Dual-Nut Screw Actuator for Failover Load Path Engagement
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Solution Overview
Problem
Existing actuators for aircraft, such as trimmable horizontal stabiliser actuators, face challenges in avoiding undesired loading of secondary load paths during normal operation and ensuring reliable transition to secondary load paths upon primary path failure, which can lead to inefficiencies and potential structural issues under aerodynamic loads like flutter.
Innovation Solution
The actuator design incorporates a secondary nut with axially movable portions and a resilient member, along with fuse pins and movable abutments, to prevent unintended engagement of the secondary load path during normal operation and ensure efficient load transfer upon primary path failure, utilizing a screw shaft and nut assembly with specific thread configurations and clearances to manage load distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a secondary load path is provided in the actuator, then reliability is improved upon primary path failure, but undesired loading of the secondary load path occurs during normal operation
Solution Approach 1:
The fuse pins are pre-installed in the fuse pin holes to prevent relative movement between the first and second portions during normal operation. These pins act as preliminary protective measures that will only fail (shear off) when the load exceeds a predetermined threshold, at which point the secondary load path is activated. This preliminary action ensures the secondary path remains inactive during normal operation while being ready for emergency activation.
Solution Approach 2:
The design changes the mechanical state parameter of the secondary load path from 'engaged' to 'disengaged' during normal operation by using fuse pins to prevent relative movement between portions. Upon primary path failure, the parameter changes back to 'engaged' as the fuse pins shear and allow relative movement, enabling load transmission through the secondary path. This parameter control prevents undesired loading while ensuring reliability.
2Reliability
If the secondary nut is designed to engage the screw shaft, then load transmission is enabled upon primary path failure, but inadvertent movement occurs prior to predetermined shear force
Solution Approach 1:
The fuse pins are pre-positioned in the fuse pin holes to maintain the relative positions of the first and second portions before failure occurs. This preliminary constraint prevents inadvertent movement during normal operation and only allows movement after the pins shear at the predetermined load threshold, ensuring stable composition until emergency activation is required.
Solution Approach 2:
The fuse pins act as intermediary elements between the first and second portions of the secondary nut. They mediate the relationship between these portions by maintaining their relative positions during normal operation and allowing relative movement upon failure. This intermediary mechanism enables controlled transition from stable configuration to load-transmitting configuration.
3Adaptability or versatility
If clearances are provided between components, then excessive loading is accommodated, but backlash exceeds certification requirements
Solution Approach 1:
The secondary load path components are pre-configured with specific clearances and geometric features that eliminate backlash while accommodating excessive loading. The first and second portions are designed with predetermined clearance distances and engagement geometries that ensure zero backlash during normal operation, yet allow for load accommodation through the resilient member and controlled engagement upon failure.
Solution Approach 2:
The design optimizes the clearance parameter between components to achieve a balance: small enough to eliminate backlash and meet certification requirements (0.034° maximum), yet sufficient to accommodate excessive loading through the resilient member. The geometric parameters of the engagement surfaces are specifically designed to maintain this optimal clearance under varying load conditions.
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 effectively prevents undesired loading of the secondary load path during normal operation while ensuring reliable transition to the secondary load path upon primary path failure, enhancing the actuator's reliability and meeting certification requirements for backlash, such as not exceeding 0.034° in flight control surface movement.
Implementation Method 1
The actuator may further comprise a resilient member (e.g., a spring such as a coil spring located concentrically within the first and second portions) biased between the first portion and the second portion and configured to cause said relative movement of the first and second portions to engage/contact the screw shaft.
Implementation Method 2
The screw thread on the first portion may be biased in a first axial direction and against one side of a screw thread of the screw shaft, and the screw thread on the second portion may be biased in a second, opposite axial direction and against the other side of the screw thread of the screw shaft.
Data Source
AI summary
There is provided an actuator comprising screw shaft and a nut assembly. The nut assembly comprises a primary nut for transmitting load through the actuator along a primary load path, and a secondary nut for transmitting load through the actuator along a secondary load path. The secondary nut comprises first and second portions movable relative to one another. As load is transmitted through the actuator along the primary load path the secondary nut does not transmit load through the actuator, wherein upon failure of the primary load path the first and second portions move relative to each other, such relative movement causing the first and second portions to engage the screw shaft and enable transmittal of load through the secondary nut of the actuator along the secondary load path.