Dual Secondary Nut Screw Actuator for Low-Backlash Load Transfer
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Solution Overview
Problem
Screw actuators for trimmable horizontal stabilizer actuators in aircraft face challenges in meeting the 0.034° angular stability requirement due to excessive backlash when the secondary load path is engaged, leading to potential flutter and structural issues under aerodynamic loading.
Innovation Solution
A nut arrangement with a primary nut and two secondary nuts connected via a screw thread with a lower pitch, allowing the first secondary nut to rotate relative to the second, reducing initial backlash between the secondary nuts and the screw shaft when the primary load path fails, thereby minimizing overall backlash and maintaining stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If backlash is introduced to space apart the secondary load path components, then the secondary load path is kept unloaded while the primary load path is engaged, but the backlash increases when the secondary load path is engaged, leading to excessive flutter beyond the 0.034° certification requirement
Solution Approach 1:
The patent applies preliminary action by pre-positioning the secondary nut and thread connection in such a way that when the primary load path fails and the secondary load path engages, the first secondary nut rotates relative to the second secondary nut to automatically reduce the initial backlash. This preliminary arrangement of components ensures that backlash reduction occurs automatically upon engagement of the secondary load path, without requiring additional active control mechanisms.
2Reliability
If the secondary nut has a thinner thread and is held by the primary nut, then the secondary load path is kept unloaded during primary load path operation, but significant backlash is present when the secondary load path is engaged
Solution Approach 1:
The patent applies dynamics by making the first secondary nut rotatable relative to the second secondary nut through a thread connection with a different pitch than the screw shaft thread. This dynamic capability allows the first secondary nut to rotate and adjust the axial spacing between the secondary nuts when the secondary load path engages, automatically reducing backlash in response to loading conditions rather than maintaining a fixed configuration.
3Stability of the object's composition
If a screw thread connection with a lower pitch is used between the first and second secondary nuts, then the first secondary nut rotates relative to the second when the primary load path fails, reducing initial backlash, but the device complexity increases
Solution Approach 1:
The patent applies universality by designing the first secondary nut to serve multiple functions: it transmits load from the screw shaft thread, rotates relative to the second secondary nut through the thread connection, and simultaneously adjusts the axial spacing to reduce backlash. This multi-functionality is achieved within the existing nut and thread structure without adding separate backlash adjustment mechanisms, thereby reducing overall device complexity while achieving backlash reduction.
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 effectively reduces backlash when the secondary load path is engaged, ensuring the trimmable horizontal stabilizer remains within the 0.034° certification requirement, enhancing structural integrity and safety during flight.
Implementation Method 1
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, the first secondary nut rotates relative to the second secondary nut (this one is blocked in rotation) as a result of the screw thread connection
Data Source
Figure 1
Figure 2A~2B
Figure 3
AI summary
A screw actuator having a screw shaft and a nut arrangement is described herein, the nut arrangement comprising: a primary nut (54); a first secondary nut (56) having a first thread (60); a second secondary nut (58) having a second thread (64); and a first attachment means (52). 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 (59) 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 (66) having a pitch lower than a pitch of a screw thread (62) of the screw actuator shaft. When the primary nut fails under load, the threads (60,64) of the first and second secondary nuts (58,58) load against the screw thread of the screw shaft (102) which causes the first secondary nut to rotate relative to the second secondary nut and, as a result of the screw thread connection (66), the first and second secondary nuts (56,58) move from the first axial spacing between the first and second secondary nuts to a second, different, axial spacing between the first and second secondary nuts. A method of manufacturing this arrangement as well as a method of reducing backlash in such an arrangement is also described herein.