Short Hollow Shaft Geometry for Accurate Torque Sensing
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Solution Overview
Problem
Current torque measurement technologies, such as strain grid gauges and magnetorestrictive sensors, face challenges in accurately measuring torque on rotating shafts due to elliptical deformation caused by bending loads, leading to errors in torque measurement, especially in applications like bicycle cranksets where bending loads are significant.
Innovation Solution
The use of a short hollow shaft with strategically designed dimensions and a flange to minimize elliptical deformation, combined with a shear pattern strain grid arrangement, allows for accurate torque measurement with less than 5% error, even in the presence of torsional and bending loads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a strain grid gauge is applied directly to the shaft in a shear pattern, then torque measurement is achieved, but measurement accuracy deteriorates due to bending strain interference
Solution Approach 1:
The shaft is segmented into multiple regions with different functions: a measurement region with optimized geometry for torque sensing and a support region with increased stiffness to minimize bending. This segmentation allows the measurement zone to be isolated from bending strain effects while maintaining overall structural integrity.
Solution Approach 2:
The shaft exhibits local quality variations through different geometric characteristics in different regions. The measurement region has specific diameter and length proportions optimized for torque measurement, while the support region has increased wall thickness or diameter to provide bending resistance. This local differentiation enables the shaft to simultaneously achieve measurement sensitivity and bending resistance.
2Measurement precision
If the shaft length is increased to reduce elliptical deformation, then measurement accuracy improves, but device complexity and space requirements increase
Solution Approach 1:
The shaft geometry parameters are optimized within a compact length constraint. By adjusting the ratio of measurement region length to shaft diameter, and optimizing wall thickness distribution, the shaft achieves sufficient stiffness to minimize elliptical deformation without requiring excessive length. The parameter optimization allows accurate torque measurement on short shafts where conventional designs would fail.
3Measurement precision
If magnetorestrictive material quantity is increased to enhance field strength, then sensing capability improves, but shaft length must be increased
Solution Approach 1:
The patent replaces magnetorestrictive sensing with a mechanical strain measurement approach using strain grid gauges on an optimized shaft geometry. This substitution eliminates the need for magnetorestrictive material and its associated requirements for long shaft lengths and steel construction, while achieving comparable or superior measurement accuracy through geometric optimization alone.
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
This configuration reduces elliptical deformation-related errors, enabling precise torque measurement on short shafts, suitable for various applications including bicycle spindles, by optimizing the inner and outer diameters of the hollow shaft and incorporating additional strain grids to cancel out bending strains.
Implementation Method 1
one of the most ubiquitous measurements of torque is by a strain grid gauge (referred to herein alternatively as 'strain grid' or 'strain gauge') directly applied to the shaft
Implementation Method 2
one type of external technology is a magnetorestrictive material whose magnetic field is proportional to the strain present in the shaft. Torque applied to the shaft causes the magnetic field to change
Data Source
AI summary
A shaft characterized by its length to diameter ratio being less than about 1.75 having a drive connection at one end where the wall thickness of the shaft is selected to be thick enough to avoid ellipticalization strain error in torsional measurement of less than 5%. One specific application is for a crankset spindle that can be used to measure a cyclist right, left, and total leg torque.