Influence of presetting on fatigue lifetime of torsion bars

Torsion bar springs are dynamically loaded machine elements which can break due to fatigue. High strength fine-grain 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 elastic-plastic 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.