Everted Ball Screw Assembly for Compact Eccentric-Load Actuators

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Solution Overview

Problem

Conventional ball screw assemblies are bulky, requiring significant space and prone to buckling under eccentric loads due to their small effective radius, which affects the accuracy and durability of linear motion in actuators.

Innovation Solution

The everted ball screw assembly features a hollow screw shaft with an internal thread and a ball cylinder with external threads, allowing for a larger effective radius and reduced length, enabling the assembly to withstand eccentric loads without additional support structures, by utilizing a continuous ball circulation pathway and internal return path within the ball cylinder.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Volume of moving object

If a conventional ball screw assembly is used, then linear motion can be achieved, but the assembly requires significant space and has a small effective radius

Engineering Contradiction:

Improveassembly sizeVSAvoidresistance to buckling

Core Design Contradiction:

Volume of moving objectVSReliability

Solution Approach 1:

The patent inverts the conventional ball screw configuration by placing the ball circulation pathway outside the nut rather than inside. This everted design allows the balls to travel along an external helical pathway, which fundamentally changes the spatial arrangement and enables a larger effective radius without increasing overall assembly size.

Inventive Principle:

Principle #13The other way round (Inversion)

Solution Approach 2:

The invention utilizes the radial dimension more effectively by positioning the ball circulation pathway externally. This dimensional reorganization allows the ball nut to have a larger effective radius while maintaining compact overall dimensions, resolving the contradiction between size and structural stability.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

2Stability of the object's composition

If the ball nut is made larger to accommodate the return pathway, then the pathway can be contained, but the diameter increases

Engineering Contradiction:

Improvereturn pathway containmentVSAvoidball nut diameter

Core Design Contradiction:

Stability of the object's compositionVSArea of moving object

Solution Approach 1:

Instead of containing the return pathway within the nut volume, the patent inverts the design by routing the ball circulation pathway externally. This allows the ball nut to maintain a smaller diameter while still providing a complete return path for the balls through the external helical pathway.

Inventive Principle:

Principle #13The other way round (Inversion)

3Reliability

If additional support structures are added to prevent buckling, then accuracy is preserved, but device complexity increases

Engineering Contradiction:

Improveaccuracy under eccentric loadVSAvoidsupport structures

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The everted ball screw design inherently increases the effective radius by inverting the ball circulation arrangement. This structural inversion provides natural resistance to eccentric loads and buckling without requiring additional support structures, thereby maintaining accuracy while avoiding increased complexity.

Inventive Principle:

Principle #13The other way round (Inversion)

4Reliability

If the shaft is made longer to increase effective radius, then eccentric load resistance improves, but total length increases

Engineering Contradiction:

Improveeccentric load resistanceVSAvoidshaft length

Core Design Contradiction:

ReliabilityVSLength of moving object

Solution Approach 1:

The patent achieves a larger effective radius by inverting the ball circulation pathway to an external configuration rather than extending the shaft length. This allows the radial dimension to be increased without proportionally increasing the axial length, maintaining compact overall dimensions while improving eccentric load resistance.

Inventive Principle:

Principle #13The other way round (Inversion)

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

This design allows for more compact linear actuators that maintain high mechanical efficiency and precision, capable of withstanding eccentric loads without buckling, thus improving the operational reliability and space efficiency compared to conventional ball screw assemblies.

Implementation Method 1

A conventional ball screw assembly includes an elongate threaded shaft, a ball nut with matching threads

Methodology Applied

Scientific EffectScrew mechanism: Screw

Implementation Method 2

a plurality of load-bearing balls that travel along the helical pathway created between the shaft threads and the nut threads

Methodology Applied

Scientific EffectBall screw mechanism: Screw

Data Source

PatentUS11041553B2Everted ball screw drive

Publication Date: 2021.06.22 AMETEK INC

AI summary

Linear actuators are described herein. In particular embodiments, a linear actuator includes an everted ball screw assembly comprising a hollow screw shaft with internal threads, a ball cylinder concentrically positioned therein and having external threads, and a plurality of bearing balls sized to fit in a substantially contiguous series along a working pathway defined by the space between the internal and external threads. The linear actuator may include a support bearing for rotatably supporting the screw shaft in a housing. The ball cylinder in particular embodiments includes an internal return path along which the balls return to the working pathway. The everted ball screw assembly may include a lash reduction assembly and one or more scoop inserts that are sized and shaped to guide the balls into the internal return path and back onto the working pathway.