Lubrication Simulation Can Stop Those Grinding Gears
Shawn Wasserman posted on April 04, 2014 |

Phenomena: Oil Flow in a Gear-Box

Potential Industry Applications:

  • Energy
  • Oil & Gas
  • Automotive
  • Manufacturing
  • Aerospace

Main Software: STAR-CCM+

Analysis Type: CFD

Models:

  • Volume of Fluid: Eulerian calculations, immiscible phases (share temperature, pressure & velocity fields), entrapment of air, turbulence
  • Multi-Body Motion: Overset (Chimera) Method, grid overlaps

Findings:

  • Torque, pressure & flow successfully studied between teeth & around the gear box
  • Influence of oil height on flow & distribution can be studied
  • Rotation rate (after 2 rotations) seems to have comparable small influence on the distribution & gear flanks

If your industry performs some sort of power transmission then you are likely concerned with the lubrication of your gear boxes. Proper lubrication ensures that your gears won’t jam, pit, skip or lose a tooth during your high production cycles.


Oil splatters as the gear rotate.

To ensure proper lubrication during gear rotations, CFD SCHUCK’s specialist Dr. Christine Klier devised a simulation to study the flow of oil and oil distribution on gear flanks in a rotating spur gear system. Using the CFD model, you can study lubrication based on oil fill height and gear rotation speed. This allows for better gear housing design and power transmission while reducing friction and energy losses.

“To simulate the multiphase fluid flow of the system we used the VOF (Volume of Fluid) method,” explains Klier. “This method uses an Eulerian framework which assumes that immiscible fluids share temperature, pressure, and velocity fields. Using this method you can also assess turbulence  regimes and entrapment of air in the oil.”

“The motion of the gears is simulated using the overlapping overset (aka chimera) method,” continues Klier. “Using this technique, every moving body is represented by one grid. The new option of the method is that it allows overset meshes to overlap each other while the solution is computed on all grids simultaneously.


Overlapping meshes. Yellow: Active Cells, Red: Passive Cells, Blue: Acceptor Cells.
For the overset method, the simulation mesh has to contain three different cell types. These cell types are:
  • Active Cells (yellow) where the simulation equations are solved
  • Passive Cells (red) where the cell is briefly deactivated and no calculations are performed
  • Acceptor Cells (blue) contain information to calculate active cell center values at thier boundaries and face fluxes

Once the mesh was created, the simulation was able to calculate the flow of oil between the cogs and throughout the gearbox.

“This is just a case study. It demonstrates that the simulation can be used to compare oil flow in a gearbox at different fill heights. For further model validation we need experimental data,” stressed Klier. “For instance, in this simulation we assumed the temperature was at 100 ͦC (212 ͦF), by changing the oil temperature you would change the oil flow.”


Oil distribution of the gear box after one rotation.

At present, the simulation is able to study the torque, pressure and flow of the oil between the gear teeth and throughout the gear-box.

In this case study, however, the teeth appear to be well lubricated after the first full rotation. Additionally, the rotation speed of the gears at the beginning had comparable little effect on the oil fraction present on the flanks of the gears. However, the effects of the oil fill height were seen in both the oil flow and the volume fraction along the flanks of the gears.

In the future, Klier hopes to add thermo analysis to the simulation: “The oil could act as a coolant on the gears. It would be interesting to see the cooling effect on the box, walls and gear flanks. I would also like to study the effects of oil viscosity and the overall geometry.”

Klier’s employer is CFD SCHUCK. They are an engineering service consultant company which employs 35 engineers and other employees. They specialize in CAD-data preparation, design optimization, CFD simulations, post processing and consulting within various software platforms.

Resource:

Eberhard Schreck, Milovan Perić and Deryl SnyderCD-Adapco, Overset Grids Technology in STAR-CCM+: Methodology and Applications

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