Gearbox Assembly Worm Shaft Pivot Rattle Reduction
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Solution Overview
Problem
Electric power-assisted steering gearboxes with worm and wheel configurations are prone to rattle due to external torsional vibrations and mechanical noise from sudden torque reversals, which existing solutions like the 'Sprung Worm' mechanism do not adequately address.
Innovation Solution
A gearbox assembly design featuring a torsionally flexible pivot axle with a biasing means that allows the worm shaft to pivot radially, reducing rattle by applying a biasing force through a torsion bar that combines with any additional spring force to maintain engagement with the gear wheel, while minimizing variations in biasing force due to component tolerances and temperature.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the worm shaft is rigidly fixed to eliminate pivoting movement, then the gearbox structure becomes simpler and more stable, but gear rattle increases due to inability to accommodate torsional vibrations and torque reversals
Solution Approach 1:
The patent applies the Dynamics principle by allowing the worm shaft to pivot dynamically around its axis within a controlled angular range (typically less than +/- 0.5 degrees) rather than being rigidly fixed. This dynamic capability enables the worm shaft to accommodate torsional vibrations from the road wheels and sudden torque reversals from the driver, thereby reducing gear rattle at the engagement points while maintaining stable gear engagement through the biasing force from the leaf spring.
2Object-generated harmful factors
If the worm shaft is allowed to pivot to reduce gear rattle, then gear engagement stability improves, but the bearing assembly complexity increases to control the pivoting movement
Solution Approach 1:
The patent applies the Segmentation principle by dividing the bearing support system into distinct functional components: the leaf spring provides biasing force, the main bearing assembly supports radial loads and allows controlled pivoting, and the tail bearing assembly provides axial location and additional pivoting control. This segmentation allows each component to be optimized for its specific function, managing the complexity through modular design rather than a single complex bearing assembly.
Solution Approach 2:
The patent applies the Parameter changes principle by carefully controlling the pivoting angle parameter to be less than +/- 0.5 degrees from the nominal position. This limited angular parameter change enables the worm shaft to accommodate vibrations and torque reversals while maintaining effective gear engagement. The tail bearing assembly is designed to move by less than +/- 0.5 mm to control this parameter, balancing rattle reduction with acceptable bearing assembly complexity.
3Object-generated harmful factors
If a biasing force is applied to the worm shaft to maintain engagement, then gear rattle is reduced, but the force variations due to tolerances and temperature affect engagement consistency
Solution Approach 1:
The patent applies the Feedback principle through the elastic properties of the leaf spring, which automatically adjusts the biasing force applied to the worm shaft. As the worm shaft pivots or experiences load variations, the leaf spring deforms elastically and provides a restoring force that maintains consistent engagement pressure. This elastic feedback mechanism compensates for force variations due to component tolerances and temperature changes, ensuring reliable gear engagement without requiring complex active control systems.
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 design effectively reduces gear rattle by allowing controlled pivoting of the worm shaft, providing secure location with minimal lash and reduced risk of rattle, while maintaining torque transfer efficiency.
Implementation Method 1
a torsion bar that combines with any additional spring force to maintain engagement with the gear wheel
Data Source
AI summary
A gearbox assembly for an electric power assisted steering apparatus comprises a gearbox housing which houses a worm shaft and a gear wheel, the worm shaft being supported relative to the housing by a main bearing assembly at an end closest to the motor and by a tail bearing assembly at an end furthest from the motor. The main bearing assembly comprises an inner race and an outer race separated by bearing elements, the outer race being supported by a carrier that is supported by an axle that is in turn secured to gearbox housing, the axle defining a pivot axis for the worm shaft that is located on the side of the worm shaft closest to the wheel gear and extends in a direction orthogonal to the axis of the worm shaft and parallel to the axis of the wheel gear.