Everted Ball Screw Assembly for Compact Eccentric-Load Resistance
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Solution Overview
Problem
Conventional ball screw assemblies require significant space and are prone to buckling under eccentric loads due to their small effective radius, which affects the accuracy and durability of linear motion in applications requiring high mechanical efficiency and precision.
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.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a conventional ball screw assembly is used, then linear motion with high mechanical efficiency is achieved, but the assembly requires significant space due to large diameter and total length
Solution Approach 1:
The ball screw assembly inverts the conventional configuration by making the screw shaft hollow with internal threads instead of solid with external threads. The ball nut becomes externally threaded and fits inside the hollow shaft, creating an 'everted' configuration. This inversion allows the return pathway to be located inside the ball nut rather than outside, significantly reducing the assembly's diameter and overall volume while maintaining the helical ball circulation pathway for efficient linear motion conversion.
2Ease of operation
If a conventional ball screw assembly is used, then linear motion is achieved, but the assembly is long requiring housing length equal to three times the stroke length
Solution Approach 1:
The everted ball screw assembly nests the ball circulation pathway within the hollow shaft structure. The working pathway and return pathway are both contained within the internal volume of the hollow shaft, with balls circulating through internal passages. This nesting eliminates the need for external return pathways and reduces the required housing length to approximately twice the stroke length, compared to three times the stroke length in conventional assemblies.
3Area of stationary object
If a conventional ball screw with small effective radius is used, then compact radial dimension is achieved, but eccentric loads generate high forces causing buckling
Solution Approach 1:
By inverting the configuration to create a hollow shaft with internal threads, the effective radius is increased. The ball circulation pathway is positioned farther from the central axis, creating a larger moment of inertia and increasing resistance to buckling under eccentric loads. The hollow shaft structure provides greater radial stiffness while maintaining a compact overall radial dimension, eliminating the need for additional support structures.
4Ease of operation
If conventional ball screw assembly is used, then linear motion conversion is achieved, but additional support structures are required to prevent bending under eccentric loading
Solution Approach 1:
The inverted everted configuration inherently increases the effective radius and moment of inertia of the screw shaft, providing sufficient structural strength to withstand eccentric loads without additional support structures. The hollow shaft with internal threading creates a more robust structural profile that resists bending and maintains linear motion accuracy under load, eliminating the need for extra bracing or support elements required in conventional designs.
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 and robust linear actuators that maintain high mechanical efficiency and precision, capable of withstanding eccentric loads without buckling, thus improving the reliability and space efficiency of ball screw systems.
Implementation Method 1
A conventional ball screw assembly includes an elongate threaded shaft, a ball nut with matching threads
Implementation Method 2
A conventional ball screw assembly includes an elongate threaded shaft, a ball nut with matching threads, and a plurality of load-bearing balls that travel along the helical pathway
Implementation Method 3
a plurality of load-bearing balls that travel along the helical pathway created between the shaft threads and the nut threads
Data Source
AI summary
Linear actuators driven by everted ball screw assemblies are described herein. In some examples, the everted ball screw assembly includes a hollow screw shaft with internal threads, a ball cylinder concentrically positioned therein and having external threads, and a plurality of balls sized to fit in a substantially contiguous series along a working pathway that is defined by the space between the internal and external threads. One or more limiters prevent the ball cylinder from rotating. A motor rotates the hollow screw shaft, thereby driving the balls along the pathway and imparting linear motion to the ball cylinder, which is coupled to a connecting rod for actuating a load. The ball cylinder in some examples includes an internal return path with scoop elements that are sized and shaped to guide the balls into the internal return path and then back onto the working pathway.