Mold pattern technological CAD design of the compressor cross-head.

Eftimie, Lucian ; Tierean, Mircea ; Baltes, Liana 等

1. INTRODUCTION

Reciprocating compressors (fig. 1) usually compress gas inside of cylinder, which inner volume is diminished due to piston movement. The compressed gas goes out through the discharge valves. The new cycle is starting with the admission of the gas, through the suction valve, as a result of the depression caused by the expansion of the inner volume of the cylinder. All the variable volumes of the inner cylinder are obtained as a result of a linear movement of the piston. If the piston is not moving following a straight line, the different supporting elements will wear.

The target of this paper is to show how is generated the additional shallows to reach the necessary shape of a pattern, especially for the compressor's cross head application used in hydrogen processing.

2. APPLICATION FOR CROSS HEAD STRUCTURE

The cross-head body is a complex part which allows compensating all efforts of the connecting rod transferring only the axial movement relative to the piston rod.

During the function the cross-head slides, on the cylindrical side faces, covered by low friction material. It is necessary to use a strong, but light design, to force on minimum the oil sliding film. Such a design is presented in the Fig. 2 using CATIA V5 software (Balc et al., 2004).

The part will be made by steel, using green sand casting technology, which requires specific steps to be followed as we show on a simple sample.

Let's suppose that the part is as simple as a cylinder, and applying all the following technological steps.

We analyze the body and choose the splitting plane as symmetry into the horizontal plane (see fig. 3). Having a symmetry system, we can work only on a half body, and we choose to work on the upper side generating the final assembly by symmetry. We add all around the side surface the necessary material to compensate the thermal contraction (figure 3).

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Machining will be necessary to obtain a smooth surface. We apply a new layer of material to allow the safe machining of the surface (see fig. 3) (Chu et al., 2006).

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The moulding technology uses cline or taper walls to the extraction direction minimizing the friction to the walls mould cavity (fig. 3) (Madan et al., 2007).

To prevent contraction cracks on the edges of the casting steel when cooling, we apply required fillets on the inner or outer edges (fig. 4). Applying symmetry, we can reconstruct the full body of the sample.

The final analysis will be made on the cross-section of the full body of the sample.

We have applied all the necessary technological steps on the finished part design shown in figure 1, and we have obtained the final shape of the cross-head body, as shown in figure 6. Into this moulding technology, caused by the complex empty inner part, we have applied also some spatial support--red colour represented--for the mould core. This red marked supports will be included both in pattern and core.

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By symmetry, we can extend the CAD part to the full body (fig. 7) (Cooper, 2001).

The pattern was produced by wood, because it is cheaper for only one part, using CAM technology on a vertical machining centre Fadal 4525, as shown in figure 8.

It can be observed that all the round surfaces connected to the splitting plane, are not modified as a cline surface because they are enough curved and they are not modifying the cavity when extracting (Homburg et al., 2006).

The full body was oriented one against the other--half by half--using two pins crossing the splitting plane.

The core box was realised using the same technology. It means that we have applied all the necessary layers to allow the thermal contraction, the machining layers, radius to prevent cracks and setting extensions to fix the core inside the mould.

3. CONCLUSION

Using CAD design and CAM technology can be obtained complex and precise patterns.

This is a simple method to produce other more complex parts. The method is applicable to the green sand moulding technology. It is necessary to respect all conventional marking colours for outer surfaces, for inner core marks and splitting plane.

The mould must have correct sized risers to feed rapidly the melted steel into the casting case during solidification. The casting designer will take in consideration the steel flows into the mould, to avoid turbulent flow as well as rapid fill of the mould. Gates and risers are added during the moulding process.

Foundry manufacturing methods specific to green sand moulds, applied for this moulding technology, improves quality of the final products.

4. REFERENCES

Balc, N. & Campbell, R.I. (2004) From CAD and RP to innovative manufacturing, Computing and solutions in manufacturing engineering, COSME '04

Chu, C.H.; Song, M.C. & Luo, V.C.S. (2006) Computer Aided Parametric Design for 3D Tire Mold Production, Computers in Industry, Volume 57, Issue 1, p. 11-25

Cooper, K. (2001). Rapid prototyping technology: selection and application, Marcel Dekker, New York

Homburg, N. & Wellbrock, E. (2006) Knowledge-based manufacturing strategy and methods for foundries, RTejournal, Ausgabe 3, p. 1-10

Madan, J.; Rao, P. V. M. & Kundra, T. K. (2007) Computer Aided Manufacturability Analysis of Die-cast Parts, Computer-Aided Design & Applications, Vol. 4, No. 1-4, pag. 147-158