Steer-by-Wire Ball Screw Anti-Rotation Mechanism
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Solution Overview
Problem
In electric power steering (EPS) systems, particularly in rack electric power steering (REPS) systems without input shafts, the addition of a pinion gear leads to an over-constraint condition due to the engagement of the rack and pinion mesh, resulting in friction variations when parts are out of alignment, which is undesirable.
Innovation Solution
An anti-rotation mechanism is introduced that includes a yoke with a bearing journal and bearings positioned on the journal, which move along a running surface within the rack housing, preventing rotation of the ball screw while avoiding over-constraint by using a biasing member and slider to maintain contact with one side of the running surface under low torque and switch to the other side under high torque, thus minimizing friction and noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a pinion gear is added to engage rack teeth for position sensing and anti-rotation, then the ball screw position sensing is improved and anti-rotation is provided, but an over-constraint condition occurs leading to friction variation
Solution Approach 1:
The patent extracts the anti-rotation function from the pinion gear mesh and implements it through a dedicated anti-rotation mechanism with a bearing and running surface. This separates the position sensing function (retained by pinion gear) from the anti-rotation function, eliminating the over-constraint condition that caused friction variation.
Solution Approach 2:
The patent segments the combined functions of the pinion gear system into separate components: the pinion gear remains for position sensing only, while a distinct anti-rotation mechanism with bearing journals and running surfaces handles the anti-rotation function. This segmentation prevents the over-constraint conflict between mesh engagement and anti-rotation requirements.
2Force
If two ball nuts are used on the same ball screw to achieve required output force, then the output force is improved, but the addition of rack and pinion mesh leads to over-constraint condition
Solution Approach 1:
The patent extracts the anti-rotation function from the rack and pinion mesh and places it in a dedicated anti-rotation mechanism. This allows the rack and pinion mesh to serve only for position sensing, reducing the constraint conditions while maintaining the force output capability provided by two ball nuts.
3Reliability
If a bearing is positioned to contact and move along a running surface, then the anti-rotation function is achieved, but the bearing must accommodate side-to-side movement without generating side loads
Solution Approach 1:
The patent employs a spherical bearing journal that contacts the running surface at a single point or small area. This spherical geometry allows the bearing to accommodate the ball screw's side-to-side movement while maintaining anti-rotation function, preventing the generation of harmful side loads through the curved contact interface.
Solution Approach 2:
The bearing is designed to dynamically adjust its contact position on the running surface as the ball screw moves side-to-side. This dynamic adaptation allows the anti-rotation mechanism to remain effective while accommodating positional variations without creating rigid constraints that would generate side loads.
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 anti-rotation mechanism effectively prevents the over-constraint of the ball screw axis, reduces friction variations, and minimizes noise by allowing the ball screw to move side-to-side without generating side loads, ensuring stable and efficient steering operations.
Implementation Method 1
The anti-rotation mechanism effectively prevents the over-constraint of the ball screw axis, reduces friction variations, and minimizes noise by allowing the ball screw to move side-to-side without generating side loads
Data Source
AI summary
A steer-by-wire steering system for a vehicle includes a rack moveable in an axial direction. The steer-by-wire steering system also includes an anti-rotation device disposed proximate an outer surface of the rack. The anti-rotation device includes a yoke having a bearing journal extending therefrom. The anti-rotation device also includes a bearing disposed on the bearing journal. The anti-rotation device further includes a running surface disposed within a rack housing and extending in a longitudinal direction of the rack, wherein the bearing is positioned to move along the running surface during operation.