen, Germany