EPS Worm Gearbox Coupling With Load-Adaptive Anti-Rattle Damping
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Electric power-assisted steering gearboxes of the worm and wheel type are prone to rattle due to external torsional vibrations and mechanical noise from sudden torque reversals, which existing solutions fail to adequately address, particularly at low torque transmission levels.
Innovation Solution
A gearbox assembly design featuring a worm shaft with external helical teeth, a main bearing assembly, a tail bearing assembly that allows radial movement, a sliding piston with a tapered shoulder and an o-ring that compresses to increase friction resistance as axial load increases, and a coupler for preventing rotational movement while allowing angular deflection, which enhances damping and reduces rattle.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the worm shaft is rigidly connected to the motor output shaft, then the gearbox structure is simple and easy to manufacture, but rattle and mechanical noise occur due to torsional vibrations and sudden torque reversals
Solution Approach 1:
The connection between the motor output shaft and worm shaft is changed from rigid to dynamic through the introduction of a compliant mechanism. The motor output shaft features a rounded tip that fits into a socket in the coupler, allowing limited angular deflection and movement. This dynamic connection absorbs torsional vibrations and reduces rattle while maintaining structural simplicity.
Solution Approach 2:
The degree of freedom in the connection is controlled by specific geometric parameters: the radius of the rounded tip (R1) is designed to be between 0.5-2mm, and the socket dimensions are carefully selected to permit limited angular deflection. These parameter changes enable the connection to be compliant enough to reduce rattle while maintaining sufficient rigidity for torque transmission.
2Object-affected harmful factors
If an anti-rattle spring is used to bias the worm shaft into dual-flank contact, then rattle between gear teeth is suppressed, but the connection requires freedom of movement that complicates the assembly
Solution Approach 1:
The anti-rattle spring function is merged with the compliant connection mechanism. The rounded tip in socket connection inherently provides the freedom of movement needed for the spring to function, while the spring simultaneously maintains dual-flank contact to suppress gear tooth rattle. This merging eliminates the need for separate mounting features for the spring.
Solution Approach 2:
The compliant connection acts as an intermediary between the motor output shaft and worm shaft, allowing the anti-rattle spring to function effectively. The rounded tip in socket mechanism mediates the interaction by providing the necessary movement freedom while maintaining torque transmission, simplifying the overall assembly.
3Object-affected harmful factors
If the worm shaft is allowed to move radially away from the wheel gear axis, then rattle is reduced through increased freedom of movement, but precision and stability of the gearbox decrease
Solution Approach 1:
The worm shaft is allowed partial movement - specifically limited angular deflection and small radial displacement - rather than complete freedom of movement. The rounded tip radius (0.5-2mm) and socket dimensions are designed to permit just enough movement to reduce rattle while maintaining sufficient precision for proper gear engagement and torque transmission.
Solution Approach 2:
The compliance is localized to specific regions: the rounded tip in socket connection allows movement at the motor-worm shaft interface, while the bearing assemblies maintain precise positioning of the worm shaft. This local quality approach enables rattle reduction without compromising overall gearbox precision.
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 rattle and mechanical noise by providing additional damping through increased friction resistance at higher axial loads, maintaining a fixed preload and accommodating component tolerances without excessive damping at low loads.
Implementation Method 1
a sliding piston with a tapered shoulder and an o-ring that compresses to increase friction resistance as axial load increases
Data Source
AI summary
A gearbox assembly for power take off from an electric motor of an electric power assisted steering apparatus comprises a gearbox housing which houses a worm shaft and a gear wheel. The worm shaft incorporates one or more external helical worm teeth. A main bearing assembly supports the worm shaft at an end closest to the motor. A tail bearing assembly supports the worm shaft at an end furthest from the motor. At least the tail bearing assembly is free to move relative to the housing through a limited range of motion that enables the worm shaft to move radially away from an axis of the wheel gear. A piston is slidingly received within a bore in an end of the wormshaft and has a has a head at an end facing the motor which connects with an output shaft of the motor. An interface between the recess of the piston and the protrusion of motor shaft defines a pivot axis of the worm shaft. A spring located within the bore in the end of the worm shaft and is compressed between the worm shaft. The piston has a tapered shoulder located within the bore that increases in diameter from an end furthest from the motor towards an end nearest the motor. The gearbox assembly further includes an annular o-ring that sits on the tapered shoulder, the spring acting on the piston through the o-ring whereby movement of the wormshaft towards the motor shaft compresses the spring which in turn drives the o-ring along the tapered shoulder until the o-ring becomes wedged between the piston and the inner wall of the bore of the wormshaft.