Concentric Ball Screw Rotary Actuator Without External Ball Return
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Solution Overview
Problem
Conventional rotary actuators face challenges with high friction and inefficiency in torque transmission, particularly in applications requiring compact and high-efficiency rotary actuation, such as aerodynamic flight control surfaces and other industries.
Innovation Solution
A compact rotary actuator design featuring two concentric ball screws with an interlaced ball circuit and shared ball paths, where pistons translate within an outer cylinder to recirculate ball bearings, reducing friction and enhancing torque efficiency by converting linear motion into rotary motion without an external ball return path.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional acme screw assemblies are used to impart forces to a driven load, then the actuator can be mechanically simple, but high friction along interfacing screw threads results in low torque efficiency and possible failure modes
Solution Approach 1:
The patent employs nested ball screws where an inner ball screw is positioned within an outer ball screw, both sharing common ball paths. This nesting arrangement allows the inner shaft and outer cylinder to rotate concentrically while sharing the same ball bearing circuit, reducing friction through ball bearing recirculation rather than sliding contact, thereby improving torque efficiency without excessive complexity
Solution Approach 2:
The patent merges the ball bearing recirculation function into the screw thread structure itself by creating shared ball paths that are common to both inner and outer ball screws. The ball bearings recirculate through these shared paths, combining the functions of torque transmission and ball bearing recirculation into a unified structure, eliminating the need for separate ball return mechanisms
2Volume of moving object
If ball bearings are recirculated through external ball return paths in conventional actuators, then the ball screw mechanism can function, but the actuator size increases and compactness is reduced
Solution Approach 1:
The inner ball screw is nested within the outer ball screw, with both sharing common ball paths that are contained entirely within the outer cylinder envelope. This nesting eliminates the need for external ball return paths, as the ball bearings recirculate internally through the shared paths, significantly reducing the actuator's overall volume while maintaining functional complexity
Solution Approach 2:
The patent transitions from a conventional single-ball-screw architecture to a concentric dual-ball-screw arrangement, utilizing the radial dimension to nest the inner ball screw within the outer ball screw. This dimensional reorganization allows ball bearing recirculation to occur within the existing envelope without requiring external return paths, effectively utilizing three-dimensional space optimization
3Force
If low thread pitch is used in ball screw assemblies to realize high linear force and translation, then force output is improved, but input torque requirements increase and efficiency decreases
Solution Approach 1:
The nested concentric ball screw arrangement with shared ball paths creates a mechanically efficient system where ball bearings recirculate without sliding friction. This reduces the energy loss in the thread interface, allowing for more efficient torque-to-force conversion. The system can achieve high force output with reduced input torque requirements compared to conventional acme screws, as the ball bearing recirculation eliminates the high friction that would otherwise require excessive input torque
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 provides high torque efficiency and low friction, enabling compact and efficient rotary actuation suitable for various applications, including aerodynamic flight control surfaces, by eliminating external ball return paths and utilizing a high thread pitch for improved performance.
Implementation Method 1
the outer and inner ball screws translate in opposite axial directions, with the translational motion and resulting rotation of the piston(s) ultimately causing the ball bearings to recirculate between the inner and outer ball screws
Implementation Method 2
one or two pistons translate within an outer cylinder in response to admitted fluid pressure from an external pressure supply
Implementation Method 3
The outer cylinder defines a set of helical or spiral grooves forming female threads of the outer ball screw. The piston, which translates within the outer cylinder by virtue of differential pressure across a piston face, has an outer diameter that includes male threads of the outer ball screw
Data Source
AI summary
A rotary actuator for a hinged panel assembly, e.g., of a fixed-wing aircraft, includes ball bearings, an outer cylinder, a piston, and an inner shaft. The outer cylinder admits fluid pressure from a fluid pressure supply. The piston is circumscribed by the outer cylinder. The piston, outer cylinder, and ball bearings collectively forming an outer ball screw. The piston translates along a longitudinal center axis in response to the fluid pressure. The inner shaft is circumscribed by the piston, with the inner shaft, piston, and ball bearings collectively forming an inner ball screw that is concentric with the outer ball screw. The ball screws form an interlaced ball circuit with one or more shared ball paths. Piston translation rotates the piston and inner shaft, and to thereby recirculates the ball bearings between the outer ball screw and the inner ball screw through the interlaced ball circuit.