Hub Motor Housing and Axle Design for an E-Scooter

The problem

Hub motors are the most common drive on Indian electric scooters because they remove the chain or belt, but they put the motor, bearings, axle and often the brake inside a small wheel that takes every pothole. Students often model the motor as a solid cylinder and miss the details that decide whether it can be built and serviced: bearing seats, cable exit through a hollow axle, sealing, brake integration and axle flats that carry torque into the frame.

Objectives

• Package an outer-rotor hub motor, two bearings, a brake drum and a tyre rim within an example 10-inch wheel envelope.
• Model the housing, end covers and stepped axle as parametric parts driven by global variables.
• Select bearings from catalogue data and model fits, shoulders and retaining features correctly.
• Size the axle for combined bending and torque by hand and confirm with an optional static study.
• Produce a fully dimensioned drawing set with fits, tolerances and an exploded assembly.

What you need

Key concepts

Outer-rotor hub motor

The stator is fixed to the axle and the rotor with magnets forms the hub shell, so the wheel rim is bolted or cast onto the rotating housing.

Fixed axle with flats

The axle does not rotate. Machined flats or a D-profile fit into the dropout slots and react the motor torque into the frame or swingarm.

Bearing fits

The rotating housing carries the outer races, so the housing bore gets an interference fit and the axle a transition or sliding fit, following the bearing maker's guidance.

Unsprung mass

Mass not supported by the suspension. A heavy hub wheel raises it and worsens ride on rough roads, so mass is a design output to report.

Step-by-step guide

1. 1

   Set requirements and the wheel envelope

   Write a requirements table for an example scooter: 10-inch rim, 90/90-10 tyre, example 1500 W continuous motor, 150 kg total vehicle-plus-rider load with 60 per cent on the rear, a 130 mm drum brake and a 160 mm dropout spacing. Sketch the envelope as a 2-D section on the Right plane showing rim seat, tyre, drum and dropouts. Treat every number as an example; take real values from a measured wheel if you have one.

   Checkpoint: A requirements table and an envelope sketch with every limit dimensioned and fully defined.

2. 2

   Create global variables

   Open Tools > Equations and add global variables for rim seat diameter, stator outer diameter, magnet thickness, air gap, stack length, bearing bore, axle diameter and dropout spacing. Link sketch dimensions to these variables rather than typing numbers. Add an equation for rotor inner diameter as stator diameter plus twice the air gap. This lets you test a longer stack or a larger bearing later by editing one value, and keeps the housing, axle and covers consistent with each other.

   Checkpoint: Changing stack length in the Equations dialog rebuilds the envelope sketch without errors.

3. 3

   Model the stepped axle

   Revolve the axle from a half-section sketch with shoulders for both bearings, a central knurled or splined seat for the stator, threaded ends and a bore or slot for the phase and Hall cables. Cut flats at the dropout ends with an Extruded Cut. Add a cosmetic thread for the end nuts. Apply a plain carbon steel such as AISI 1045 from the material library and note that it is an example choice, not a recommendation for production.

   Checkpoint: Mass Properties reports a sensible axle mass and the cable bore is large enough for the example cable bundle.

4. 4

   Model the rotor housing and covers

   Revolve the hub shell with a magnet-carrier bore, bearing seats in each end cover, rim flanges and an integral brake drum on the left side. Cut pockets to save mass while keeping a minimum wall, for example 4 mm for cast aluminium. Add a bolt circle with Hole Wizard to clamp the covers to the shell, and an O-ring groove on each joint to keep water and dust out of the motor.

   Checkpoint: Section View shows uniform walls, no sharp internal corners and bearing shoulders that match the bearing abutment height.

5. 5

   Insert bearings and fasteners

   From Toolbox insert deep-groove ball bearings sized to the axle and the expected load, plus circlips and axle nuts. Mate the bearings concentric and coincident to their shoulders. Check the bearing static load rating against your peak radial load with a margin, using the bearing maker's catalogue values rather than assumed ones. Add a simple stator part as a placeholder body mated to its seat on the axle so the air gap can be checked.

   Checkpoint: The assembly is fully mated, the rotor spins about the axle when dragged and only the intended degree of freedom remains.

6. 6

   Check fit and interference

   Run Interference Detection with the option to treat coincidence as interference turned off. Look for clashes between stator and magnets, brake shoes and drum, and the cable exit and dropout. Then use Clearance Verification to confirm the air gap and the drum-to-shoe gap are above your minimum. Fix any clash in the parts themselves, not by suppressing mates or hiding bodies, and rerun both checks after every fix.

   Checkpoint: Interference Detection finds no interferences other than intentional press fits, and the air gap clearance matches the global variable.

7. 7

   Size the axle by hand

   Model the axle as a beam supported at the dropouts with the wheel load applied at the bearings, plus a bump factor such as 3 g for a pothole, labelled as an assumption. Compute bending moment, torque from peak motor torque and the combined von Mises stress at the shoulder, including a stress concentration factor from a standard chart. Compare with the material yield strength and report the factor of safety in a short calculation sheet.

   Checkpoint: A hand calculation sheet giving a static factor of safety of at least 2 at the critical shoulder under the example bump load.

8. 8

   Confirm with a static study

   Run SimulationXpress on the axle part, or a Static study in SOLIDWORKS Simulation, fixing the flats and applying the bearing loads as bearing loads or split-face forces. Add a mesh control at the shoulder fillets and refine until peak stress changes by less than 5 per cent between meshes. Compare the result with your hand value and explain any difference, which usually comes from the stress concentration estimate or the fixture choice.

   Checkpoint: Simulated peak stress is within about 15 per cent of the hand calculation after the mesh converges.

9. 9

   Prepare drawings and the report

   Make drawings of the axle, shell and covers with ISO 2768-m general tolerances, specific tolerances for bearing seats, surface finish on sealing faces and a runout tolerance on the bearing seats to the axle datum following ASME Y14.5 or ISO GPS. Add an exploded view with a bill of materials and balloons. Report mass, estimated unsprung mass, the clash results and the factor of safety, and list what you would test physically.

   Checkpoint: A drawing set a machinist could quote from, and a short report with mass, factor of safety and open risks.

Testing & validation

• Interference Detection reports no unintended clashes in the final assembly.
• Axle stress from hand calculation and simulation agree within about 15 per cent after mesh convergence.
• Bearing static load ratings exceed the peak example load with a stated margin.
• Changing a global variable rebuilds the full assembly without errors.

Expected results

A parametric hub motor wheel assembly with housing, covers, axle, bearings and brake drum, a checked axle with a stated factor of safety, a mass and unsprung-mass estimate, and a manufacturing drawing set.

Take it further

• Run a Thermal study on the stator-to-axle heat path to see how much heat leaves through the axle.
• Design a cast version with draft and check it with Draft Analysis.
• Compare a 10-inch and a 12-inch wheel for unsprung mass and packaging.
• Add a disc brake mount and recheck the axle under braking torque.

Viva and review questions

Learn it at DIYguru

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