Nested Harmonic Drive Unit for Narrower Pedelec Cranksets
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Solution Overview
Problem
Existing drive units for vehicles like pedelecs are too wide due to the space requirements of rotary bearings, freewheels, and electric motors, which complicates comfortable long-distance travel and increases the overall width beyond desired dimensions.
Innovation Solution
A compact drive unit design incorporating a harmonic drive transmission with a wave generator, flexspline, and ring gear, where the electric motor is partially or fully housed in the flexspline cavity, and rotary bearings are arranged to overlap radially, reducing axial width and allowing for flexible control and efficient energy transfer between human muscle power and electric assistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional drive units with rotary bearings and freewheels are used, then reliable power transmission is achieved, but the axial width of the drive unit becomes too large
Solution Approach 1:
The patent applies nesting by placing the electric motor inside the hollow cavity of the flexspline. The motor's rotor is positioned within the flexspline's internal space, allowing the motor to be housed within the transmission components rather than alongside them. This nested arrangement significantly reduces the overall axial width of the drive unit while maintaining both the motor and transmission functions.
Solution Approach 2:
The patent transitions from a conventional linear arrangement of components to a three-dimensional nested configuration. By utilizing the radial and axial spaces more efficiently through the harmonic drive's inherent three-dimensional geometry, the design accommodates all necessary components (motor, flexspline, wave generator, bearings) in a compact volume with reduced axial projection.
2Ease of operation
If the drive unit is made narrower to improve comfort, then installation space is reduced, but it becomes difficult to accommodate necessary components like rotary bearings and freewheels
Solution Approach 1:
The patent resolves the space constraint by nesting the electric motor within the flexspline cavity. This eliminates the need for separate mounting spaces for the motor and transmission components, allowing all necessary elements to coexist in a compact arrangement that maintains riding comfort without requiring complex spatial management.
Solution Approach 2:
The flexspline serves multiple functions: it acts as both the flexible transmission element of the harmonic drive and as a housing structure that contains the electric motor. This multi-functionality reduces the number of separate components and simplifies the overall assembly, making the narrow design feasible without excessive complexity.
3Ease of manufacture
If the electric motor is placed outside the transmission components, then ease of assembly is improved, but the overall width of the drive unit increases beyond desired dimensions
Solution Approach 1:
The electric motor is positioned within the hollow cavity of the flexspline, creating a nested configuration where the motor is housed inside the transmission component rather than being mounted externally. This integration reduces the drive unit's overall width while maintaining assembly feasibility through modular component design.
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 results in a narrower drive unit that effectively transfers power while minimizing axial width, enabling comfortable pedaling and efficient energy use, with the ability to adapt speed and torque to operating conditions, thus enhancing the usability of pedelecs and similar vehicles.
Implementation Method 1
a harmonic drive transmission with a wave generator (20), a flexspline (19) and a ring gear (14)
Implementation Method 2
an electric drive motor (27) arranged around the rotation axis and having a stator (28) and a rotor (31), wherein drive energy of the electric drive motor (27) can be transmitted to the output shaft (12) via the harmonic drive transmission (18)
Data Source
AI summary
A means of transport, in particular a pedelec, which can be driven by means of drive energy produced electromotively and by human muscular power, comprising a drive unit in which an electric drive motor and a harmonic drive transmission are nested one inside the other, thereby saving space.