Dual-Nut Screw Actuator for Smooth Backup Load Transfer
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Solution Overview
Problem
Screw actuators in safety-critical roles, such as horizontal stabilizer trim actuators in aircraft, face issues with wear and potential catastrophic failure due to nut slippage along the screw shaft, necessitating frequent inspection and the use of secondary load paths that are not immediately activated upon primary path failure.
Innovation Solution
A screw actuator design with a primary and secondary nut configuration where the secondary nut is in contact with the screw threads during normal operation, utilizing anti-rotation elements to prevent relative rotation and allow axial movement, enabling seamless transfer of load to the secondary nut upon primary failure, with a monitoring system to detect such failures promptly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a secondary nut is provided as a backup nut to take load if the primary nut fails, then reliability is improved, but device complexity increases due to additional components and load transfer mechanisms
Solution Approach 1:
The patent combines the primary and secondary nuts into a single integrated assembly where the secondary nut is positioned adjacent to the primary nut on the screw shaft. Both nuts are prevented from rotating relative to the screw shaft simultaneously, and they work together as a unified load-bearing system, eliminating the need for separate complex load transfer mechanisms.
Solution Approach 2:
The secondary nut is pre-configured in position adjacent to the primary nut during assembly, with both nuts initially sharing the load. This preliminary arrangement ensures that the secondary nut is ready to immediately assume the full load if the primary nut fails, without requiring additional activation mechanisms or complex load transfer devices.
2Reliability
If a shear pin or preloaded index device is used to secure the primary nut to the secondary nut, then load transfer is enabled, but device complexity and potential failure points increase
Solution Approach 1:
The patent removes the shear pin or preloaded index device from the system entirely. Instead of using these additional components to enable load transfer, the design relies on the direct mechanical configuration of the two nuts positioned adjacent to each other on the screw shaft, both prevented from rotating, allowing seamless load transfer without intermediate elements.
3Force
If the secondary nut is not in contact with the screw thread in normal operation, then the primary load path is maintained, but the transition to secondary load path causes shock loading
Solution Approach 1:
The patent ensures continuous load-bearing action by having both the primary and secondary nuts simultaneously engaged with the screw shaft threads and both prevented from rotating. This continuous engagement allows for a smooth, progressive transfer of load from the primary to the secondary nut upon failure, eliminating sudden shocks and ensuring uninterrupted useful action.
4Ease of manufacture
If the thread on the nut requires disassembly of the screw actuator for inspection, then manufacturing is simplified, but maintenance difficulty increases
Solution Approach 1:
The patent enables the nuts to effectively inspect themselves through their operational behavior. The monitoring system detects when the primary nut fails by observing changes in the rotational characteristics of the nut assembly. This self-detection capability eliminates the need for disassembly and manual inspection, allowing the system to service itself by automatically identifying when inspection or replacement is needed.
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
Ensures rapid and smooth transition to the secondary load path upon primary failure, reducing the risk of sudden shock loading and enabling immediate detection of failures, thereby enhancing safety and reliability in safety-critical applications.
Implementation Method 1
A ballscrew actuator is a form of screw actuator, where ball bearings are located between a thread of the nut and the thread of the screw shaft. These ball bearings help to reduce friction between the nut thread and the screw thread
Implementation Method 2
A typical screw actuator comprises a screw shaft having a screw thread, and one or more nuts disposed on the screw shaft. When the screw shaft is rotated about its axis, the nut is forced to move along the axis of the screw shaft by the interaction of the two threads
Data Source
AI summary
A screw actuator includes a primary ball nut, a secondary nut, and a ball screw. The primary ball nut and secondary nut are both in threaded engagement with the ball screw, in which the inner functional plays of the screw actuator are configured such that in normal operation the threads of the secondary nut are in contact with the ball screw threads.