Captive Screw Assembly Unit with Spring Arm Holder
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Solution Overview
Problem
Existing assembly units with elastomer collars suffer from low retaining force and restricted lateral assembly clearance, making them unreliable for captive screw fixation during transport and handling, and they are limited by the use of plastic materials.
Innovation Solution
An assembly unit featuring a disk-shaped flange holder with a flange opening and spring arms that provide axial and radial mobility, ensuring captive retention and allowing for axial offset compensation through flexible retaining elements and spring arms, which can be produced efficiently using a punching-bending process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If an elastomer collar is used as the holder, then the screw is held by non-positive or frictional engagement, but the retaining force is low and captive fixing is unreliable
Solution Approach 1:
The holder is segmented into a flange body and multiple spring arms that can independently deflect. This segmentation allows the holder to provide reliable retaining force through the spring arms' elastic engagement with the screw shank, while the flange body provides structural support. The spring arms can be manufactured separately and attached to the flange, maintaining ease of manufacture while significantly improving reliability compared to a monolithic elastomer collar.
Solution Approach 2:
The spring arms are designed to be elastically deformable, allowing them to dynamically adapt to the screw shank during assembly and operation. This dynamic capability enables the holder to maintain reliable retaining force under varying loads and misalignments, overcoming the static limitation of elastomer collars while remaining manufacturable through standard spring forming processes.
2Reliability
If the collar bears closely against the wall of the through opening and the threaded portion, then the screw is retained, but lateral assembly clearance is barely present
Solution Approach 1:
By segmenting the holder into a flange and separate spring arms, the design creates gaps between the spring arms and the through opening wall. This segmentation maintains reliable retaining force through the spring arms' elastic engagement with the screw, while simultaneously providing lateral assembly clearance that allows the screw to accommodate axial offsets between the through opening and threaded bore of the basic structure.
Solution Approach 2:
The spring arms are designed with localized elastic properties, being stiff enough to provide reliable retaining force when engaged with the screw shank, yet flexible enough to allow lateral movement and clearance. This local quality differentiation enables the holder to simultaneously achieve high retaining force and adequate lateral assembly clearance, resolving the contradiction between reliability and adaptability.
3Adaptability or versatility
If a disk-shaped flange holder with spring arms is used, then axial and radial mobility are ensured and lateral assembly clearance is enabled, but the device complexity increases
Solution Approach 1:
The spring arms provide the necessary axial and radial mobility through their inherent elastic properties, eliminating the need for complex mechanical guidance systems. The dynamic deflection of the spring arms automatically accommodates lateral assembly clearance requirements while maintaining simple construction that can be manufactured using standard processes, thus achieving high adaptability without proportionally increasing device complexity.
4Ease of operation
If the screw is pushed out of the through opening during assembly, then the assembly side can lie on the mating surface, but the holder must allow sufficient axial displacement
Solution Approach 1:
The spring arms provide the necessary axial displacement capacity through their elastic deflection, allowing the screw to be pushed out during assembly without requiring the holder to have excessive projecting length. The dynamic elastic properties of the spring arms enable sufficient travel distance within a compact holder structure, facilitating easy assembly while maintaining a reasonable holder length that does not protrude excessively from the assembly part.
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 assembly unit ensures reliable captive screw fixation and axial positioning, even with small assembly parts, and allows for radial movement to compensate for axial offsets, enhancing handling and assembly precision while being cost-effective and material-efficient.
Implementation Method 1
Onto the outer rim of the flange are molded a plurality of spring arms, which are spaced apart in the peripheral direction thereof and extend away toward the head-remote shank end
Implementation Method 2
the inner rim forms a first retaining element or at least bears a first retaining element, wherein the latter juts into an undercut space extending between the head of the screw and a shank projection axially distanced therefrom
Implementation Method 3
In a first axial end position of the holder, the second retaining element or elements make contact with a stop element, projecting radially inward from the wall of the through opening
Data Source
AI summary
An assembly unit has an assembly part with an assembly side and an opposite outer side. A through opening extends from the outer side to the assembly side. A screw has a head that juts radially over the through opening on the outer side and a shank which reaches through the through opening with radial clearance. The screw has a threaded portion and a non-threaded portion disposed between the threaded portion and the head. A holder fixes the screw captively in the through opening. The holder is held axially movably in the through opening and axially movably on the non-threaded shank portion of the screw. The holder has a disk-shaped flange with a flange opening reached through by the non-threaded shank of the screw.