Possibilities for enhancing pressing force of pull down hydraulic press.

Masek, Bohuslav ; Hronek, Pavel ; Stadler, Ctibor 等

1. INTRODUCTION

The analysed forging press (Fig. 1.) is a pull down, two-column construction, with a direct oil drive. The movable closed O-frame slides through guides in the middle crossbeam, which is fastened on a foundation. The movement of the O-frame is ensured by the combination of the press and return cylinders, which are situated below the floor of the forging shop, between the middle and lower crossbeams of the O-frame. On the floor of the forging shop a longitudinal and transverse table for the bottom swage holder slides on the middle crossbeam. The holder for the top swage is fixed to the upper crossbeam of the O-frame.

[FIGURE 1 OMITTED]

2. DEMAND FOR PRESSING FORCE ENHANCEMENT

The aim of this paper is to analyse the stress-strain on the press structure, which consists of the O-frame and the middle crossbeam.

This structure is designed and used for a nominal force of 6.3 MN, which is generated with one press cylinder. The pressing force could be enhanced up to 8 MN without significant redesign of the press structure. Frames of earlier presses can be used for higher pressing forces under certain conditions. It is indeed necessary to perform detailed analysis of load-carrying parts and explore pressing force enhancement possibilities. FEM was used for this analysis.

3. PROBLEM SOLUTIONS

One way to increase the pressing force is to utilize the return cylinders to generate force in the working stroke. The other way of achieving pressing force enhancement is by increasing the pressure in the hydraulic circuit. In this case however, reconstruction requires purchasing a hydraulic power supply, changing tubes and sealing elements. This variant does not constitute a significant redesign of the press structure, but it is much more expensive. To analyse the structure, a FEM model of the basic load-carrying parts was made, i.e. O-frame and middle crossbeam. This model was completed with swage holders, including swages and the forging piece, for a more realistic distribution of force. In the area of the swages, the system is supported by a solid figure representing the forging piece, which gives the model its required eccentricity.

We set several conditions for carrying out the analysis. In the analysis, we did not consider the self-weight effect of the O-frame and middle crossbeam or the weight of the forging piece. In comparison with pressing force, these can be ignored. All analyzing models are solid and omit unnecessary details. These details do not have cardinal dominance in a state of stress but they complicate the making of FEM meshes.

A look at the stress distribution in the whole assembly of the press structure reveals that evidently (Fig. 2.) average values are moving under the fatigue limit and only in certain local extremes, which can be dangerous for structural solidity. The analyses detected a few places with increased stress concentrations, the highest of which were in the lower crossbeam of the O-frame (Fig. 3.) in places where the pressing force is transferred to the O-frame. In these places, the material fatigue limit was exceeded.

The first important and potentially dangerous place is the connection between the O-frame and the main hydraulic cylinder. There is a power transfer between the working cylinder by means of a swivel spherical lens to the O-frame where the stress is 179 MPa. This stress is under the yield limit, however, it is over the fatigue limit. It is necessary to add that there is not a pronounced stress increase compared to the current state and it is locally concentrated in a small volume of the compact O-frame material. The stress around this place decreases rapidly, which means that it is not dangerous.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The second place is where we have pivoted return cylinder drawbars. These drawbars are placed in exchangeable capsules, which are set into eyes cast within the O-frame. The eyes have the biggest stress concentration, which can reach up to 194 MPa.

4. CONCLUSION

The stress analysis found that increasing the pressing force to 8 MN using return cylinders in the working stroke is possible. From the deformation viewpoint, the press structure is suitable. The only problematic places (Fig. 3.) in the press structure are the eyes for fastening the return cylinders. These were cast with the O-frame, and are dimensioned just to lift its weight. However, their modification does not mean a significant intervention into the press structure.

5. ACKNOWLEDGEMENTS

This paper includes results created within the project 1M06032 Research Centre of Forming Technology. The project is carried out within the framework of the Research Centres (1M) programme and subsidised from specific resources of the Czech state budget for research and development.

6. REFERENCES

Heinz Tsaetsch, (2005). Metal forming practise, Springer-Verlag, ISBN 3-540-33216-2, Berlin

Lasova, V. & Kosnar, M. (2008). Mechanical Properties of CKV 7200 Press at Pressing Force 105MN--Current State & New Modifications, University of West Bohemia, Pilsen

Schuler, (1998). Metal forming handbook, Springer-Verlag, ISBN 3-540-61185-1, Berlin

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