Rotationally Stiff Key for Misaligned Shaft Coupling
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Solution Overview
Problem
Coupling motor and position sensor shafts with their own pairs of bearings poses alignment challenges, leading to binding, increased friction, and wear, especially in applications where space is limited, such as in missile systems, and existing solutions like planar flexures or rubber keys fail to provide adequate rotational stiffness and accuracy.
Innovation Solution
A rotationally stiff metallic key with reliefs and a protrusion is used to couple non-parallel shafts, allowing for misalignment without increasing friction, providing high torsional stiffness, and fitting into standard keyways or flats, which maintains accurate angle measurements and reduces axial loads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If rigid coupling is used between shafts, then rotational stiffness is improved, but misalignment causes binding and increased friction
Solution Approach 1:
The patent employs a flexible membrane structure that can deform to accommodate misalignment between shafts while maintaining rotational stiffness. The membrane acts as a flexible coupling element that transmits torque without rigid constraints, preventing binding and reducing friction caused by misalignment.
Solution Approach 2:
The patent changes the physical state and properties of the coupling material from rigid to flexible, allowing it to deform elastically under misalignment conditions. This parameter change enables the coupling to accommodate angular and parallel misalignments while maintaining sufficient torsional stiffness for accurate angle measurement.
2Adaptability or versatility
If planar flexures are used to accommodate misalignment, then flexibility is improved, but distance between motor and position sensor increases
Solution Approach 1:
The patent nests the flexible membrane coupling within a compact housing structure that fits between the motor and position sensor shafts. This nested configuration allows the flexible coupling to accommodate misalignment while maintaining a compact overall size, eliminating the need for extended planar flexure structures.
Solution Approach 2:
The patent transitions from two-dimensional planar flexures to a three-dimensional membrane structure that can deform in multiple directions simultaneously. This dimensional change allows the coupling to accommodate misalignment in both angular and parallel directions within a compact volume, reducing the axial distance between components.
3Ease of manufacture
If rubber keys are used for coupling, then ease of manufacture is improved, but rotational stiffness is insufficient leading to angle errors
Solution Approach 1:
The patent uses a composite structure combining a flexible membrane with a rigid housing and shaft interfaces. The membrane provides flexibility for misalignment accommodation, while the rigid components maintain rotational stiffness for accurate angle measurement. This composite approach achieves both ease of manufacture and sufficient rotational stiffness.
Solution Approach 2:
The patent applies different material properties to different parts of the coupling: the membrane is made flexible for misalignment accommodation, while the shaft interfaces and housing are made rigid for maintaining rotational stiffness and measurement accuracy. This local differentiation of material quality resolves the contradiction between manufacturability and rotational stiffness.
Data Source
AI summary
A stiff key provides rigid coupling between a pair of shafts, such as a motor shaft and a rotary position sensor. The key may have a main body and a protrusion from the main body, such as from a middle portion of the main body. One of the protrusion or the main body may engage one part of one of the shafts being coupled together, and the other of the protrusion or the main body may engage a part of the other of the shafts. For example the protrusion may be in a keyway of one shaft and the main body may engage a recess or flat of the other shaft. The key and the coupling may be used in applications where space is at a premium, for example in a missile.