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 scale
Global scale
Mixed lubrication model
• Modified Reynolds equation


 h3
u h 

   p (h)
Phd       s (h)

12

2




 pxx (h)  pxy (h) 
  sx (h) 



 p ( h)  
,  s (h)  


 pyx (h)  pyy (h) 
 sy (h) 
– Incorporating the effects of the local scale
fluid flow through flow factors
Mixed lubrication model-friction
calculation
– Friction is calculated as the sum
of friction from asperity contact
and hydrodynamic friction
Ftot   LC  FH ,
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