Electric Assist Bicycle Motor Drive Unit Internal Derailleur
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Solution Overview
Problem
The single-shaft motor drive unit in electric assist bicycles faces issues with chain durability due to smaller dimensions, interference with battery mounts, reliability concerns from external derailleur exposure, and reduced torque transmission efficiency compared to planet gear mechanisms.
Innovation Solution
An electric assist bicycle with a motor drive unit featuring a crank shaft receiving human driving force, a cylindrical human-power transmission member, a combined-force transmission member, and a speed reduction mechanism with pairs of reduction gears and a selection clutch, which combines and transmits human and auxiliary driving forces efficiently, eliminating the need for a thin chain and external derailleur.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a single-shaft motor drive unit with external derailleur is used, then gear changing is achieved, but chain durability decreases due to smaller dimensions and thinner chain required
Solution Approach 1:
The patent moves the derailleur mechanism from an external lateral position to an internal axial position within the motor drive unit. Instead of shifting the chain laterally between sprockets as in conventional external derailleurs, the invention uses a sun-and-planet gear mechanism where planetary gears engage with a sun gear on the crankshaft, achieving gear changes along the axial dimension rather than laterally. This eliminates the need for a thin chain and improves chain durability.
2Adaptability or versatility
If an external derailleur is provided, then gear switching is enabled, but reliability decreases due to exposure to external interference and obstacles
Solution Approach 1:
The patent nests the derailleur mechanism (sun-and-planet gear system) inside the motor drive unit housing. The planetary gears are contained within the motor drive unit case, protecting them from external obstacles, debris, and interference. The entire gear changing mechanism is integrated into the protected internal space of the motor drive unit, eliminating exposure to external environmental factors that would reduce reliability.
3Power
If planet gear mechanism is used for speed reduction, then torque transmission efficiency improves, but device complexity increases
Solution Approach 1:
The patent combines multiple functions into the sun-and-planet gear mechanism: it serves as both the speed reduction mechanism and the derailleur (gear changing mechanism) simultaneously. The planetary gears provide torque multiplication through speed reduction while also enabling gear switching by engaging/disengaging with the sun gear. This multi-functionality achieves high torque transmission efficiency without adding separate complex mechanisms, thereby controlling overall device complexity.
4Adaptability or versatility
If driving sprockets are laterally disposed for derailleur operation, then gear change is achieved, but chain life expectancy decreases due to smaller dimensions
Solution Approach 1:
The patent transitions from lateral gear changing (external derailleur) to axial gear changing (internal derailleur with planetary gears). The chain remains in a consistent lateral position while gear changes occur through axial engagement of planetary gears with the sun gear. This eliminates the need for the chain to accommodate lateral shifts between multiple sprockets, maintaining larger chain dimensions and extending chain life expectancy.
Data Source
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AI summary
Provided is an electric assist bicycle that can apply a sufficient auxiliary driving force with a satisfactory gear change, preclude problems caused by interference between a mount or a battery and a front derailleur, and prevent a reduction in torque transmission efficiency. A cylindrical human-power transmission member that receives a transmitted human driving force is disposed on the outer periphery of a crank shaft, and a combined-force transmission member that receives the combined force of a human driving force and an auxiliary driving force from a motor is disposed on the outer periphery of the crank shaft. The electric assist bicycle includes a speed reduction mechanism that has pairs of reduction gears and a selection clutch engageable with the reduction gears, combines the human driving force and the auxiliary driving force, and changes gears for the combined force of the human driving force and the auxiliary driving force.