Ball Screw Actuator Backup Nut Engagement for Faster 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, requiring frequent inspection and a secondary load path that is not effectively engaged until primary path failure, leading to potential hidden failures.
Innovation Solution
A screw actuator design with a primary and secondary ball nut configured for inner functional axial play, where the secondary nut is in contact with the ball screw threads in normal operation, utilizing anti-rotation elements to prevent relative rotation and allow smooth load transfer from the primary to the secondary nut upon failure, along with a distance monitoring system to detect primary load path failures promptly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a secondary nut is provided as a backup nut to take load if 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 nut and secondary nut into a single integrated assembly where both nuts are mounted on the same screw shaft. The load transfer mechanism is integrated within this assembly, allowing seamless transition from primary to secondary nut without external intervention. This merging reduces the need for separate backup systems and simplifies the overall device structure while maintaining reliability.
Solution Approach 2:
The secondary nut is pre-positioned and pre-configured in the assembly, ready to immediately assume the load if the primary nut fails. The load transfer mechanism is pre-established through the integrated design, eliminating the need for complex activation sequences or additional components. This preliminary preparation ensures immediate failover while keeping the device structure relatively simple.
2Reliability
If shear pin or preloaded index device is used to secure primary nut to secondary nut, then load transfer is enabled, but manufacturing precision requirements increase due to precise engagement features
Solution Approach 1:
The integrated nut assembly design allows the system to automatically transfer load from primary to secondary nut through inherent mechanical features. The load transfer is self-actuating based on the failure condition, eliminating the need for externally controlled shear pins or preloaded index devices. This self-service approach reduces manufacturing precision requirements while ensuring reliable load transfer.
3Reliability
If functional play is configured so secondary nut is not in contact with screw thread in normal operation, then primary load path is maintained, but detection of primary load path failure is delayed
Solution Approach 1:
The patent incorporates a monitoring system that continuously tracks the position and load status of both primary and secondary nuts. This feedback mechanism provides real-time information about the operational state of the load path, enabling immediate detection when the primary nut fails and the secondary nut engages. The feedback loop eliminates detection delays while maintaining proper functional play during normal operation.
4Loss of time
If secondary nut is in contact with screw thread in normal operation, then failure detection is improved, but device complexity increases due to active engagement requirements
Solution Approach 1:
The patent merges the monitoring function into the integrated nut assembly structure. The monitoring system is built into the assembly itself, utilizing the existing mechanical interfaces and load paths. This integration allows the secondary nut to remain in contact with the screw thread for immediate failure detection without requiring separate, complex monitoring devices or additional active engagement mechanisms.
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 enables faster engagement of the secondary load path upon primary nut failure, reducing the risk of catastrophic events by ensuring smooth load transfer and immediate detection of primary load path failures, thus enhancing safety and reliability.
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 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
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AI summary
A screw actuator (16) is disclosed. The screw actuator (16) includes a primary ball nut (18), a secondary nut (20), and a ball screw (22). The primary ball nut (18) and secondary nut (20) are both in threaded engagement with the ball screw (22), in which the inner functional plays of the screw actuator (16) are configured such that in normal operation the threads of the secondary nut (20) are in contact with the ball screw threads.