Steer-by-Wire Ball Screw Anti-Rotation Mechanism
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Solution Overview
Problem
Existing electric power steering (EPS) systems with ball screw based rack electric power steering systems face issues with over-constraint and friction variation due to the addition of a rack and pinion mesh, particularly in large vehicles, leading to undesirable friction changes when parts are out of alignment.
Innovation Solution
A steer-by-wire steering system with an anti-rotation mechanism that includes a yoke with a bearing journal, a bearing on the yoke, and a running plate structure within a rack housing, which moves along the surface of the running plate during rack movement, along with a biasing member and slider to apply torque and prevent rotation, eliminating the need for a pinion gear and addressing over-constraint.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a rack and pinion mesh is added to a ball screw based REPS system to provide anti-rotation and position sensing, then the anti-rotation function is improved, but friction variation and over-constraint occur when parts are out of alignment
Solution Approach 1:
The patent removes the pinion gear from the system entirely. Instead of using a rack and pinion mesh for anti-rotation, the invention employs a dedicated anti-rotation mechanism with a brake assembly that applies friction directly to the ball screw. This extraction eliminates the gear meshing interface that causes friction variation when misaligned, while still achieving the anti-rotation function through the brake mechanism.
Solution Approach 2:
The patent introduces a brake assembly as an intermediary mechanism between the ball screw and the housing. This brake assembly includes a brake pad that contacts the ball screw surface and a brake actuator that applies the braking force. This intermediary provides anti-rotation control without requiring direct gear meshing, thereby eliminating the friction variation problem associated with misaligned rack and pinion interfaces.
2Reliability
If a rack and pinion mesh is added to achieve anti-rotation, then the anti-rotation capability is improved, but the system becomes over-constrained leading to friction variation
Solution Approach 1:
The patent extracts the pinion gear component entirely from the system. The anti-rotation function is achieved through a separate brake assembly that applies friction to the ball screw, rather than through gear meshing. This simplification removes the over-constraint condition created by combining ball screw drive with rack and pinion engagement, while maintaining the necessary anti-rotation capability.
Solution Approach 2:
The patent segments the anti-rotation function from the drive function. The ball screw handles the linear drive function, while a separate brake assembly handles the anti-rotation function. This segmentation allows each component to perform its specific function without creating over-constraint conditions, as the brake assembly independently controls rotation without interfering with the ball screw-rack linear motion.
3Force
If two ball nuts are used on the same ball screw for large vehicles 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 removes the pinion gear that would create the over-constraint condition. With the pinion extracted, the system can use two ball nuts on the same ball screw without the additional constraint from gear meshing. The brake assembly provides anti-rotation control without interfering with the dual ball nut configuration, allowing the system to achieve the required output force without over-constraint.
Solution Approach 2:
The patent segments the anti-rotation control from the drive train components. The brake assembly operates independently on the ball screw, allowing multiple ball nuts to be used for force multiplication without creating over-constraint conditions. This segmentation enables the dual ball nut configuration to function freely while the brake provides necessary rotational control.
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 solution effectively resists rotation of the ball screw, reduces friction variation, and prevents over-constraint, ensuring smooth and stable steering operations by transferring torque through friction and maintaining contact with the running plate structure, thus enhancing the reliability and performance of the EPS system.
Implementation Method 1
The anti-rotation mechanism also includes a biasing member in contact with the yoke. The anti-rotation mechanism further includes a slider member to apply a torque on the yoke during operation.
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 mechanism disposed proximate an outer surface of the rack at the mounting location of the rack. The anti-rotation mechanism includes a yoke having a bearing journal extending therefrom. The anti-rotation mechanism also includes a bearing disposed on the bearing journal of the yoke. The anti-rotation mechanism further includes a running plate structure disposed within a rack housing and extending in a longitudinal direction of the rack, wherein the bearing and the yoke are positioned to move along a surface of the running plate structure during movement of the rack in the axial direction.