Combined study on mechanical behavior of scroller shafts.

Rusu-Casandra, Aurelia ; Iliescu, Nicolae ; Baciu, Florin 等

1. INTRODUCTION

Since scroller shafts are machine elements often used in practice, means for calculating stresses and strains that occur under loading conditions are important in structural design. This paper describes the results from a combined computational and experimental study, in order to design scroller shafts of special shapes in a new structural view and continues the research of a previous work (Rusu-Casandra et al., 2009). Calculated and experimental results regarding the stress state in the flanks of two types of scroller shafts are compared to assess the accuracy of the computational method developed.

The scroller shafts have the function to transport the material to be processed from the supply to the injecting or cutting compartment, by developing a certain compression force that provides the required pressure for a constant flow rate evacuation (Mott, 2005). Therefore they are made with variable geometry (the step is decreasing and the height of the flanks is increasing towards the compression compartment).

The two models (Fig.1 and Fig.2) were designed as structures with metallic insertion, composed of central steel shafts with hexagonal sections on which the scrollers of thermoplastic material were injection-moulded. Experiments have been carried out for an extreme situation met in practice, the self-locking of the shaft.

2. NUMERICAL CALCULUS

A finite element analysis was performed using SOLIDWORKS software (*** 2009). Two models (type A and type B) subjected to variable pressure, with a Poisson's ratio value applicable to photoelastic materials, have been supplementary loaded with a compression force P=30 N and a torque T=2 Nm applied on the crank of the scroller shaft.

Both the applied loads and boundary conditions used for the finite element models were chosen to be similar to those of the photoelastic models. The contour plots of the principal stresses difference ar a2 in the two models obtained using the finite element method, are presented in Fig. 3 and Fig. 4.

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3. PHOTOELASTIC INVESTIGATION

The experimental investigation was conducted using the three-dimensional photoelasticity technique (Iliescu & Atanasiu, 2006; Paipetis, 1990). Details of the methods for casting, testing and analysing the two models (Fig. 5 and Fig. 6) are given in a previous paper (Rusu-Casandra et al., 2009) and consequently only a short description of those techniques will be included.

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The models were made from Araldite by cold casting and the stresses in the shaft were fixed using the "freezing technique". The material to be processed was introduced into the feeder and transported into the processing compartment using a crank. The assembly have been fed in each case until the shaft was self-locked. Both models were compound loaded, being compressed with a force P=30 N and a torque T=2 Nm through a weight placed on the crank. The stress photoelastic constant for the model was determined to be [f.sub.[sigma]] = 55 x [10.sup.-3] MPa / fringe.

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Fringe patterns obtained for the two investigated sections of models type A and B respectively are shown in Fig. 7 and Fig. 8. The curves of the principal stresses difference [[sigma].sub.1] - [[sigma].sub.2] on the boundary of the two models are plotted in Fig. 9 and Fig. 10.

4. CONCLUSIONS

Comparison of the results of the finite element analysis (Fig. 3 and Fig. 4) with the photoelastic results (Fig. 9 and Fig. 10) regarding the stress state in the flanks of two types of scroller shafts led to the conclusions:

1. The value of the stresses is much lower in model type B compared with model type A. Thus the stresses on the surface of the first coil flank that compresses the material in the processing compartment are 50% lower in type B compared to type A. On the other side of the same coil of the type B the stresses are 75% lower compared to type A.

2. Small curvature radii (2 mm) between the flanks and the body of the shaft in type B have no significant influence on the distribution of stresses, therefore this area Is not a stress concentrator.

3. The coils in type B are oversized, therefore one can estimate that the thickness of these coils may be reduced by up to 30%, maintaining the same curvature radii without any significant rise in the value of stresses.

4. No noticeable discrepancies occur between theoretical and experimental results. The maximum error was less than 10% with the calculated results being smaller than the photoelastic results.

The main conclusion of the study leads to the correction of the flanks geometry, in this way an optimal shape for a new type of scroller shaft can be designed.

5. REFERENCES

Iliescu, N.; Atanasiu, C. (2006). Metode tensometrice in inginerie (Stress Analysis Techniques in Engineering), Editura AGIR, ISBN 973-720-078-0, Bucuresti

Mott, R. (2005). Machine Elements in Mechanical Design, Pearson Prentice Hall, ISBN 0130618853, United Kingdom

Paipetis, S. (1990). Photoelasticity in Engineering Practice, Routledge, ISBN 978-0853343639, United Kingdom

Rusu-Casandra, A.; Iliescu, N; Baciu, F. (2009). Comparison of Three- dimensional Finite Element and Photoelastic Results for a Scroller Shaft, Proceedings of the 20th International DAAAM Symposium, Vienna

*** (2009) Solidworks User Manual, Dassault Systemes SolidWorks Corp, Concord, MA, USA