Modelling and simulation of hydraulic motor tribology

Patrik Isaksson

Agenda

• Introduction

• Mixed lubrication model

• Test rig-validation

• Micro EHL

• Tribology modeling - Project leakage reduction

Introduction-tribological interfaces

• Piston assembly

• Distributor plate

• Seals• Rolling

element bearings

• …

Tribology of hydraulic motor

– Wide speed and load range

– High loads-EHL– All lubrication

regimes has to be considered

Modelling tribology of hydraulic motors

• Fluid dynamics– Thin film approximation of Reynolds

equation is used• Structural deformation

– High loads• Fluid property model

– Viscosity dependence• Surface roughness

-when film thickness is small

Mixed lubrication-modelling strategy

• Separate the problem into two different scales– Local scale representing surface roughness– Global scale representing the piston bearing structure– Deformation is calculated on both scales

Local scaleGlobal scale

Mixed lubrication model

• Modified Reynolds equation

– Incorporating the effects of the local scale fluid flow through flow factors

2)(

12)(

3 huhP

hh shdp

)(

)()(,

)()(

)()()(

h

hh

hh

hhh

sy

sxs

pyypyx

pxypxxp

Mixed lubrication model-friction calculation

– Friction is calculated as the sum of friction from asperity contact and hydrodynamic friction

,HCtot FLF

Simulation software

• Commercial software package-COMSOL– Based on finite element method– Facilitates flexibility as a design

tool– Fluid dynamics and other

needed equations can easily be implemented

– Full system solution approach

Test rig-validation

• Friction, leakage and oil film pressure is measured in the roller-piston interface

Validation-modeling test rig setup

• Mixed lubrication model applied in roller/piston interface

Validation-Oil film pressure

Validation-Leakage

Validation-mixed lubrication

Movie clip

Lift off zone

Not always very good correlation, some effect and/or input is missing

Micro EHL

Deterministic EHL model of high pressure zone with transverse ”roughness”

Micro EHL

Micro EHL will influence transition between full film and mixed/boundary

Tribology modeling - Project leakage reduction

Task:

• Reduce leakage in piston to 50%

• Keep low friction

Project leakage reduction-result

• New piston design in production