Case

Case Highlights

A new-energy vehicle OEM was developing a front-drive e-axle for a C-segment passenger EV. The original prototype reducer used standard helical gears, but whine noise at highway speeds and mesh efficiency were not yet at target.

DD Gear delivered a two-stage small-module helical gear set with optimized micro-geometry and case-hardening steel (18CrNiMo7-6), a material widely used in high-load automotive gears thanks to its strong, tough core and wear-resistant case. After tuning on DD Gear prototypes, the project achieved:

• Quieter mesh in key speed ranges (improved interior NVH impression).

• Higher mesh efficiency, contributing to better energy consumption. Research shows that properly optimized helical gears and surface finishing are essential for high-efficiency EV reducers.

Customer Background & Project Scope

• Customer industry: New-energy passenger vehicle OEM.

• Application: Single e-axle with a two-stage helical reduction between a high-speed PMSM traction motor and the differential.

• Target vehicle: Front-wheel-drive compact EV used for city and highway commuting.

• Key requirements:

  • High efficiency over WLTP-type drive cycles.

  • Very low gearbox whine, because EVs lack engine masking; tonal gear noise is easily heard.

  • Consistent quality at automotive mass-production volumes.

The OEM’s baseline design already used helical gears, but gear micro-geometry, material grade and grinding needed to be improved to meet aggressive NVH and durability targets.

Challenges

1. Gear whine around cruising speeds

   • Interior tests showed noticeable tonal noise from the reducer at typical mesh frequencies during 80–110 km/h cruising.

   • Even though helical gears inherently run smoother and quieter than spur gears, transmission error (TE) and manufacturing deviations can still excite annoying tones in EVs.

2. Efficiency & thermal performance

   • The reducer needed better mesh efficiency to support competitive kWh/100 km figures.

   • Literature indicates that finely ground or polished helical gear flanks and optimized micro-geometry are key to reducing losses and avoiding micropitting in EV gearboxes.

3. Durability at high speed & torque

   • High input speed and peak torque demanded a deep case-hardened alloy steel with good core toughness. 18CrNiMo7-6 / 17CrNiMo6 case-hardening steels are commonly used in heavy-duty industrial and automotive gears for this reason.

DD Gear Solution

1. Helical Gear Design & Micro-Geometry

• Two-stage helical layout

  • Maintained the overall ratio requested by the OEM, but adjusted tooth counts and helix angles to improve contact ratio and mesh kinematics.

  • Helical gears are preferred in EV transmissions because gradual tooth engagement reduces impact, vibration and noise compared with spur gears.

• Multi-objective micro-geometry optimization

  • Applied profile and lead modifications (crowning, end-relief) based on contact and dynamic simulations to minimize loaded transmission error over the main torque/speed window. Similar studies show that careful tooth modification is an effective method to reduce vibration and noise in helical gear transmissions.

  • Focused on those gear pairs contributing most to audible whine, following EV transmission noise analysis practices.

2. Materials & Heat Treatment

• Case-hardening steel

  • Selected 18CrNiMo7-6 for high-load gears; this case-hardening steel offers excellent surface hardness, core strength and toughness for gears and shafts in demanding transmissions.

  • Case depth and hardness were specified to match the OEM’s load spectrum.

• Heat treatment & finishing

  • Carburizing + quenching followed by fine gear grinding; grinding is recommended in EV gear design guidelines to achieve high efficiency and reduce micropitting risk.

  • Surface finishes were tuned to support low-viscosity oils used for efficiency in modern EV transmissions.

3. Manufacturing & Gear Accuracy

• ISO 1328 accuracy

  • Tooth profile, helix deviation and pitch controlled to ISO 1328 Grade 4–5, a high accuracy class suitable for low-noise, high-speed helical gears.

• Process control

  • SPC on critical dimensions, flank deviations and runout.

  • Routine gear-pair tests for TE, loaded contact pattern, and backlash before release.

Results

After adopting the DD Gear gearset and running multiple prototype loops:

• NVH

  • Interior measurements showed significant reduction in tonal gear noise in the previously critical speed bands.

  • The cabin sound profile became more dominated by road and wind noise, which is the typical design goal for refined EVs.

• Efficiency & thermal behavior

  • Bench tests indicated a measurable improvement in reducer efficiency (in line with studies that show optimized helical gear design and finishing can boost efficiency).

  • Temperature rise of the gearbox oil under steady-state highway operation was reduced compared with the baseline design.

• Durability & robustness

  • Endurance testing under the OEM’s duty cycles showed stable contact pattern and no early micropitting or scuffing in the test window.

  • The OEM approved the DD Gear design for SOP and listed DD Gear as a key gear supplier for this EV program.

Typical Technical Specifications

Representative; DD Gear customizes parameters for each EV platform.

Case Summary

By combining high-grade case-hardening steel, EV-oriented helical gear micro-geometry, and ISO Grade 4–5 gear accuracy, DD Gear helped this EV OEM:

• Reduce gear whine and tonal noise in the e-axle.

• Improve mesh efficiency and thermal behavior under real driving cycles.

• Validate a robust, series-production gearset for its new EV platform.

This approach is suitable for passenger EVs, hybrid drive units and light commercial EVs that demand high efficiency and low NVH.

Developing a new e-axle, motor reducer or auxiliary EV drive?

Contact DD Gear’s engineers to co-design small-module helical gears that balance efficiency, NVH and durability.