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Procedia Engineering 10 (2011) 213–218

ICM11

Influence of presetting on fatigue lifetime of torsion bars
Vinko Mo ilnika, Nenad Gubeljakb, Jožef Predanb
b

a
ERD d.o.o., Engineering, Research & Development, rne e 186, 2370 Dravograd, Slovenia University of Maribor, Faculty of Mechanical Engineering, Smetanova ul. 17, 2000 Maribor, Slovenia

Abstract
Torsion bar springs are dynamically loaded machine elements which can break due to fatigue. High strength finegrain steel, grade VCN is normally used for springs or shafts. Springs are used as shock absorbers in caterpillar machines, where the diameters and lengths of the springs are limited. Springs should meet the requirements regarding spring constant (applied torque vs. twist angle), twist angle magnitude, and overcome the prescribed fatigue lifetime (number of fatigue cycles to failure). Using twist angle, it is possible to increase the elastic-plastic presetting of a torsion bar. A spring made as a torsion bar may exhibit different fatigue behaviour regarding to different elasticplastic presetting torques. Experimentally obtained results show that fatigue lifetime strongly depends on the ratio between the preset elastic-plastic torque and the applied fatigue loading range. A model for fatigue lifetime and shear stress level is proposed in spite of relative small number of tested specimens. © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of ICM11 Keywords: Torsion bars, Presetting, Multi-axial stress state, Micro cracks, Fatigue, Twist angle, Lifetime, Torsion ;

1. Introduction
Round bar springs are common used as high strength torsion springs for vehicles and caterpillar machines. The quality of spring has an effect on fatigue lifetime in the cases of low-cyclical and highcyclical fatigue. A spring’s quality is determined by the material’s microstructure, and the thermomechanical treatment of the springs during the manufacturing process. The final cold rolling defines the final shape and surface roughness of a spring. The process of cold rolling reduces any stress concentration caused by surface roughness. It is well known that it is possible to achieve a higher suspension angle by presetting the spring [1, 2]. Compression residual stress caused by presetting appears at the outer boundary of the spring. It is accepted that the extension of a torsion spring’s lifetime is achieved by coldrolling its surface. Surface defect can cause additional stress concentration, and a reduction of lifetime. Properly performed presetting of the torsion bar spring usually leads to higher elastic twist angle, in contrast to a spring bar without presetting [1, 2, 3]. In order to enlarge the angle of elasticity, the springs are torsion-deformed into a plastic during the manufacturing process. The preferred method is elasticplastic presetting of the bar. The bar is twisted within the plastic shear zone over a specified angle, unloaded, and twisted at least twice to the same twist angle. The measurement for plastic presetting is the * Corresponding author. Tel.: +386-2-220-76-61; fax: +386-2-220-79-96. E-mail address: nenad.gubeljak@uni-mb.si.

1877-7058 © 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11 doi:10.1016/j.proeng.2011.04.038

Vinko Mocilnik et al. / Procedia Engineering 10 (2011) 213–218 ˇ

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angle size over the yield shear-angle. Presetting causes plastic shear strain at the outer boundary of the spring bar, meanwhile the material in the middle of the bar only remains elastically-deformed. It causes compressed residual stress at the outer boundary of the spring bar. This paper aims to analyses the effects of different presetting twist angles on fatigue lifetime under differently applied strains. Nomenclature

A0
gmin
gmax
m
Mt
Mtmin
Mtmax
Nf
R
Rp0.2
Rm
e
t
m
t
ps
m
t
ps
e

- Coefficient of the power law equation...
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