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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