Preloaded Lead Screw Assembly for Spool-Up Load Protection
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Solution Overview
Problem
Lead screws in applications like Electrically Driven Thrust Reverser Actuation Systems face high spool-up loads that can cause damage and premature failure, limiting their use due to the need for expensive materials with high mechanical properties, whereas existing solutions like ball screws are inefficient in terms of power consumption and cost.
Innovation Solution
A lead screw assembly with a machined spring providing axial preloading and a secondary nut that engages to transfer excess loads, allowing the primary nut to handle only predetermined loads, thereby reducing peak loads and enabling the use of lower-cost, lower-mechanical-property materials like polymers and composites.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a lead screw is used in TRAS applications, then cost savings of approximately 66% compared to ball screws are achieved, but the screw parts are vulnerable to damage from spool-up loads that can be two to three times the maximum dynamic operating loads
Solution Approach 1:
A machined spring is pre-installed in the nut assembly to apply a predetermined axial preload to the lead screw threads before operation. This preliminary action ensures that during normal dynamic operation, the threads remain engaged and share the load, preventing the nut from reacting the full magnitude of spool-up loads that could cause premature failure.
Solution Approach 2:
The machined spring acts as a cushioning element that absorbs and limits peak loads before they can damage the lead screw threads. By being pre-positioned and pre-loaded, the spring provides beforehand protection against the harmful spool-up loads, allowing the use of cost-effective lead screw materials while maintaining reliability.
2Reliability
If the lead screw nut is designed to handle high spool-up loads, then reliability is improved, but the cost increases due to the need for materials with high mechanical properties
Solution Approach 1:
The machined spring is pre-installed to provide a predetermined axial preload, ensuring that during normal operation including spool-up events, the lead screw threads remain engaged and share the load. This preliminary action allows the use of lower-cost materials while maintaining the ability to withstand high spool-up loads.
Solution Approach 2:
The machined spring acts as an intermediary element between the external load and the lead screw threads. It mediates the load transfer by maintaining thread engagement and preventing the nut from reacting the full magnitude of spool-up loads, thereby enabling the use of cost-effective materials while ensuring reliability.
3Use of energy by moving object
If a ball screw is used instead of a lead screw, then power consumption efficiency is improved with approximately 95% efficiency, but the cost increases significantly compared to lead screws
Solution Approach 1:
The invention modifies the operational parameters of the lead screw system by introducing a predetermined axial preload through the machined spring. This parameter change optimizes the lead screw's performance within its efficiency range (45%-75%), allowing it to reliably handle spool-up loads while maintaining the cost advantage over ball screws. The spring preload ensures consistent thread engagement that maximizes the lead screw's inherent efficiency characteristics.
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 limits static loads on the lead screw nut, reducing the risk of damage and enabling cost savings by allowing the use of less expensive materials while maintaining operational efficiency, particularly beneficial in TRAS applications.
Implementation Method 1
a machined spring arranged to provide an axial preloading X of the primary nut
Data Source
AI summary
A leadscrew assembly includes a screw shaft along which is formed a first helical groove and a primary nut along which is form a second helical groove. The first helical groove and the second helical groove cooperate to define a track. The assembly also includes a secondary nut along which is formed a third helical groove. The first helical groove and the third helical groove cooperate to define a track 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.