String Trimmer Line Feed Mechanism Using Motor Braking

Solution Overview

Problem

Existing auto-feed line pay-out mechanisms in string trimmers require precise manufacturing of a reference weight, leading to potential errors in line feeding, and are prone to force imbalances causing unintended line payout due to delicate balancing.

Innovation Solution

A string trimmer with a spool assembly and line pay-out mechanism that uses an electric motor and controller to generate a third moment opposing the inertial forces of the line segments, allowing controlled payout only when the sum of moments exceeds a certain threshold, eliminating the need for a tightly balanced reference weight.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of operation

If a reference weight is used in the auto-feed line pay-out mechanism, then line payout control is achieved, but manufacturing precision requirements increase and reliability decreases due to delicate balancing

Engineering Contradiction:

Improveline payout controlVSAvoidreference weight tolerance

Core Design Contradiction:

Ease of operationVSManufacturing precision

Solution Approach 1:

The patent replaces the mechanical reference weight balancing system with an electric motor-driven spool rotation system controlled by a controller. The controller selectively rotates the spool in first and second directions to pay out and rewind line, eliminating the need for precisely manufactured reference weights and their delicate inertial force balancing.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent introduces a controller as an intermediary between the user input and the spool rotation mechanism. The controller manages the motor-driven spool rotation, providing precise control over line payout and rewind operations without relying on mechanical balancing of reference weights.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Ease of operation

If a reference weight is used in the auto-feed line pay-out mechanism, then line payout control is achieved, but reliability decreases due to force imbalances from minor impacts

Engineering Contradiction:

Improveline payout controlVSAvoidoperation stability

Core Design Contradiction:

Ease of operationVSReliability

Solution Approach 1:

The patent replaces the mechanical reference weight balancing system with an electric motor-driven spool rotation system controlled by a controller. The controller selectively rotates the spool in first and second directions to pay out and rewind line, eliminating the need for precisely manufactured reference weights and their delicate inertial force balancing.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent employs a controller that can detect the operational state of the spool assembly and adjust motor rotation accordingly. This feedback mechanism ensures reliable line payout control by preventing unintended rotation from minor impacts while maintaining controlled payout when needed.

Inventive Principle:

Principle #23Feedback

3Manufacturing precision

If motor braking/reversal technique is used to pay out line, then reference weight precision requirements are eliminated, but device complexity increases

Engineering Contradiction:

Improvereference weight toleranceVSAvoidcontrol system complexity

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

The patent replaces the mechanical reference weight balancing system with an electric motor-driven spool rotation system controlled by a controller. The controller selectively rotates the spool in first and second directions to pay out and rewind line, eliminating the need for precisely manufactured reference weights and their delicate inertial force balancing.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent introduces a controller as an intermediary between the user input and the spool rotation mechanism. The controller manages the motor-driven spool rotation, providing precise control over line payout and rewind operations without relying on mechanical balancing of reference weights.

Inventive Principle:

Principle #24Intermediary (Mediator)

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

Ensures accurate and controlled line payout, preventing excessive or inadequate line feeding, and mitigating the impact of minor forces, thereby improving operational reliability and reducing the risk of unintended line payout.

Implementation Method 1

an electric motor, a rotatable output member driven by the electric motor

Methodology Applied

Scientific EffectElectromagnetic induction: Electromagnetic Induction

Implementation Method 2

The line pay-out mechanism is configured to control rotation of the spool relative to the output member to pay-out the trimming line from the spool in response to a difference between a first moment and a second moment. The first moment is generated by inertia of the one or more line segments. The second moment is generated by inertia of a reference weight and opposes the first moment.

Methodology Applied

Scientific EffectInertia: Inertia

Data Source

PatentEP3025573B1Line feed mechanism for string trimmer

Publication Date: 2019.10.02 BLACK & DECKER CORP

Figure 1~2
Figure 3~5
Figure 6A~7

AI summary

A string trimmer (10) with a controller (20) that brakes an electric motor (32) so that the electric motor can selectively generate a moment that is used as a control signal for a line pay-out mechanism (64).