Screw Actuator Nut Arrangement for Low-Backlash Secondary Load Path
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Solution Overview
Problem
The existing attachment systems for trimmable horizontal stabilizer actuators in aircraft, particularly the lower attachment, experience significant backlash when the secondary load path is engaged, which can exceed the 0.034° certification requirement for stabilizer flutter, due to design requirements for redundancy and wear prevention, leading to potential instability and structural issues.
Innovation Solution
A screw actuator nut arrangement with a primary nut, two secondary nuts, and a first attachment means, where the secondary nuts are connected via bearings and a screw thread with a lower pitch than the screw shaft, allowing the first secondary nut to rotate relative to the second, reducing backlash by changing axial spacing when the primary load path fails, and a latch mechanism to lock the position.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the secondary nut is designed with a thinner thread and held by the primary nut such that the thread of the secondary nut does not touch the thread of the screw shaft while the primary load path is engaged, then the secondary load path components are spaced apart to avoid wear and fatigue, but significant backlash is introduced in the system when the secondary load path is engaged
Solution Approach 1:
The patent makes the spacing between the secondary nut and screw shaft dynamic rather than static. During normal operation, the secondary nut is held spaced from the screw shaft by the primary nut to prevent wear. When the primary load path fails and the secondary load path engages, the secondary nut is allowed to move axially to contact the screw shaft, dynamically adjusting the spacing to eliminate backlash while maintaining wear prevention during normal operation.
Solution Approach 2:
The patent pre-positions the secondary nut in a retracted state where it is held spaced from the screw shaft by the primary nut during normal operation. This preliminary positioning prevents wear accumulation before the secondary load path is needed. When failure occurs, the secondary nut is already in place and can immediately engage the screw shaft to eliminate backlash without requiring additional adjustment mechanisms.
2Reliability
If backlash is introduced to sufficiently space apart the secondary load path components so that they do not experience loading while the primary load path is engaged, then wear and fatigue in the secondary load path are avoided, but the cumulative backlash can lead to the 0.034° certification requirement not being met when the secondary load path is engaged
Solution Approach 1:
The patent implements a dynamic spacing mechanism where the secondary nut's axial position changes based on load path engagement status. During normal operation, the secondary nut is held retracted to prevent wear. When the primary load path fails, the secondary nut moves axially to contact the screw shaft, dynamically eliminating the backlash that would otherwise cause angular position errors and violate the 0.034° certification requirement.
3Measurement precision
If a latch mechanism is used to lock the position of the secondary nut when the secondary load path is engaged, then the angular position stability is improved to meet certification requirements, but the device complexity increases
Solution Approach 1:
The latch mechanism is designed to engage automatically when the secondary load path is activated. As the secondary nut moves axially to contact the screw shaft and eliminate backlash, the latch mechanism self-activates to lock the angular position. This self-service approach provides the necessary angular position stability to meet certification requirements without requiring external control systems or complex actuation 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 configuration reduces overall backlash in the actuator system when the secondary load path is engaged, ensuring the stabilizer remains within the 0.034° angular limit, enhancing stability and preventing structural damage from excessive flutter.
Implementation Method 1
the first secondary nut is mounted to the first attachment means via bearings that allow the first secondary nut to rotate relative to the first attachment
Implementation Method 2
the first secondary nut is connected to the second secondary nut via a screw thread connection having a pitch different from a pitch of a thread of the first secondary nut; and wherein, when any primary load path component fails, when the primary nut fails the threads of first and second secondary nut load against a screw thread of the actuator, the first secondary nut rotates relative to the second secondary nut
Data Source
AI summary
A screw actuator having a screw shaft and a nut arrangement is described herein, the nut arrangement comprising: a primary nut; a first secondary nut having a first thread; a second secondary nut having a second thread; and a first attachment means. The first nut, first secondary nut, and second secondary nut are connected to the first attachment means with a first, initial, axial spacing between the first and second secondary nuts. The first secondary nut is mounted to the first attachment means via bearings that allow the first secondary nut to rotate relative to the first attachment. The first secondary nut is connected to the second secondary nut via a screw thread connection having a pitch lower than a pitch of a screw thread of the screw actuator shaft.