Coaxial Bicycle Drive Unit Design for Weight Reduction
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Solution Overview
Problem
Existing electrically assisted bicycle drive units are bulky and heavy due to their long casing, which limits design freedom and increases weight, while also inefficiently utilizing motor assistance due to complex transmission mechanisms.
Innovation Solution
A compact and lightweight bicycle drive unit is designed with a motor integrated into the crank axle, a transmission mechanism with selectable gear ratios, and a torque combining member to efficiently combine pedaling and motor forces, featuring a coaxial crank and motor axis, a one-way clutch to prevent back-torque transmission, and a reduction gear mechanism for efficient motor operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If three separate shafts (crank axle, transmission shaft, output shaft) are used in the drive unit, then the transmission mechanism can function properly, but the casing becomes long and bulky, increasing weight and reducing design freedom
Solution Approach 1:
The patent merges the crank axle and motor output shaft into a single coaxial structure. The motor is positioned such that its output shaft coincides with the crank axle, eliminating the need for separate transmission shafts and reducing the number of components. This integration directly reduces the drive unit's weight and casing length while maintaining transmission functionality.
Solution Approach 2:
The crank axle serves multiple functions: it acts as both the crank mechanism component and the motor output shaft. This multi-functionality eliminates redundant components and simplifies the overall structure, contributing to weight reduction without compromising the transmission mechanism's operation.
2Reliability
If three separate shafts are used in the drive unit, then the transmission mechanism can operate, but the casing length increases, reducing design freedom
Solution Approach 1:
By combining the crank axle and motor output shaft into one coaxial structure, the patent significantly reduces the casing length and simplifies the overall layout. This creates more design freedom for positioning other components and configuring the bicycle frame, while the transmission mechanism continues to operate effectively through the integrated structure.
3Device complexity
If the motor output is directly connected to the crank axle without a one-way clutch, then the structure is simpler, but the crank torque is transmitted to the motor, reducing efficiency
Solution Approach 1:
The patent introduces a one-way clutch as an intermediary component between the motor output and the crank axle. This clutch allows the motor to provide assistance during pedaling but prevents back-torque from the crank from being transmitted to the motor, thereby protecting the motor from unnecessary load and maintaining energy efficiency. The one-way clutch adds minimal complexity while significantly improving energy utilization.
4Adaptability or versatility
If the planetary gear mechanism operates frequently, then gear ratio changes are achieved, but the transmission mechanism lifespan is reduced
Solution Approach 1:
The patent employs a dynamically switchable transmission system where the planetary gear mechanism can be engaged or disengaged based on operational needs. By using a one-way clutch and selective gear engagement, the system minimizes unnecessary planetary gear operation, reducing wear and extending the transmission mechanism's lifespan while still providing gear ratio changes when required.
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 more compact, lightweight drive unit that efficiently utilizes motor assistance, allowing for smooth gear changes and extended transmission mechanism lifespan by reducing the frequency of complex planetary gear operation.
Implementation Method 1
the output of the motor to be transmitted to the torque combining member through a one-way clutch
Implementation Method 2
the output of the motor to be transmitted to the torque combining member through the reduction gear mechanism
Data Source
AI summary
A bicycle drive unit includes a motor, a transmission mechanism and a torque combining member. The motor includes a crank axle receiving hole. The transmission mechanism includes a plurality of selectable gear ratios. The torque combining member is operatively coupled to the motor and the transmission mechanism to combine an output of the motor and an output of the transmission mechanism together.