The determination of the strain state in the bed of a press.

Gheorghiu, Horia ; Dinu, Gabriela ; Baciu, Florin 等

1. INTRODUCTION

The aim of this work was to determine the strain state in the bed of a press for curing the component elements of a rubber belt with a metallic insertion.

The blocking--tension group that includes the press in service belongs to the technological line for manufacturing and curing transport belts with metallic insertions. The tool has a double role:

--To block the metallic cables between this group and the blocking--tension device;

--To tension the cables during the curing process of the belt. The group is made of bed, blocking unit, guidance system using rollers, tensioning group and tensioning unit. Owing to the constructive symmetry, only half of the structure was analyzed: the external plate, the supports of the cylinders and the middle plate for which a thickness of 20 mm was considered (half of the thickness of the plate). The bed is made of three closed frames made of steel sheet, stiffened with members and lateral plates with two windows for the blocking unit and tensioning unit. The bed fixed with screws on two profiles. A lateral support is also provided for taking over the tensioning force of the cables. The blocking unit is made of two plates: the upper one (fixed at the top side of the window) and the lower one. The guidance system with rollers is located in front of the entrance in the blocking unit and features the possibility of adjustment for the battery of vertical guidance rollers. The system ensures the ordered conducting of the metallic cables at the entrance in the blocking unit. The tensioning group is made of four batteries of rollers mounted on the tensioning head, fixed at the upper part of the big window of the bed. The tensioning unit is made of 164 rollers, each driven by a plunger. The pressure on the sections of the plunger is ensured from a common bar; their bores are drilled in a common block that can move vertically under the action of two double effect hydraulic cylinders (Dinu, 1998).

The equipment of the press consists of the following main units:

--the stand, consisting of two closed frames made of steel sheet of 80 mm thickness stiffened by two lateral plates with a thickness of 20 mm. The stand is sustained on the platform of the unit on which it is fixed with screws by means of some supports;

--the lower cup which is displaceable, its displacement being obtained by means of the pistons of two hydraulic cylinders on which it is supported;

--the higher cup is fixed on the top of the window of the bed;

--Two main hydraulic cylinders, with simple effect, that are used to produce the pressing force; they are placed between the two frames of the bed;

--The guiding system of the lower cup ensures the plane-parallel displacement of the cup.

Technical characteristics/performance:

--Maximum pressing force: 107.5 x 105 N

--Maximum pressing surface: (2150 x 1000) [mm.sup.2]

--Stroke of the lower cup: 300 mm.

--Maximum pressure in the pressing cylinder, produced by the main hydraulic cylinders with simple effect: 29.7 MPa.

2. THE FINITE ELEMENT ANALYSES

For the calculation of the bed, the authors used the code SolidWorks by means of which both static and dynamic analyses can be performed.

The model of the bed was meshed with tetrahedral elements.

2.1 The model

The stand of the press, consisting of two identical plates, features symmetry in two perpendicular planes. For this reason, it was sufficient to study only one fourth of the structure.

For the nodes situated in the plane of symmetry some degrees of freedom were constrained so that the symmetry conditions were fulfilled. Thus, for the nodes on the plane 1 the displacement on Ox and the rotations around Oy and Oz were blocked, and, for the nodes on plane 2 the displacement on Oz and the rotations around Ox and O were blocked. (Fig 1)

The external load was applied as concentrated forces in the nodes placed on the top of the cylinders supports. The load for a half of a plate is P/4 of the load P, generated by the two cylinders and the load for a support is half of the force P/4 which means P/8 (Dinu et al, 2008).

The support of the structure was modeled by inserting a contour element with a very high elasticity constant in node nr. 48.

This first stage of the calculation was dedicated only to the determination of the areas with higher stresses and the maximum strain increase.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The force P for which the strain calculation was made has the value of 7.2345 x [10.sup.6] N and it is used for producing the band conveyer of a maximum width.

2.2 The calculation results

Using the code SolidWorks (UserGuide), the values of the principal strains in the nodes were obtained. Based on these values, the strains in the middle of the element sides placed on the inner contour of the plate (Fig. 2), in the most stressed area, were determined.

3. EXPERIMENTAL TESTS

In order to check the model used for calculation, experimental tests have been performed to obtain the strain state. For this purpose, resistive strain gauges manufactured by Hottinger were used, having the following characteristics: resistance 120 D, gauge constant 2.03, base 10mm.

The gauges were placed in the points 2, 3, 4, and 5 (fig.3).

The measurements have been made with a type MK bridge manufactured by H.B.M- RFG (Dinu et al, 2008), (Hadar et al, 2008).

In Table 1, are listed both the values of the normal strains resulted on the basis of the unit deformations measured using strain gauges as well as the values of the strains calculated by finite element analyses.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

4. CONCLUSIONS

The stress distribution is emphasized through the components yielding from the loads in the plane of the plates (membrane stresses) and the bending stresses.

The highest stresses appear in the areas having the smallest width. The stress variation in the area of the stress raiser is shown in Fig. 4.

From the data presented in Table 1, one can conclude that the values of the calculated strains are about 1.2 times higher than the measured ones.

The results obtained by calculation and measurements show that the most stressed part of the bed is the area with stress concentrator.

In this way, the numerical model is validated, and the calculation has a high degree of reliability.

5. REFERENCES

Dinu, G. (1998). Researches regarding the influence of the contact deformations-strains on the accuracy of the machine-tools, PhD thesis, "Politehnica" University of Bucharest

Dinu, G.; Atanasiu, C.; Baciu, Fl.; Nutu, E. (2008). Study of deformations in ram-boring spindle, Proceedings of the 19th international DAAM Symposium, Katalinic, B. (Ed.), pp. 379-380, Trnava, Slovakia

Dinu, G.; Baciu, Fl.; Vlasceanu, D.; Draghici, S. (2008). Study the strength and stiffness of a boring and milling machine bed, Proceedings of the 19th international DAAM Symposium, Katalinic, B. (Ed.), pp. 381-382, Trnava, Slovakia

Hadar, A.; Motomancea, A.; Szabo, A.; Gheorghiu, H.; Marin, D.; Ocnarascu, C. (2008). Consideration about One-way Hydrocylon with Ring Shaped, The 19th International DAAAM Symposium, ISSN 1726-9679, pp.1035, Trnava, Slovakia

*** (2009) UserGuide of SolidWorks

Tab. 1. Values of strain

 The principal strain

Gauge Measured Calculated Calculated
 nr. [MPa] [MPa] Measured

2 124 156,8 1,26
3 122 156,8 1,28
4 198,9 233,6 1,14
5 -265 -297 1,12