Contributions to the improvement of the dynamic characteristics of the hydrostatic springs.

Otlacan, Dimitrie ; Kaposta, Iosif ; Tusz, Francisc 等

Abstract: The paper presents, in a critical way, the state-of-the-art and the problems of the dynamic characteristics of the hydrostatic springs necessary for the shock absorbers used in the construction of the railway wagons. From this analysis, our original contributions at the development of the hydrostatic technology for the construction of the shock absorber result. The study of the compressibility of some fluids was performed. A new hydrostatic shock absorber was conceived, one that allows the achievement of any dynamic characteristics, at least at the level of the best hydrodynamic shock absorbers, without having their complexity.

Key words: shock-absorbers, hydrostatic springs, compressible fluids, dynamic regime.

1. HYDROSTATIC SHOCK ABSORBERS

A usual application of the hydrostatic spring is the construction of the buffers for railway vehicles. In this case, both the properties regarding the compressibility of the fluid and those regarding its viscosity are important, so that, in the dynamic regime, through the accumulated energy, the protection of the wagon may be achieved (Dixon, 1998).

In figure 1, the principle of this spring for the buffers is presented. One may notice that the fluid is forced to pass through a ring hole around the piston and this way a hydraulic resistance is achieved, with regard to the displacement velocity of the piston and of the collision velocity of the vehicles. The hydrostatic springs (Fig. 1) rely on the compressibility of some fluids, the most used being those that have certain elastic properties. Their main characteristics are: viscosity: [10.sup.7] to 2x107 cSt; compressibility: 15%, at a pressure of 4000bar; energy consumption due to the hysteresis at a cycle: 5 to 10%; response time: from 0.01 to 0.1s. These characteristics are not strongly related to the deformation speed.

The hydrostatic shock absorber is placed in an uncompressed state and it is linked with another spring (usually made of rubber) in order to control the force necessary in the first part of the deformation of the buffer. This way, a static and a dynamic characteristic F = f (d) is obtained, that is according to the standard of the International Union of the Railways (UIC 526-1). The typical characteristics of the hydrostatic springs used nowadays for the buffers of the railway vehicles are presented in figure 2; the dynamic diagram was obtained through the impact of two wagons, of 80t each.

One may see the significant force step that occurs once the displacement of the additional spring ends and the hydrostatic shock absorber mounted in the uncompressed state starts to function. This produces, when the wagons pass through curves, high transversal forces, with undesired influences on the wear of the plates, of the railway, and even on the safety of the traffic, appear. If the pre-compressing force of the hydrostatic shock absorber was reduced, it would not be possible to obtain in the buffer an accumulated energy according to nowadays regulations. The decrease of the size of the hole through which the hydraulic resistance is appield would affect the static and the dynamic behavior at low temperatures, making the buffer to fall outside the admitted limits, according to the present regulations.

This is the reason why, although the buffer would be fit, at the limit, with the standard of the C category buffer (minimum 70kJ of accumulated energy), after the manufacturing and the use of thousands of pieces throughout Europe, now more and more wagons manufacturers gave up its services. The larger of the two factories that manufactured these shock absorbers already stopped the production in that which concerns the rail ways applications. One notice that the additional spring does not accumulate in the dynamic regime supplemental energy with regard to the one accumulated in the static regime.

2. CONSTRUCTION OF A NEW SHOCK ABSORBER

Our first contribution was the identification, with experimental studies on several substances, of a rubber, made in Romania, with superior characteristics regarding the compressibility and the behavior at extreme temperatures extreme, in comparison to the materials used on an international scale. Obtaining a 15% compressibility at a 3000 bar pressure with regard to a 15% compressibility at a pressure of 4000 bar, which characterize the fluid used in the products existent on the market nowadays leads to the possibility of an increase of the practical applications of the hydrostatic technology (Otlacan et al, 2006). Assuming the technological and functional limits of the hydrostatic technology, we achieved a spring-shock absorber able to use the advantages of the hydrostatic technology, but to benefit of functional characteristics at the level of the hydrodynamic technology. In figure 3, the principle scheme of the new shock absorber is presented (Otlacan, 1999).

In another constructive solution (Fig. 5a), the shock absorber can be achieved in order to obtain a step-by-step static characteristic, while in a third one (Fig. 5b), with special applications at the automatic couple of the wagons, the shock absorber is provided with a central hole, in order to simplify the transmission of the forces of elongation and compression at the vehicle. The constructive solutions developed are protected through the RO113229 patent (Otlacan, 2000).

The typical static and dynamic diagrams of the spring produced according to our new solutions (Fig. 5), with the dimensions for a buffer of the C category, according to UIC 526-1 (ORE 1991, 1995), are presented in figure 4.

3. CONCLUSION

The main technical and economic problem solved by the newly designed and manufactured shock absorber is that they allow for the achievement of any dynamic characteristics, at least at the level of the best hydrodynamic shock absorbers, without having their complexity.

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This is possible by a proper adjustment of the displacement "s" between the head of the piston and the cylinder. This variation according to the necessities of the functioning can be achieved through an adequate variation of the interior diameter of the cylinder on the portion specific to the displacement of the piston. It also has a constructive simplicity, low cost, small dimension and weight; its maintenance is easy. In the cylindrical solution, it allows for a simpler mounting on the wagon of the automatic couple. It is also an opportunity for the development of the hydrostatic technology.

4. REFERENCES

Dixon, J. (1998). The shock absorber handbook, Society of Automotive Engineers, Warendale, Pa.

ERRI B12 RP 17 (1994) Wagons. Program of tests to be carried out on wagons with steel under-frame and body (suitable for being fitted with the automatic buffing and draw coupler) and on their cast steel frame bogies, Ed. 8, Utrecht

ORE B12 RP 49 (1991) Bases de calcul pour l'etablissement des diagrams de la fiche UIC 530-2, Utrecht

ORE B36 RP 33 (1991) Study of suitable measures for improving the wear behavior of buffer head (application of high tensile-strength manganese)

ORE B51 RP 28 (1995) Testing the life of hydrodynamic and hydrostatic buffers

Otlacan, D.; Tusz, F.; Kaposta, I.; Jurca, S. Hydrostatic springs. Contributions to the improvement of their dynamic characteristics, Sc. and Tech. Bull. of the Aurel Vlaicu Univ. of Arad, Series: Mech. Eng., Vol.2, No.1, (April 2006), pp. 40-46, ISSN 1584-918X

Otlacan, D. (1999) Patent RO119142 (WO 03/067116; AU 2001/297520)

Otlacan, D. (2000) Shock absorber, Patent RO113229