Lavrenko Ia., Babenko A.

National technical university of Ukraine «Kievsky politechnical institut», Kiev, Ukraine

The lifetime research of laboratory centrifuge

To the high-speed laboratory centrifuge is put forward a number of requirements, the main one is to provide the necessary lifetime, the second, the guaranteed service life should be ensured fail-safety and reliability. These requirements are satisfied by the elements of data structures must not break down during the guaranteed period of work and safety is guaranteed by the presence of emergency shell [1].

Laboratory centrifuges are used in medical, biomechanical and in chemical laboratories. The main tasks of design is to ensure reliability, capacity and ease of use. Because of the different conditions of operation, the industry supports a wide range of rotors and adapters for laboratory centrifuges (Fig.1). The rotor (I) of lab centrifuge

Fig. 1. Rotor and cap of lab centrifuge

which rotates is one of the most loaded constructive elements who use at variable loading is destruction represents danger. The cup (II) with different forms for beakers (III) fixes on pins and changes the position at speed up and speed down. During the investigation it was found out that the destruction takes place cups in place of contact with pins, and through detachment bottom of the cup [2]. Structural elements, which is the danger of destruction, is a cup.

In this paper the cups as one main constructive element is mounted to a laboratory centrifuge rotor, Figure 1 was considered. To improve the accuracy of predicting the lifetime of the machine and, consequently, more accurate selection of the parameters of anti-crash container in order to facilitate their weight. To predict the lifetime of constructive elements, it is necessary to determine more accurately their stress-strain state (SSS) and mechanical properties of the material structure.

The lifetime of construction can be defined as experimental and simulation course. Actual testing of the acceleration and braking of centrifuge take a lot of time and are expensive, so to predict the construction resource the simulation of stress-strain state (SSS) centrifuges at different types of loads and complex of experimental studies was carried out. In this case, the dynamic calculations are performed by FEM [3]. Definition of the stress-strain state made within the collaborative work NTU "KPI" and the Institute of Mechanics, Otto-von-Guericke University Magdeburg (Germany) by means of widespread software package ANSYS [4, 5]. The maximum stresses and deformations which arose in a cup under the influence of a cup body weight were considered first of all. Identified three levels of stresses (470 MPa, 490 MPa, 500 MPa), which arise in dangerous points of cups lab centrifuge. To determine the influence of stress concentration the parameter which is called as theoretical coefficient of stress concentration is used [6,7].

Fig.2 Cyclic load	Fig.3 Cyclic creep	Fig.4 Creep

To the test used hydraulic test stand MTS 810 (Fig.5). Mechanical properties of samples were determined by their stretching with speed of 10 mm/min. Tests made according to GOST 1497-84. The character of deformation studied on diagram points according to the methods [8].

Fig.5 Samples and test stand MTS 810 to determine the mechanical properties of materials

To the test used hydraulic test stand MTS 810 (Fig.5), through which conducted three phases of research. In the first phase of research were tested cylindrical specimens of aluminum alloy 7075 on low cycle (Fig.2) with sawtooth cycle fatigue loading (loading sample to the required level of stresses occurred at 5sec and unloading 5sec). In the second phase were tested at constant load to fracture (Fig.4). Laboratory centrifuges work in different conditions, therefore the time the centrifuge is in the operating conditions could be different. In this paper, the time chosen on the basis of the optimal time of testing samples. The third phase of testing - the study sample at program load (Fig.3) (load sample 5sec, holding at constant load 120sec and unloading 5sec).

Fig.6 Specimens failure probability 1%

Plot the fatigue curves was performed by the standard method according to GOST [9, 10]. On the basis of the distribution curves of fatigue life curves of equal probability was built. At the diagrams curves of fatigue specimens of aluminum alloy 7075 for the probability of destruction are shown. Fig. 6 shows the fatigue curves for different types of loading at a probability of 1%. It can be concluded that the maximum lifetime have specimens that tested at low-cycle fatigue.

Conclusions

Researches have shown that the unsuccessful choice of manufacturing techniques of preparation is one of the reasons which promotes destruction in the form of branch of a bottom of a the cup shell. In all cases the stress-strain state simulation has shown that the maximum stresses take place in a point of contact of a pin of a rotor and cups of a laboratory centrifuge. It leads to cup destruction in a place of contact of a pin and a cup.

As a result of experimental studies were built durability and fatigue distribution curves for aluminum alloy 7075. These data allow to conclude a satisfactory correlation of experimental and theoretical calculations. The approach proposed by Lemaitre counting of interaction damage under cyclic creep gives satisfactory results, but requires further research.

References

[1] Lemaitre J., Desmorat R. Engineering Damage Mechanics, Springer, 2005, p.380.

[2] Babenko A., Lavrenko Ia., Strackeljan J. Estimation of a centrifuge rotor strength and analysis of the influence of constructive and technological parameters. // 10. Magdeburger Maschinenbau-Tage. 27.-29. September 2011.

[3] Babenko A., Lavrenko Ia. Vyznachennja napruzheno-deformovanogo stanu chashky laboratornoi centryfugy pid dijeju dynamichnogo navantazhennja. // Vseukrainskyj shhomisjachnyj naukovo-tehnichnyj i vyrobnychyj zhurnal. Mashynoznavstvo. – L.: - 2011. - №7-8. - s. 7-10.

[4] Lavrenko Ia. I. Do pytannja pro vyznachennja resursu konstruktyvnyh elementiv pry zminnyh navantazhennjah. Visnyk NTUU “KPI”, Mashynobuduvannja. - K.: Yzd-vo KYT, 2009 . - №. 56., s.88-92.

[5] Stolarski T., Nakasone Y., Yoshimoto S. Engineering Analysis with ANSYS Software, 2006.

[6] Neuber H and Hann H. Collection of Translations, Mechanics, №4, 1967.

[7] Pisarenko G. S. Strength of materials - 5th ed. Rev. And add. - K.: Highest sc. Head Press, 1986.

[8] Pavlov I. M., Shelest A.E., Tarasevich Y.F. The study of discontinuous deformation patterns of some alloys after thermomechanical processing. - In Sat: Plastic deformation of refractory metals and special alloys. Nauka, Moscow, 1970, p.111-125.

[9] GOST 28785-90 «Ultracentryfugy ta rotory preparatyvni. Zagalni tehnichni vymogy ta metody vyprobuvan».

[10] GOST 25.502-79 «Metody mehanichnyh vyprobuvan metaliv. Metody vyprobuvan na vtomu».