Interlocked Crank-to-Axle Spline for Anti-Withdrawal Bottom Brackets
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Solution Overview
Problem
Crank arms in bottom bracket assemblies of micromobility vehicles can loosen or withdraw from the axle, rendering the bicycle unusable due to improper torque, deformation, or wear, leading to single-point failures that affect usability and maintenance requirements.
Innovation Solution
A mechanically interconnected bottom bracket assembly with angled surfaces and splines on the axle and crank arm, combined with a fastener system that secures the crank arm to the axle through recesses and threaded engagement, preventing withdrawal and enhancing engagement to resist loosening.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If hardware is torqued to secure crank arms to axle, then connection strength is improved, but hardware may loosen or withdraw due to improper torque or compromised torque over time
Solution Approach 1:
The connection system is divided into multiple independent retention mechanisms: splines for rotational engagement, recesses with fasteners for axial retention, and geometric interlocking features. This segmentation ensures that failure of one mechanism does not compromise the entire connection, addressing the reliability issue while maintaining connection strength.
Solution Approach 2:
The axle and crank arm features are designed with asymmetric geometries - the splines have specific angular profiles, the recesses are positioned at particular locations, and the fastener holes are oriented in specific directions. This asymmetry prevents withdrawal in the axial direction while allowing controlled rotational engagement, resolving the contradiction between strength and retention.
2Strength
If crank arms are press fit onto axle, then connection strength is improved, but contact force may cause unintended deformation of crank arm material compromising the press fitting
Solution Approach 1:
The connection design applies different mechanical properties to different regions: the splines are designed with specific material properties for rotational engagement, the recesses are positioned to avoid high-stress areas, and the fastener holes are located where they provide retention without compromising the overall structural integrity. This localized optimization prevents unintended deformation while maintaining connection strength.
3Power
If splines are used to engage crank arm and axle, then power transmission is improved, but splines may wear until they do not transfer power
Solution Approach 1:
The splines are pre-configured with specific geometric profiles and material properties during manufacturing to ensure optimal engagement from the start. The recesses and fasteners are positioned to maintain consistent contact pressure, preventing wear from the outset. This preliminary optimization extends the operational duration while maintaining power transmission capability.
Data Source
AI summary
A bottom bracket assembly with interconnected axle and crank arms. Each end of the axle has a plurality of angled surfaces and a recess formed with a depth through at least the axle perimeter and at least partially around the perimeter of the axle. Each crank arm has a slot formed a distance along the length such that the end of the crank arm is separated into two deflectable jaws. A bore is formed orthogonal to the slot and a fastener is installed in the bore, wherein the fastener has a length to span the slot in the crank arm to engage both deflectable jaws. The fastener has a shank radius that extends to a depth of the recess. Removal of the crank arm from the axle end is opposed when the fastener is in the bore.