Purdue University

Purdue University

Mechanical Engineering
Tribology Laboratory

Flow Visualization in a Surface Modified Thrust Washer

Andrew Cross


Surface modification of thrust faces allows for the formation of a hydrodynamic lubrication film between the surfaces. This lubricant film reduces friction and wear. Surface pockets achieve hydrodynamic lubrication via the geometric wedge effect. The converging edge of the pocket generates fluid pressure, separating the thrust faces. The leading, diverging edge causes the pressure to drop and the lubricant cavitates.

The pressure drop theoretically should be equal and opposite to the pressure generated by the pocket but if this were so, no lifting force would be generated. Instead, the pressure drop causes the air entrained in the oil to be released. This is referred to as gaseous cavitation. If the pressure were to drop further, the lubricant would boil and vaporous cavitation would occur.



  • As the oil enters the diverging region of the pocket the pressure drops and the oil cavitates
  • A clear reformation boundary forms as the pressure rises above the saturation pressure of the lubricant






  • The Blue curve shows the cavitation area growing with increasing bearing speed
    • Increasing the speed and/or viscosity causes the cavitation ratio to increase,
    • Increasing the load reduces the cavitation ratio
  • The Red line is known as the Stribeck Curve
    • At low speeds, the bearing does not generate sufficient pressure to separate the surfaces
    • At higher speeds, the hydrodynamic film forms and the friction becomes dependent on the viscosity of the lubricant






Numerical Simulations

ANSYS FLUENT software was used to develop a model using the full three-dimensional Navier-Stokes equations to corroborate with the experimental results. This model was used as a tool to help measure variables, such as lubricant film thickness and pressure distribution that could not be found experimentally. The model represents one pocket of the fifteen pocket array.
image_thumb[10] image_thumb[11]


Experimental Results

Cavitation Area 59.7%

Cavitation Area 61.6%

Load 561.00N

Load 551.38N

Film Thickness 19.3μm

Pressure Profile image_thumb[12]

  • The pressure profile diagrams the interactions between the pockets
    • Size and number of pockets play a large factor in optimizing the washer design
    • If the pocket is too large, pressure generated in the leading pocket will be canceled by the diverging region of the following pocket
  • Future work will concentrate on mapping the pressure profile experimentally using thin film sensors.

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