Watch Case-Lug Connection Using Undercut Tab and Recess Lock

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Solution Overview

Problem

Existing watch case and bracelet attachment methods lack a secure and irreversible connection mechanism, which can lead to detachment and potential damage during wear or handling.

Innovation Solution

A caseband-lug connection device featuring a first connector with a tab and a recess, and a second connector with a groove and an undercut portion, allowing for irreversible integration of the case middle and horn when the undercut portion is seated in the recess under compression.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If traditional attachment methods (claws or hollows with notches or protrusions) are used to connect the case middle and horn, then the device complexity is reduced and ease of manufacture is improved, but the reliability and strength of the connection deteriorate, allowing detachment under force

Engineering Contradiction:

Improveconnection reliabilityVSAvoidconnector structure complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The connector is divided into two distinct parts: a tab with a recess on the horn and a groove with an undercut portion on the case middle. This segmentation allows each part to have a specific function - the tab provides a positive engagement feature while the groove with undercut provides the locking mechanism, together achieving irreversible connection without excessive overall complexity

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The connector design employs asymmetric geometry where the tab has a recess on one side and the groove has an undercut portion creating a non-symmetric engagement. This asymmetric configuration ensures that the horn can be inserted and locked in one direction but cannot be removed by simple reverse force, providing irreversible connection and preventing detachment under normal wear conditions

Inventive Principle:

Principle #4Asymmetry

2Strength

If reversible connection devices are used for caseband-lug attachment, then ease of operation for assembly and disassembly is improved, but the strength and stability of the connection deteriorate, allowing unintended detachment during wear

Engineering Contradiction:

Improveattachment strengthVSAvoidassembly and disassembly ease

Core Design Contradiction:

StrengthVSEase of operation

Solution Approach 1:

The connector design incorporates preliminary action through the undercut portion geometry that creates automatic locking during the insertion process. As the tab is inserted into the groove, the undercut portion engages with the tab's recess and prevents withdrawal, providing irreversible connection. This eliminates the need for separate locking operations while ensuring strong attachment that cannot be accidentally detached during wear

Inventive Principle:

Principle #10Preliminary action

3Reliability

If simple attachment means are used without undercut portions or recesses, then device complexity is reduced and manufacturing is easier, but the reliability and irreversible connection capability deteriorate, allowing detachment under compression or force

Engineering Contradiction:

Improveirreversible connection reliabilityVSAvoidconnector manufacturing difficulty

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The design replaces complex mechanical fastening systems with a simplified geometric interlocking mechanism. The undercut portion and recess create a mechanical lock through their geometric configuration alone, eliminating the need for threads, screws, or multiple fastening elements. This substitution maintains manufacturing simplicity while achieving reliable irreversible connection

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

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 solution provides a secure and irreversible attachment of the case middle and horn, preventing detachment and mitigating the risk of damage when forces are applied, ensuring a stable and durable watch assembly.

Implementation Method 1

the tongue is configured to elastically deform upon insertion into the groove, the tongue and the undercut portion being configured such that the tongue substantially returns to its rest position, preferably returns to its rest position, when the undercut portion is housed in the recess of the tongue

Methodology Applied

Scientific EffectElastic deformation: Elasticity

Data Source

PatentEP3798742B1Device for connecting case body and lugs for a watch

Publication Date: 2025.05.28 ETA SA MFG HORLOGERE SUISSE

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AI summary

A case-lug connection device for a watch, a method for attaching a clasp comprising at least one lug to a case, and a case-lug assembly. The connection device comprises a clasp including at least one lug having a first connector and a case having a second connector, the first connector being configured to cooperate with the second connector to attach the clasp to the case, wherein: ∘ the first connector includes a tab with a recess; ∘ the second connector includes a groove with an undercut portion, the undercut portion being configured to fit into the recess of the tab; and ∘ the case and the clasp are made fixed together when the undercut portion of the second connector is fitted into the recess of the tab of the first connector.