Planetary Mixer FEA: Blade, Shaft and Gear Strength from CFD Mixing Loads

A process equipment manufacturer was developing a double planetary mixer for a very thick, sticky material. The machine has a centre blade and two side blades that turn on their own axes while the planetary plate carries them around the bowl, all driven through one gear train. Nobody had a reliable number for the torque the blades would see in such a viscous mix, and the shafts, blade roots and gears had been sized on an estimate.

Shirsh Technosolutions worked out the real loads first, with a CFD simulation of the blades moving through the material, and then used those loads in a planetary mixer FEA in ANSYS Mechanical. Over several rounds with the maker's design team the weak zones were found, the design was changed, and the final blade and shaft assembly was checked against the allowable stress of each material.

Loads from CFD instead of an estimate. The mixing chamber and blades were simulated in CFD with the viscous material, and the reaction moments on the centre blade and side blades were read from the solution. These came out much higher than the torque the client had first estimated, so the CFD values were agreed with the client and carried into the structural work. Torques at the motor, the eccentric gear and the blades were later reported in a consistent set of units for the maker's drive selection.

Assembly model and mesh. The blade assembly CAD was cleaned of fasteners, small holes and cosmetic chamfers, then meshed with second-order solid elements (hexahedral and tetrahedral) and shell elements where suitable. Element quality, aspect ratio, Jacobian ratio and skewness were checked before solving. Stainless steel and structural steel parts were assigned their own properties and allowables.

Supports and loads that match the machine. Cylindrical supports at the bearing seats left each shaft free to rotate while holding it radially and axially, and fixed supports held the gear end. The CFD moments were applied to each blade about its own axis, and the side-blade moments were also applied about the central axis to represent the planetary motion.

Iterating with the design team. The first rounds showed stress above yield at a few local regions and high shear at the blade-to-shaft joint. Shirsh reported the hot spots with stress and safety-factor plots, marked areas below the target factor, and suggested changes such as larger shafts, fillets at the blade roots and changes to the blade. The maker revised its CAD after each review and the model was re-run, until the final assembly was within the allowable stress everywhere.

Shear pin check. Using the peak moment from the analysis and the gear geometry, the shear force on the drive pin was calculated so the maker could size the pin.

Common questions

Why use CFD to find the load for a mixer FEA? In a viscous mix the torque on each blade depends on the material, the blade shape and the speeds, and simple estimates can be far off. A CFD run of the blades in the actual material gives the reaction moments directly, so the structural check starts from a realistic load.

What does design iteration with FEA look like for mixer blades? Each round shows where stress or the safety factor misses the target. The designer changes shaft size, fillets or blade shape at those spots, and the updated model is solved again, until every part meets its allowable before metal is cut.