Maximizing Compactness: Axial Space Optimization Strategies in E-Drive Systems

In the era of electric vehicle innovation, the miniaturization and integration of drive systems are paramount. While radial dimensions are often constrained by power density and material strength, the axial (Y-direction) space offers a critical avenue for optimization. Through advanced structural design, process innovation, and integrated solutions, significant axial space reduction can be achieved without compromising—and often enhancing—transmission efficiency and reliability. This knowledge brief outlines key strategies for compressing the axial footprint of e-drive shaft systems.

1. Input Shaft and Motor Shaft Integration

A primary method for axial space saving involves integrating the motor shaft with the gearbox input shaft, eliminating interfaces and components.

• Gear Sleeve on Motor Shaft: The gear is directly mounted onto the motor shaft via a fine-tolerance spline fit (typically H/h). An auxiliary smooth bearing section on the shaft with a minimal clearance (~0.05 mm) provides additional support. A locknut is preferred over a circlip for final fixation to prevent axial play (which can be 0.1–0.2 mm with a circlip) during force reversals.

• One-Piece Shaft Design: This advanced approach fully integrates the motor and input shafts. It eliminates the coupling spline and can remove the need for at least one bearing, substantially saving axial length. Designs may use two or three bearing arrangements. Crucially, one bearing must be allowed to float to resolve statical overdeterminacy. Careful validation is required when combining ball and cylindrical roller bearings to assess potential side-frequency issues. For manufacturing, gear honing is used, with the distance from the gear face to the chucking position recommended not to exceed 140 mm for process stability.

2. Intermediate Shaft Assembly Space Reduction

Optimizing the intermediate shaft and its associated gears presents another major opportunity for compactness.

• Spline Press-Fit with Optional Circlip: Traditional spline press-fit assemblies require certain axial length for stability. The press force is typically controlled under 20 kN for hot pressing. An external spline chamfer aids assembly. A circlip may be added for anti-loosening when the axial forces on the intermediate shaft gear and the first reduction gear act in opposite directions.

• Tapered Roller Bearing Mounted on Gear: By integrating the bearing seat directly onto the gear, the housing can be recessed. This can compress the bearing width by approximately half, saving around 10 mm. The bearing location on the first reduction gear must be finish-turned with high precision.

• Integrated Forging of Large Gear and Shaft: Forging the large gear and shaft as a single piece is a highly integrated solution. The smaller gear is then press-fitted. This process relies on the high precision of the spline (requiring runout within 0.04 mm) to ensure the final gear alignment, eliminating post-assembly honing.

• Twin-Gear Manufacturing Process: This involves machining the large and small gears as an integrated unit. Pre-heat treatment, the large gear is hobbed, and the small gear is shaped (e.g., by skiving). Post-heat treatment, the large gear is ground, and the small gear is honed. The axial distance between the two gears is precisely calculated, typically between 9–12 mm. A key consideration is managing the difference in carburization depth between the gears of differing modules during heat treatment.

3. Summary and Industry Application

The strategic compression of axial space is a cornerstone of modern, high-power-density e-drive development. The techniques outlined—from shaft integration and bearing optimization to advanced forging and machining—demonstrate a systematic approach to saving every millimeter. Implementing these strategies allows engineers to design more compact, efficient, and robust electric drive systems, directly contributing to improved vehicle packaging, weight reduction, and performance. Mastery of these optimization principles is essential for staying at the forefront of electrification technology.