Landing Gear Shock Strut Retraction With Non-Back-Driving Outstop
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Solution Overview
Problem
Aircraft landing gear shock absorber struts require a mechanism to shorten for stowage without extending accidentally, which could damage the landing gear bay or inhibit deployment, especially when the strut is longer than standard to increase ground clearance.
Innovation Solution
A non-back-driving screw thread mechanism is implemented, where a rotatable member with a threaded surface drives a mechanical outstop to compress the shock absorber strut, preventing accidental extension by ensuring the spring force cannot rotate the rotatable member, thus allowing controlled shortening and extension.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If a longer shock absorber strut is used to increase ground clearance, then ground clearance is improved, but the likelihood of accidental extension during stowage increases which could damage the landing gear bay
Solution Approach 1:
The patent applies preliminary anti-action by designing a non-back-driving screw thread mechanism that prevents the spring force from rotating the rotatable member and causing accidental extension. The screw thread geometry (helix angle less than 45 degrees) creates a mechanical lock that anticipates and counteracts the potential harmful action of the spring force before it can cause damage.
Solution Approach 2:
The patent uses an intermediary mechanical outstop that engages with an abutment surface on the inner cylinder to limit extension. This outstop acts as a mediator between the spring force and the inner cylinder, allowing controlled compression while preventing uncontrolled extension that could damage the landing gear bay.
2Reliability
If a non-back-driving screw thread mechanism is used to prevent accidental extension, then safety is improved, but the device complexity increases due to additional components
Solution Approach 1:
The rotatable member serves multiple functions: it compresses the shock absorber strut during stowage by driving the outstop axially, and it prevents accidental extension during retrieval by acting as a non-back-driving screw thread mechanism. This multi-functionality reduces the need for separate safety mechanisms, thereby limiting overall device complexity.
Solution Approach 2:
The patent merges the compression function and the safety function into a single integrated mechanism. The screw thread mechanism that compresses the strut also inherently prevents back-driving, combining two functions that could have been separate into one unified system.
3Length of moving object
If the screw thread mechanism is used to compress the shock absorber for stowage, then shortening capability is improved, but the risk of damage to external parts increases if the mechanism fails
Solution Approach 1:
The patent implements beforehand cushioning by providing a mechanical outstop that limits the extension of the shock absorber. This outstop acts as a safety cushion that prevents the inner cylinder from extending beyond a safe position, thereby protecting the landing gear bay from damage even if the screw thread mechanism fails.
Solution Approach 2:
The non-back-driving screw thread design applies preliminary anti-action by preventing the spring force from rotating the rotatable member in the first place. This eliminates the need for the inner cylinder to extend at all during accidental scenarios, proactively preventing potential damage to the landing gear bay.
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 enhances safety by preventing accidental extension of the shock absorber strut during stowage, ensuring the landing gear can be lowered without damage and maintaining deployment functionality, even if external parts are damaged.
Implementation Method 1
a rotatable member distinct from the inner cylinder and defining a threaded surface comprising a plurality of threads configured to drive the outstop axially as the rotatable member is rotated
Implementation Method 2
the inner cylinder being biased by a spring force to assume the second condition
Implementation Method 3
the rotatable member being arranged such that the spring force cannot rotate the rotatable member to allow the shock absorber strut to extend
Data Source
Figure 1
Figure 2a
Figure 2b
AI summary
An aircraft landing gear shock absorber strut (24) comprising: an outer cylinder (26) having a bore defining an opening; an inner cylinder (28) having a first end region movably coupled within the bore and a second end region which projects out of the opening, the inner cylinder being arranged to move along a longitudinal axis of the bore between a first condition in which the shock absorber strut is compressed and a second condition in which the shock absorber strut is extended, the inner cylinder being biased by a spring force to assume the second condition; a mechanical outstop (OS) arranged to engage an abutment surface of the inner cylinder to limit extension of the shock absorber strut; and a rotatable member defining a threaded surface (TS) configured to drive the out stop axially to move the inner cylinder from the second condition towards the first condition to compress the shock absorber strut, the rotatable member being arranged such that the spring force cannot rotate the rotatable member to allow the shock absorber strut to extend.