Ball Screw Electric Power Steering Load Distribution

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Solution Overview

Problem

In electric power steering apparatus of the ball screw type, the load is concentrated on the outermost ball within the inner peripheral screw groove due to the bending of the ball nut, leading to durability issues and noise/vibration generation.

Innovation Solution

The design includes a rack bar with a spiral outer peripheral screw groove, a ball nut with a corresponding spiral inner peripheral screw groove, and an end cap with a ball circulation passage, where the center radius of the inner peripheral screw groove gradually increases toward the inlet, distributing the load and eliminating the step between the screw groove and the end cap inlet.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If the inner peripheral screw groove has a constant center radius, then the structure is simple, but the load is concentrated on the outermost ball causing durability deterioration

Engineering Contradiction:

Improvedurability of ball nut and ballVSAvoidstructure of inner peripheral screw groove

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent applies local quality by making the center radius of the inner peripheral screw groove vary specifically in the axial direction rather than being uniform throughout. The radius is smaller at the center and larger at both ends, creating localized geometric variations that distribute loads more evenly across multiple balls during bending operations, thereby improving durability without requiring complete structural redesign

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent changes the geometric parameter of the screw groove by making the center radius variable instead of constant. Specifically, the radius decreases from the ends toward the center and then increases again, creating a non-uniform profile that optimizes load distribution. This parameter change allows the same screw groove structure to adapt to varying load conditions during ball nut bending

Inventive Principle:

Principle #35Parameter changes

2Object-affected harmful factors

If there is a step between the inner peripheral screw groove and end cap inlet, then manufacturing is easier, but noise and vibrations are generated

Engineering Contradiction:

Improvenoise and vibrationsVSAvoidmanufacturing of screw groove and end cap

Core Design Contradiction:

Object-affected harmful factorsVSEase of manufacture

Solution Approach 1:

The patent applies spheroidality by replacing the sharp step transition with a curved, rounded transition zone. The inner peripheral screw groove is designed to smoothly curve into the end cap inlet without abrupt changes, eliminating the step that causes impact and vibration. This curved transition reduces harmful noise and vibrations while remaining manufacturable through standard machining processes

Inventive Principle:

Principle #14Spheroidality (Curvature)

Data Source

PatentUS9266555B2Electric power steering apparatus of ball screw type

Publication Date: 2016.02.23 HL MANDO CORP

AI summary

A rack-driven auxiliary power steering apparatus prevents a ball nut and a nut pulley from idling due to a slip thereof and increases durability when the drive belt is operated by driving of a motor, and minimizes vibrations and noise transferred through the rack bar, the ball nut, and the nut pulley when the rack bar is slid while the nut pulley and the ball nut are rotating, thereby providing a comfortable steering feeling to the driver.