Lead Screw Assembly With Secondary Nut Peak-Load Bypass
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Solution Overview
Problem
Lead screws in applications like Electrically Driven Thrust Reverser Actuation Systems face damage from peak loads that are two to three times the maximum dynamic operating loads, limiting their use due to material requirements for high static load capacity, which are costly and not necessary for most operational conditions.
Innovation Solution
A lead screw assembly with a preloading system that bypasses high loads by engaging a secondary nut only when axial loads exceed a predetermined threshold, thereby limiting the load on the primary nut and allowing the use of less expensive materials.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the lead screw nut is designed to withstand peak loads (2-3 times maximum dynamic operating loads), then reliability is improved, but material cost and manufacturing complexity increase
Solution Approach 1:
A bypass mechanism acts as an intermediary element between the lead screw nut and the peak loads. When peak loads occur, the bypass mechanism engages to carry these exceptional loads, protecting the nut from damage. During normal operation, the bypass mechanism remains disengaged and does not interfere with the nut's function. This allows the nut to be designed for normal operating loads rather than peak loads, reducing material requirements and manufacturing complexity.
Solution Approach 2:
The bypass mechanism is pre-configured with a predetermined engagement threshold that activates before peak loads can damage the nut. The mechanism includes pre-set mechanical elements (such as spring-loaded components or cam-actuated systems) that automatically engage when load conditions approach dangerous levels, preventing the nut from ever experiencing damaging peak loads without requiring active control systems.
2Strength
If high-strength materials are used for the lead screw nut, then load capacity is improved, but cost increases
Solution Approach 1:
The bypass mechanism creates a localized load path that diverts peak loads away from the nut, allowing different parts of the system to have different strength requirements. The nut can be made from lower-cost materials optimized for normal operating conditions, while the bypass mechanism components are designed to handle the exceptional peak loads. This local differentiation of quality requirements optimizes overall system cost.
Solution Approach 2:
The bypass mechanism is designed as a sacrificial or replaceable component that protects the more critical nut. In some embodiments, the bypass mechanism may wear or deform under peak loads, but this is acceptable as it prevents damage to the nut and can be replaced more easily and cheaply than replacing the entire lead screw assembly.
3Ease of manufacture
If a bypass mechanism is added to limit peak loads, then ease of manufacture is improved, but device complexity increases
Solution Approach 1:
The bypass mechanism is designed to operate automatically based on load conditions without requiring external control systems, sensors, or active intervention. Mechanical elements such as spring forces, friction thresholds, or geometric constraints cause the bypass to engage and disengage automatically based on the applied load, making the system self-regulating and eliminating the need for complex control electronics or monitoring systems.
Solution Approach 2:
The lead screw assembly is segmented into distinct functional zones: the primary load-carrying path through the nut for normal operation, and the bypass path for peak loads. This segmentation allows each component to be optimized independently and simplifies the overall design by clearly defining the function and load requirements of each element, making the system easier to manufacture and maintain despite the added complexity of the bypass mechanism.
Data Source
Figure 1A~1D
Figure 2
Figure 3
AI summary
A leadscrew assembly comprising: a screw shaft (1) along which is formed a first helical groove (10); a primary nut (2) along which is form a second helical groove (20); the first helical groove and the second helical groove cooperating to define a track (10,20); a secondary nut (5) along which is formed a third helical groove (30); the first helical groove and the third helical groove cooperating to define a track (10, 30) with a nominal running clearance; a preload X is provided wherein when a load applied to the screw exceeds the predetermined preload, the secondary nut engages with the track such that load of the screw above preload X is transferred through the secondary nut and thus protecting the primary nut.