Nested-Screw Linear Actuator for Jam-Tolerant Stroke Control
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Solution Overview
Problem
Existing electric actuators are not tolerant to jam failure modes and require complex designs to bypass or dampen failed actuators, leading to increased manufacturing difficulty and weight, while hydraulic actuators are more tolerant but are less desirable for electrification demands.
Innovation Solution
A linear actuator design with a nested screw assembly system that includes a sleeve and nut configuration, allowing normal operation and a secondary mode in case of jam failure, featuring a threshold torque mechanism and jam detection means to ensure continued functionality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If electric actuators use mechanical gearing mechanisms, then they can provide precise control and positioning, but they are not tolerant of jam failure modes
Solution Approach 1:
The actuator is divided into two independent screw drive units (first and second ball screw drive units) that operate in parallel. Each unit can function independently, and if one jams, the other can continue to provide actuation. This segmentation of the drive system eliminates the need for complex bypass mechanisms while maintaining jam failure tolerance.
Solution Approach 2:
The patent changes the operational parameter from single-unit actuation to dual-unit simultaneous actuation. By having both ball screw drive units operational during a normal stroke, the system can tolerate jam failures without requiring complex structural changes or bypass mechanisms.
2Reliability
If dual redundant actuators are used for fail operative operation, then reliability improves, but weight and manufacturing complexity increase
Solution Approach 1:
The patent merges two ball screw drive units into a single integrated actuator housing, sharing common components such as the housing, seals, and control systems. This consolidation achieves fail operative capability through redundancy while minimizing weight increase compared to using two completely separate actuators.
Solution Approach 2:
The common housing and support structures serve multiple functions: providing structural support for both drive units, containing lubrication systems, and housing detection mechanisms. This multi-functionality reduces the overall component count and weight while maintaining reliability.
3Reliability
If complex bypass or damped mode designs are implemented, then fail operative operation is achieved, but manufacturing difficulty and component count increase
Solution Approach 1:
The detection system automatically detects when one ball screw drive unit jams and signals the control system to rely on the other unit. This self-detecting, self-reporting mechanism eliminates the need for complex manual bypass procedures or additional mechanical components, simplifying both manufacturing and operation.
Solution Approach 2:
The patent incorporates detection means that provide feedback on the operational status of each ball screw drive unit. This feedback mechanism enables the control system to automatically switch to or rely on the functioning unit when a jam is detected, eliminating the need for complex mechanical bypass designs.
4Length of moving object
If single actuator is used, then weight and complexity are reduced, but stroke is limited in the event of a jam
Solution Approach 1:
By segmenting the actuation function across two independent ball screw drive units, each unit can provide the full required stroke independently. If one unit jams, the other can still achieve complete stroke range, eliminating the stroke limitation that would occur in a single-unit system operating in bypass mode.
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 actuator maintains stroke functionality and detects jam failures, ensuring continued operation in secondary mode without limiting range, thus improving jam tolerance and reducing complexity and weight.
Implementation Method 1
a screw nut provided configured to engage with the third screw thread. In use, the at least one motor applies a torque to the sleeve via the screw nut
Implementation Method 2
a detector configured to identify rotation of the tube about the stroke axis, wherein the detector is configured to provide signal indicative of a jam or fault condition if rotation of the tube about the stroke axis is detected
Data Source
AI summary
A linear actuator (100) comprising a stroke axis (102), an actuator rod (104) extending along the stroke axis. The actuator rod comprises: an actuator rod outer surface (110), and a first screw thread (108) on a portion of the actuator rod outer surface; a sleeve (118) positioned around, and coaxial with, the actuator rod. The sleeve comprises: a sleeve inner surface (122), a sleeve outer surface (130), a second screw thread (120) on a portion of the sleeve inner surface, the second screw thread configured to engage with the first screw thread, and a third screw thread (128) on a portion of the sleeve outer surface. A screw nut (136) is provided configured to engage with the third screw thread. At least one motor (142a, 142b) configured to drive the screw nut. In use, the at least one motor applies a torque to the sleeve via the screw nut. When the applied torque is below a threshold value, the sleeve is restricted from rotating about the stroke axis, such that rotation of the screw nut causes linear displacement of the sleeve and actuator rod together along the stroke axis. When the applied torque equals or exceeds the threshold value, the sleeve is able to rotate about the stroke axis, such that rotation of the screw nut causes rotation of the sleeve, and such that rotation of the sleeve causes linear displacement of the actuator rod.