Direct Acting Force Limiting Actuator for Aircraft Flight Control
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Solution Overview
Problem
Aircraft flight control actuation systems face challenges in managing high torques and preventing jamming or failure of actuators or drive shafts, particularly in ensuring safe operation by limiting excessive forces without requiring heavy and complex designs.
Innovation Solution
A direct acting force limiting actuator design that causes significant drag on a rotary input shaft when excessive loads are applied, using mechanisms like friction between a worm gear and worm shaft, or hydraulic systems to engage a brake on the input shaft, thereby dissipating input torque and preventing further movement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a force limiting device is designed to accommodate high torques through traditional worm shaft and worm gear mechanisms, then the actuator can handle excessive loads, but the device becomes heavy and complex
Solution Approach 1:
The patent inverts the traditional approach by applying the force limiting function directly at the input shaft through friction brakes or viscous coupling, rather than relying on the worm gear mechanism to handle and limit high torques. This reverses the conventional torque management strategy, achieving force limiting without requiring the worm gear components to be oversized for torque accommodation.
Solution Approach 2:
The patent extracts the force limiting function from the worm gear mechanism and implements it separately through dedicated friction brakes or viscous coupling devices on the input shaft. This separation allows the worm gear to focus on torque transmission while the extracted force limiting mechanism handles excessive load protection independently.
2Reliability
If traditional force limiting mechanisms are used with worm gears, then excessive forces can be limited, but significant weight is required in the components
Solution Approach 1:
The patent replaces traditional mechanical force limiting mechanisms that rely on heavy worm gear components with alternative mechanisms such as friction brakes or viscous coupling devices. These substitution mechanisms achieve the same force limiting function with significantly reduced component weight and complexity.
Solution Approach 2:
The patent introduces intermediary elements such as friction brake pads or viscous coupling fluid between the input shaft and the load path. These intermediaries provide force limiting through friction or viscous resistance rather than requiring heavy mechanical components to directly withstand and limit the forces.
3Reliability
If the worm gear and worm shaft are designed for high torque accommodation, then the actuator can prevent jamming, but the device complexity and size increase
Solution Approach 1:
The patent inverts the jamming prevention strategy by protecting the input shaft from excessive torques before they can be transmitted to the worm gear. Instead of relying on the worm gear's inherent torque handling capacity to prevent jamming, the system preemptively limits input torques through friction brakes or viscous coupling, thereby preventing jamming with simpler components.
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 solution effectively limits forces and prevents jamming by dissipating input torque as friction, reducing the need for heavy components and enabling weight reduction while ensuring safe operation by stalling the input drive system when excessive loads are detected.
Implementation Method 1
The worm gear and worm shaft are normally kept centered to one another during operation of the actuator under normal loads. When the load on the output member becomes excessive and exceeds the pre-specified value, the load causes the worm gear to translate relative to the worm shaft and cause the worm shaft to rub on the relatively displaced worm gear. This creates a friction drag on the worm shaft that can ultimately lock the input from imparting any further load on the output member.
Implementation Method 2
The movement of the piston is restrained, for example, by opposing pre-loaded spring elements that define a force trigger. When the load exceeds the trigger set point the piston displaces to cause fluid to move and displace a brake actuator member.
Data Source
AI summary
A direct acting force limiting actuator wherein the actuator output force either directly or indirectly causes significant drag on a rotary input shaft rather than on the translating output member. Consequently, input energy is dissipated before the actuator gear train mechanical advantage amplifies the reaction forces.