E-Bike Crank Drive with Coaxial Strain Wave Gear
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Solution Overview
Problem
Electric bicycles face challenges in providing a reliable and space-efficient drive system that enables high power transfer while maintaining low noise and high efficiency.
Innovation Solution
A drive device comprising a motor unit, a first gear stage, and a second gear stage, where the second gear stage is a strain wave gear or cycloidal gear configured coaxially with the pedal crank, allowing for a compact and backlash-free torque transmission, and a shiftable radial coupling element for decoupling the pedal crank, enabling a reliable and space-saving design.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a conventional gear system is used for torque transmission, then the structure is simple, but the transmission ratio is limited and backlash occurs
Solution Approach 1:
The gear system is divided into two distinct stages: a first gear stage for initial torque reduction and a second gear stage (strain wave gear) for final high-ratio transmission. This segmentation allows each stage to be optimized for its specific function, achieving both high transmission ratio and precision without excessive complexity in a single stage
Solution Approach 2:
A belt element is introduced as an intermediary between the motor unit and the strain wave gear. This belt transmission serves as a mediator that connects the two gear stages while allowing for smooth torque transfer and reducing the complexity of direct coupling, thereby enabling the strain wave gear to achieve its full precision potential
2Power
If a high transmission ratio gear system is used, then large torques can be transmitted, but the device size increases
Solution Approach 1:
The strain wave gear is arranged coaxially with the pedal crank, with the second gear stage nested within the space defined by the pedal crank assembly. This nested arrangement allows the high-ratio gear system to be housed within the existing bicycle frame geometry, achieving compact integration without increasing overall vehicle dimensions
Solution Approach 2:
The drive system transitions from a traditional lateral gear arrangement to a coaxial arrangement around the pedal crank axis. By utilizing the radial space around the pedal crank rather than extending the gear train laterally, the system achieves high transmission ratio without increasing the lateral footprint of the drive system
3Productivity
If the pedal crank is permanently coupled to the drive system, then continuous power transmission is achieved, but drag occurs when the bicycle is pushed or during maintenance
Solution Approach 1:
The coupling between the pedal crank and drive system is made dynamic rather than static. The shiftable radial coupling element can be positioned in different states: engaged for power transmission during cycling, and disengaged when the bicycle is being pushed or during maintenance. This dynamic coupling allows the system to adapt its connectivity based on operational requirements, eliminating unwanted drag while maintaining continuous power transmission when needed
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides a low-noise, high-efficiency, and compact drive system for electric bicycles, capable of transmitting large torques with a high transmission ratio, and allows for easy decoupling to prevent drag and facilitate maintenance, resulting in a cost-effective and lightweight design.
Implementation Method 1
a deformable cylindrical steel sleeve is provided, which is referred to as a 'flexspline' and forms the output of the Harmonic Drive and is deformed in a predetermined manner by rotation of the wave generator
Implementation Method 2
an elliptical disk element which, as a so-called 'wave generator', forms the drive of the strain wave gear
Data Source
AI summary
In an embodiment a driving device for an electric bicycle includes a motor unit having an electric motor configured to drive the electric bicycle, a first gear stage and a second gear stage, wherein the first gear stage is coupled to the motor unit on the one hand and to the second gear stage on the other hand, wherein the second gear stage is configured to output a torque configured to drive the electric bicycle, and wherein the second gear stage is a strain wave gear and is arrangeable coaxially with respect to an axis of rotation of a pedal crank of the electric bicycle.