overview and present applications

Introduction Overview: Past to Present

• Michael Thonet and his Chair No. 14 • Applications of the Past • Present use

How does it work • Video Solid Wood Bending • Tensions in Bentwood

Stress Relaxation of Wood after Bending • Burger-Kelvin model & viscoelastic properties of wood • Practical application

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Content

Family operated, est. 1870 Today main business Solid Wood Bending equipment Further activities in renewable energies Bending machines for open (U) and closed (O) shapes Softening plants: steam generator & autoclaves Export worldwide

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Introduction

Michael Thonet and his Chair No. 14 • 1796 – 1871 German cabinet maker

o 1841 moving from Boppard to Vienna o 1849 developing solid wood bending (Thonet Method),

because laminating results not satisfactory (glue) o 1859 designing chair Thonet No. 14

› First mass product of industrialisation › Until 1914 > 50 mio. produced › Modular design for making multiple versions

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Overview: Past to Present (1)

Wood Bending Applications of the Past: • Chairs • Sleighs (and related) • Frames of many kinds

o Suitcases o Ships (vertical frames) o Airplanes (wood constructions until 1950ies)

• Wheel rims, steering wheels etc.

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Overview: Past to Present (2)

Wood Bending – Present Use: • Chairs • Sleighs (and related)

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Overview: Past to Present (3)

Softening • Initial moisture, optimum: 18% ± 2% • in steam, T ≈ 100 … 110 °C, t ≈ 1min/mm

Bending • Bending ratio s =

depending on wood species (degree of tolerated densification)

Stabilizing 1. t ≈ 30 … 90 min with strap & clamp 2. Drying in frame to H < 12% 3. No more spring-back (see Norimoto)

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Wood Bending – How does it work (1)

r d

Tensions in Bentwood • Wood allows tensile elongation only to ≈ 1,5% • Further elongation causes breaking • Softened Wood: Compression uncritical • System of Metal Strap with End Stops & Softend Wood

o Tensile Stress covered by Strap o Compression Stress to densify softend wooden cells

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Wood Bending – How does it work (2)

Fig. b) Wood Bending Fig. a) General Bending

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Stress Relaxation of Wood after Bending

σ0

E0 (εel)

Er ηr (εr)

η0 (εv)

σ0

Burger-Kelvin-Model

Fig.: Burger-Kelvin Model of viscoelastic material behaviour [Sae92], with: εel - elastic; εv - viscous; εr - viscoelastic (relaxing)

ε

εr Re

lief

Load

σ

σ0

εv t

t

εel

εv εr

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Stress Relaxation of Wood after Bending

Viscoelastic properties of wood

0

2

4

6

8

10

12

14

0,222 11,333 22,444 33,555 44,666

σ, N/mm²

Wood Bending - Compression Stress

t, s

Stress Relaxation Compression Stress

Machine Stop Bending

Fig.: Compression stress relaxation of solid wood during bending process [Egg95]

Practical Application • Bentwood needs to stay in the strap (end-stop),

until tensile stress is relieved. o Cutting on relaxation time = increased productivity

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Stress Relaxation of Wood after Bending

Fig.: Production flow of Solid Wood Bending line in a chair plant [Egg95-2]

Thank You

Contact: Dr.- Ing. Otto Eggert GHEbavaria Maschinen GmbH Sudetendeutsche Str. 38 80937 München +49 (0)89- 6001 3525 otto.eggert@ghebavaria.de www.ghebavaria.de

More videos available at http://www.youtube.com/user/GHEbavaria

Solid Wood Bending 11/08/2012 12

http://www.youtube.com/user/GHEbavaria

[Egg95] Eggert, Otto: Untersuchung der Einflußgrößen beim Biegen von Vollholz. Diss. Univ. Stuttgart 1995

[Egg95-2] Eggert, Otto: Materialflußoptimierung beim Holzbiegen. Stuttgart, Holz-Zentralblatt Nr. 121 (1995)

[Sae92] Saechtling, H.: Kunststoff- Taschenbuch. 25. Aufl., München: Hauser 1992

Heisel, U. & Eggert, O.: Holzbiegen- ein umweltfreundliches altes und zugleich modernes Produktionsverfahren. In: HK 1- 1994. Stuttgart, DRW-Verlag 1994

Eggert, Otto: Stress Relaxation in Solid Wood Bending. In: Proceedings of The 12th International Wood Machining Seminar 1995, Kyoto, Japan. The 12th IWMS Organizing Comitte p. 204- 214

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Literature

Introduction to��Solid Wood Bending�overview and present applicationsContentIntroductionOverview: Past to Present (1)Overview: Past to Present (2)Overview: Past to Present (3)Wood Bending – How does it work (1)Wood Bending – How does it work (2)Stress Relaxation of Wood after BendingStress Relaxation of Wood after BendingStress Relaxation of Wood after BendingThank YouLiterature