Ball Screw Rotary Actuator Ball Cage for Low-Friction Back-Drive
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Solution Overview
Problem
Existing rotary actuators, such as lead screw rotary actuators, are unsuitable for aerospace applications due to their weight, high friction, and inability to be back-driven, while ball screw actuators are typically used for linear motion conversion rather than rotary motion.
Innovation Solution
A ball screw rotary actuator design featuring ball cages to confine ball bearings in a spaced grid, reducing friction and preventing sliding, combined with indexing gears to control the position of the ball cages and bearings, allowing for efficient rotary motion conversion and back-drive capability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If lead screw rotary actuators are used, then reliability and robustness are improved, but weight increases and friction becomes high
Solution Approach 1:
The patent changes the fundamental operating principle from lead screw sliding friction to ball screw rolling friction by introducing ball bearings between the screw and nut components. This parameter change in the friction mechanism reduces the coefficient of friction from typically 0.1-0.2 for sliding to 0.001-0.005 for rolling contact, achieving weight reduction while maintaining reliability
Solution Approach 2:
The patent replaces the traditional lead screw mechanical sliding system with a ball screw system that uses rolling element mechanics. The ball bearings substitute for direct metal-to-metal sliding contact, fundamentally changing the mechanical interaction mode to achieve lower friction and reduced weight while preserving the rotary actuator's reliability
2Loss of energy
If ball screw actuators are used for linear motion, then friction is reduced, but they cannot be used for rotary motion conversion
Solution Approach 1:
The patent creates a universal ball screw rotary actuator that combines the low-friction advantages of ball screws with rotary motion conversion capability. The design integrates a ball screw mechanism with a rotary output shaft and control system, enabling the same ball screw technology to serve both linear actuation and rotary actuation applications, thus achieving multi-functionality
Solution Approach 2:
The patent segments the ball screw assembly into distinct functional components: the ball screw mechanism for friction reduction, the rotary shaft for motion conversion, and the control system for actuation. This segmentation allows the ball screw's low-friction property to be preserved while adding rotary motion capability through separate functional elements working together
3Loss of energy
If ball bearings are allowed to move freely in the ball screw, then friction is reduced, but balls slide out of position due to vibrations or gravity
Solution Approach 1:
The patent introduces a ball cage as an intermediary component between the ball bearings and the surrounding environment. The cage acts as a mediator that allows the balls to move freely for rolling motion while preventing them from sliding out of position due to vibrations or gravity, thus resolving the contradiction between friction reduction and position stability
Solution Approach 2:
The patent extracts the ball position control function from the free-moving ball bearings and assigns it to a dedicated ball cage component. By separating the rolling function (performed by balls) from the positioning function (performed by cage), the system achieves both low friction through free rolling and stability through constrained ball positions
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 a lightweight, efficient, and robust rotary actuator with reduced friction and size, suitable for aerospace applications like flight control surfaces, achieving greater than 95% efficiency and enabling back-drive functionality.
Implementation Method 1
A ball screw rotary actuator translates linear force into rotary motion with little friction. Ball screws achieve lower friction than lead screws because the rolling motion of the ball bearings along their path creates less friction than sliding motion
Data Source
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AI summary
Ball screw rotary actuator (100) with ball cage. In one embodiment, a ball screw rotary actuator (100) includes an outer cylinder (110), a piston (150), and inner shaft (190). Outer ball bearings (122) travel in helical groove between the outer cylinder (110) and position to rotate the piston (150) as it translates due to fluid pressure. The inner shaft (190) is situated radially inward of the piston (150), and straight grooves (140) are disposed between the piston (150) and the inner shaft (190). Inner ball bearings (142) travel in the straight grooves (140) and rotate the inner shaft (190) as the piston (150) rotates. The ball screw rotary actuator (100) also includes an outer ball cage (124) to position the outer ball bearings (122) in a spaced configuration, an outer indexing gear (160-1) to control a position of the outer ball cage (124), an inner ball cage (144) to position the inner ball bearings (142) in a spaced configuration, and an inner indexing gear (160-2) to control a position of the inner ball cage (144).