I am designing the drive base for a humanoid robot with:

• Two powered wheels (differential drive)
• One caster wheel
• Total robot mass ≈ 50 kg

The drivetrain layout is:

Motor → Coupling → Shaft → Bearings → Wheel

The shaft is supported using two bearings, and the wheel is mounted in an overhung configuration.

Hand Calculation

I calculated the shaft diameter using:

• Combined bending + torsion
• Von Mises failure theory

The overhung distance (distance between wheel center and nearest bearing center) is:

30 mm

Using hand calculations, I obtained:

• Required shaft diameter = 9.13 mm

For safety and manufacturability, I selected:

• Final shaft diameter = 12 mm

My ANSYS Boundary Condition Plan

Now I am trying to validate the shaft using static structural analysis in ANSYS, but I am confused about the correct boundary conditions.

My current plan is:

Loads Applied

1. Apply radial load on the wheel mounting face
2. Apply driving torque on the wheel face

Supports Planned

1. Fixed Support

At the opposite end of the shaft, I was planning to use:

• Fixed support

2. Cylindrical Support

At the bearing locations, I was planning to apply:

• Cylindrical support

with:

• Radial direction = fixed
• Axial direction = free
• Tangential direction = free

My Doubt

Is this boundary condition setup mechanically correct for simulating a shaft supported by bearings in a differential drive robot?

Or should I:

• avoid fixed support entirely?
• use cylindrical supports at both bearings only?
• constrain axial motion at only one bearing?
• model bearings differently?

Thanks.