Electric Vehicle Gearbox Design

The Question

Can a single-stage gear pair be designed to transmit 95kW at 5000 RPM with minimum mass and volume, while remaining quiet, reliable, and manufacturable — meeting a specified 12,000-hour service life?

Why It Matters

Every kilogram of unnecessary gearbox mass in an electric vehicle directly reduces range and increases unsprung/drivetrain weight. But minimising mass isn't free — it trades off against noise, vibration, harshness (NVH), manufacturing cost, and fatigue life. Getting this trade-off right, using rigorous stress analysis rather than rules of thumb, is exactly the kind of design discipline that separates a gearbox that survives its rated life from one that doesn't.

What I Did

Designed the gear pair's macro-geometry (tooth numbers, module, facewidth, helix angle) iteratively in Dontyne Systems' professional Gear Production Suite, balancing mass against ISO 6336 contact and bending stress safety factors
Chose a prime pinion tooth number (31) to minimise hunting-tooth frequency, and a helix angle giving an integer overlap ratio of 2 for quiet running
Specified micro-geometry modifications — tip relief, helix crowning, end relief — to minimise transmission error (TE), the direct driver of gear noise and vibration
Ran tooth contact analysis (TCA) to simulate the real contact stress distribution across the tooth flank under load
Accounted for real-world misalignment sources — manufacturing tolerance, gearbox case tolerance, and load-induced shaft deflection — rather than assuming perfect alignment
Selected material (20MoCr4 case-hardened steel) and manufacturing route (hobbed, carburised, induction hardened, superfinished) to meet the required hardness and fatigue strength

Tooth micro-geometry (tip relief, lead crowning) tuned to minimise transmission error — achieving just 0.08 microns peak-to-peak.

Key Result

Colour contour map of contact stress across the pinion tooth flank, peaking at 399.8 MPa, showing an even load distribution across the full face width — Simulated contact stress across the tooth flank — the even distribution (no hot spots at the tooth edges) confirms the micro-geometry corrections are working as intended.

Hand-drawn engineering cross-section of the gearbox showing the pinion and wheel shafts, bearings, and torque flow direction — Hand-drawn cross-section showing how axial thrust from the helical gears is reacted through the bearings and housing.

The final design achieved a total gear pair mass of just 14.3kg while sustaining the full 95kW rating, with contact stress safety factors of 1.24–1.25 and bending safety factors of 3.50 — both comfortably above the minimum of 1.0, even after accounting for manufacturing misalignment, shaft deflection, and dynamic loading.

Impact / What's Next

Being honest about a design's remaining weaknesses is as important as reporting its strengths: the bending safety factor (3.50) came out considerably higher than needed relative to the contact factor (1.24), meaning some further mass could likely be trimmed with a different material or geometry choice — and the gear pair's resonance ratio (0.70) sits close enough to a known problem region to flag for further vibration analysis before this design would be production-ready. Identifying exactly where a design falls short, and why, is what turns a single analysis into a design that can actually be iterated on.