Chain Tensioner Thread Geometry for Smooth Plunger Travel

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Solution Overview

Problem

Conventional chain tensioners face difficulties in smoothly and reliably moving the plunger to absorb chain looseness due to bulges formed on rolled internal threads interfering with external thread root surfaces, leading to unstable movement and potential chain slippage or flapping.

Innovation Solution

The chain tensioner features internal threads with rolled surfaces and chamfers at the intersection of protruding-side flanks and thread top faces, ensuring a surface roughness less than Ra 1.0 μm and chamfer angles of 30 to 50 degrees, allowing smooth sliding and preventing bulge interference with external threads, enabling quick and stable absorption of chain looseness.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If internal threads are formed by rolling to reduce surface roughness, then the plunger and screw rod can slide more smoothly, but bulges are formed on the thread top faces which interfere with the external thread root surfaces

Engineering Contradiction:

Improvesmoothness of plunger movementVSAvoidbulge interference

Core Design Contradiction:

ReliabilityVSObject-generated harmful factors

Solution Approach 1:

The harmful bulges generated during rolling are extracted and relocated to the chamfered portions of the internal threads, separating the beneficial smooth surface effect from the harmful interference effect. This allows the rolled surfaces to provide smooth sliding while the chamfers contain the bulges away from the external thread engagement area.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The bulges that are normally harmful interference factors are converted into a beneficial feature by providing chamfers that intentionally receive and contain these bulges. The chamfers transform the harmful bulge formation into a controlled feature that actually ensures proper thread engagement by preventing bulge interference with external threads.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

2Force

If the plunger moves significantly to absorb chain tension, then the tension is reduced, but the chain may become too slack causing flapping and tooth skipping

Engineering Contradiction:

Improvechain tension absorptionVSAvoidchain tension stability

Core Design Contradiction:

ForceVSReliability

Solution Approach 1:

The thread flank angles are changed to create asymmetric profiles where the push-in side has a larger angle and the protruding side has a smaller angle. This parameter change allows the threads to control plunger movement more effectively, enabling smooth absorption of chain tension while preventing excessive movement that would cause chain slackness.

Inventive Principle:

Principle #35Parameter changes

3Ease of manufacture

If cut surfaces are used for internal threads, then manufacturing is simpler, but the surface roughness is higher reducing sliding smoothness

Engineering Contradiction:

Improvethread formation simplicityVSAvoidsliding smoothness

Core Design Contradiction:

Ease of manufactureVSReliability

Solution Approach 1:

The manufacturing method is changed from cutting to rolling, which fundamentally changes the surface characteristics. Rolling produces smooth surfaces with low roughness that enable smooth sliding between the plunger and screw rod, while the asymmetric thread profile ensures proper functional performance.

Inventive Principle:

Principle #35Parameter changes

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 design ensures smooth and reliable movement of the plunger to quickly absorb chain looseness, preventing slippage and ensuring stable operation by minimizing interference between internal and external threads, thus maintaining optimal chain tension.

Implementation Method 1

a return spring disposed between the screw rod and the plunger, and configured to bias the plunger in a direction in which the plunger protrudes from the cylinder

Methodology Applied

Scientific EffectElastic force: Elasticity

Implementation Method 2

since a rolled surface has a smaller surface roughness than a cut surface, the plunger and the screw rod should be capable of more smoothly slide relative to each other

Methodology Applied

Scientific EffectFriction reduction: Friction

Data Source

PatentUS11231091B2Chain tensioner

Publication Date: 2022.01.25 NTN CORP

AI summary

A chain tensioner is formed with internal threads each including a push-in-side flank, a protruding-side flank, and a thread top face. The push-in-side flank has a flank angle which is larger than a flank angle of the protruding-side flank. The push-in-side flank and the protruding-side flank are rolled surfaces formed by rolling, and have a surface roughness smaller than Ra 1.0 μm. Each internal thread further includes, at a position at which the protruding-side flank intersects with the thread top face, a chamfer configured to receive a bulge resulting from excess materials pushed out from the protruding-side flank during formation of the internal thread by rolling.