Hydraulic shock absorber with force-deformation characteristic independent with regard to temperature.

Otlacan, Dimitrie ; Duma, Virgil Florin ; Kaposta, Iosif 等

Abstract: The paper approaches the problem of the variation of the damping force of a shock absorber with temperature. An experimental stall was designed and manufactured for the testing of shock absorbers. The force-deformation characteristics of such devices produced both in the EU and in Romania are studied for extreme functioning temperatures. In comparison, the force-deformation characteristics of our patented, improved shock-absorber are presented and one of its advantages is discussed: a variation smaller than 5% for the damping force with regard to temperature.

Key words: shock absorbers, damping force, temperature, force-deformation characteristics.

1. INTRODUCTION

The main role of the suspension system of a vehicle is to avoid transmitting shocks and vibrations produced by the road various defects to the main part of the vehicle (Dixon, 1998). The efficiency of the suspension system may be evaluated with regard to the ratio between the displacements of the wheels while in contact with the road and the displacements of the main structure of the vehicle, ratio commonly known as the coefficient of transmission ([tau]). At limit, like in the case of an ancient carriage with wooden wheels and no springs, [tau] = 1.

The efficiency of the suspension system has certain effects on: the comfort of the passengers, the safety of the load, the lifetime of the tires, the velocity of the vehicles and the protection of the roads, as well as of the environment (ORE 1995). Yet, the most important effect is the effect on the safety of the vehicle, by providing an optimum adherence of the wheels to the road, regardless of the traveling velocity, the specific load, the quality of the roads and the environmental temperature.

In principle, the suspension system can be modeled as a spring & damper system, parallel coupled (Fig. 1). The F = f (d) characteristic of the springs is constant with regard to the deformation velocity of the spring and is only slightly altered by this velocity or by the temperature of the environment. The F = f (d) characteristic of the damper, in order to prove efficient, has to be a function of the deformation velocity of the suspension system (UIC 1990). What is more, it would be ideal for the force to be adjustable or self-adjustable, even during the functioning, with regard to the specific load and to the quality of the road (Otlacan, 2006).

2. STUDY OF EXISTING SHOCK-ABSORBERS

Various problems are encountered during functioning, regarding comfort, adherence to the ground and vehicles rolling in extreme temperatures, especially during the cold season. In continental climate, temperatures of -10[degrees]C to -15[degrees]C are ussual during the winters, while in the summer, the temperature of the dampers may reach +60[degrees]C to +80[degrees]C. In consequence, experimental ascertainments were performed on shock absorbers of well-known automobiles companies.

[FIGURE 1 OMITTED]

The forces that appear in the shock absorbers at different temperatures and maximum velocities of the piston were studied, obtaining the [F.sub.max] = f (v) curves.

The experimental stall developed is presented in figure 2, with the following notations: A = shock-absorber to be tested; MBM = driving mechanism; TF = force traducer; TD = displacement traducer; AT = tensometric amplifier; A/D = convertor; C = computer (R = printing device).

The apparatuses used in the experimental research are: displacement traducer, type W200 (made by HBM-Germany); tensometric amplifiers, type N2314 (made by IEMI-Bucharest); force traducer, of original design (Otlacan, 2002); analog-digital convertor (National Instruments); mechanical part of the stall, achieved by the authors. Cooling of the shock absorber was performed in an isotherm chamber, with carbon dioxide; heating was performed by achieving, with the stall, several functioning cycles, with high deformation velocities.

The F = f (d) diagrams were ascertained experimentally for deformation velocities of 0,1 to 1m/s, at -15[degrees]C; +20[degrees]C; +80[degrees]C temperatures. With the maximum values obtained in these diagrams, the [F.sub.max] = f (v) curves were obtained, this time using the extreme values of the temperatures (Figs. 3,4).

From the experimental ascertainments, we concluded that, at the same value of the velocity of the piston, differences a) higher than 80% between the forces of the damper at normal temperatures of the environment (around 20[degrees]C) and the forces that appear at extremely low temperatures occur; b) up to 30% between the forces obtained at extremely high temperatures and the forces obtained at normal temperatures appear.

We may therefore conclude that the usual shock absorbers used nowadays for the automobiles, adjusted from manufacturing at a temperature of some 20[degrees]C, have a negative impact on both the comfort and the safety of the traffic in extreme conditions of temperature, due to the important variation of the maximum damping force.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

3. DEVELOPMENT OF A NEW SHOCK ABSORBER

In order to solve these problems, we developed a telescopic shock absorber with a force-deformation characteristic with a minimum variation of the damping force with regard to temperature (Otlacan, 2001). Using this patent, several prototypes were built, than tested with the same experimental stall (Fig. 2); the diagrams F = f (d) and than [F.sub.max] = f (v) for functioning at extreme temperatures were obtained (Fig. 5)

4. CONCLUSIONS

The solution patented solves the problem of the undesired variation of the maximum force in the telescopic shock absorbers on the entire domain of extreme functioning temperatures. The maximum damping forces at extreme temperatures thus has a variation smaller than 5% with regard to the maximum damping forces ascertained at temperatures of 20[degrees]C. The shock absorber developed allows, through the adjustment of constructive parameters, for a maximum damping force, smaller at lower than at higher temperatures.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

5. REFERENCES

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

Otlacan, D. (2002) Force traducer, Patent RO 86897

Otlacan, D. (2001) Telescopic hydraulic shock absorber, Patent RO 117280

Otlacan, D.; Kaposta, I.; Tusz, F.; Dobranski, J. Buffers for railway vehicles: measures for an improved velocity on the railways, Sc. and Tech. Bull. of the Aurel Vlaicu Univ. of Arad, Series: Mech. Eng., Vol.2, No.1, (April 2006), pp. 32-39, ISSN 1584-918X

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

Code UIC 520 (1990) Wagons, voitures et fourgons. Organes de traction, Ed. 6