diamond-spark grinding in forming of functional … · 2009. 11. 26. · abstract:recently...

Fiabilitate si Durabilitate - Fiability & Durability nr.2/2008 Editura “Academica Brâncuşi” , Târgu Jiu, ISSN 1844 – 640X

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DIAMOND-SPARK GRINDING IN FORMING OF FUNCTIONAL WHITE LAYERS OF INCREASED MICROHARDNESS IN STEELS

Yury GUTSALENKO

National Technical University “Kharkov Polytechnic Institute”, Ukraine Abstract:Recently increased microhardness of white layer structures calls attention of designers of steel products. To working surfaces of such steel products (for example, saw blade in woodworking) the requirements of high wear-resistance at as greater excess as possible in hardness over opposite body of working (cutting) contact are provided. Application of diamond grinding with introduction of additional energy as electric discharges (diamond-spark grinding) in the technologies of reinforced steels allows to raise homogeneity of white layer structures, that will assist to increasing of stability of finished product operation. Keywords: diamond, friction, microhardness, X-ray, roughness. 1. Introduction

The white layer (white zones) as slightly etching hard buildup, appearing on a surface of steel components during contact thermomechanical influence, for a long time draws attention of researchers from the point of view of possible increase of life-limiting characteristics of operational contact surfaces. However to the beginning of the third quarter of XX century cautious negative estimations prevailed among metallophysicists as regards practical usage of white layer phenomenon. There is characteristic point of view of world-famous scientists I.M.Lyubarskiy and L.S.Palatnik [1]: «White zone does not possess satisfactory for effective resistance to deterioration properties …, it is frequently formed as separate isolated sites on friction surface or in volume of metal, … the non-uniform areas being concentrators of stresses that results in deterioration wear-resistance are formed; the nonequilibrium white zone possesses low fatigue resistance». Here the same authors in the same work [1] emphasize: «the problem of white zone is rather actual and wide and demands statement of the further investigations for more detailed studying … » (citing in author's translation from Russian).

The subsequent investigations of various authors testify to indefatigable temptation to use effect of white layer for the practical purposes. Here we shall note only some examples of successful promotion in this direction.

Questions of prediction and purposeful quality management of the processed surface under conditions of white layer formation are investigated by the European technological school [2-4]. In the Ukrainian-Polish development of LTD. of Open Company «TRIZ» (Ukraine, Sumy) [5] white layer phenomenon is successfully used in technologies of repair of surfaces subjected to wear. Achievement of white layer microstructures during electroerosive processing of steel industrial equipment is considered as basic resource of strengthening [6]. At the Siberian University (Russia, Novokuznetsk) [7] even curing of fatigue damages of steels due to electric current pulses and practical technology based on this principle are justified on the basis of the mechanism of formation of local white zones in fatigue cracks.

Method of thermofriction strengthening of steel surfaces, using white layer phenomenon in real embodiments, relieved from the basic disadvantage quoted above on [1]: discontinuity flaw is completely eliminated and substantially - heterogeneity [8] is developed at Kharkov Polytechnic Institute.

Diamond grinding with introduction of electric current pulses in zone of processing (diamond-spark grinding) allows both to independently form the strengthened white layers in steels [9], and to support such layers by means of finishing processing after preliminary thermoclutch strengthening with increase of their uniformity [10], that holds with electropulse technologies [5,6].


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2. Diamond-spark grinding as finishing postprocessing after thermofriction strengthening

Results of research of influence of DSG on phase and stressed state of steels [9] are evidenced that control of quality of surface layer by means of combining of diamond grinding with sputter–ion influence can be more directional including control of forming of both structures of second heat hardening and structures of tempering then in traditional technologies. At that heat of machining surface is a little relative to TFP and it is equivalent to influence of backing, especially under grinding without current and by preliminarily dressed wheel.

Availability of use of diamond-spark grinding on operations of finishing following after thermofriction processing is confirmed of some practice of observation of white layer structures after diamond-spark grinding of steel [9] that can be explained exclusively thanks to action of electric discharges providing fast impulse heating of machined surface up to phase transformation temperatures. In electric contact patch maximum covering of machined surface in technologies of double speed diamond-spark grinding (with increased speeds of working motions of both diamond wheel and machined piece) it is to the point of melting and even to partial evaporating, with next enough quick cooling especially under influence of lubricoolant of efficient composition in combination with active technique of supply.

As it marked after known diamond-spark grinding experiment [9] just combination of high-speed and enough short-term heating with enough quick cooling can provide forming (even more so an effective support) of qualitative white layer in result of implementation of combined technologies.

Characteristics of white layer on working surface of wood-working knife blade from steel 65Г after thermofriction processing and next operation (finishing by grinding and backing) are presented in the Table 1. Table 1: Influence of thermfriction processing (TFP), next finishing and backing on characteristics of steel 65Г (by example of wood-working knife).

Characteristic Before TFP After TFP

After TFP and

finishing

After TFP, finishing and

backing Parent metal

(average) 5300

Dispersion - 8200 - 11200 9000 - 11000 9100 – 10100

Microhardness, MPa White

layer Moda - 10000 Parent metal Needled martensite

Microstructure White layer - Deformed granular martensite

Deformed granular tempered martensite

White layer depth, mm - 0.54 – 0.60 0.45 – 0.55

Ra 0.2 1.82 0.22 Rq 0.32 2.38 0.42 Surface roughness under Lc=0.25 mm, µm Rz 1.4 7.0 0.7


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X-ray metallography past thermofriction processing steel pieces authenticates about appearance of additional phases in its surface layer. So, predominantly martensate structure of white layer in manganese steel 65Г and in carbon steel У8А, together with small quantity of austenite and a number of ferric oxides, evidents of appearance of ∈ - carbide (∈ - Fe3C) which is hypothetically responsible for high hardness of white layer.

This hypothesis also proceeds from probable smallness of individual ∈ - carbide fragments. As such fragments there are identified discovered by electron-microscope investigation in near-surface layers of past thermofriction processing steel, moreover on surface of martensate grains, particles of right geometric form with a size approximately 0.1 – 0.2 µm [8], that practically is contignous with nanometer excursion, in the case of upper bound of that is taken on a value 100 nm, and with transformation in which of structure of materials are been connected greatest successes of modern materials science.

In light of electron-microscope investigations [8] it is noteworthy that grains – carriers of this parts with size 0.1 – 0.2 µm themselves are more roundish and greatly smaller size (about 1–2 µm) than size of needled grains of martensite of parent (before thermofriction processing) texture. Latest research allowed to determine predominant component of white layer microstructure as deformed grainular martensite [8]. 3. Conclusions

Diamond-spark grinding, especially high-speed processing with cooling, is active mean of support of white layer after thermofriction processing. It is possible efficiently use diamond-spark grinding by combined control of electrical and mechanical operating modes taking into account technical requirements to accuracy and roughness, with short-term ironing sparking-out without current if necessary. References: [1] Lubarskiy I.M., Palatnik L.S. Metallophysics of Friction – M.: Mechanical Engineering, 1976. – 176 p. (in Russian). [2] Abrao A.M., Aspinwall D.K. The surface integrity of turned and ground hardened bearing steel. Wear 196 (1996), pp. 279-284. [3] Brinksmeter E., Brockhoff T. White layers in machining steel. High steel Machining: 2nd Internation German and French Conference, Darmstadt 1999, pp. 7-13. [4] Chou Y.K., Evans C.J. White layer and thermal modeling of hard turned surfaces. International Journal of Machine Tool & Manufacture 39 (1999), pp. 1863-1881. [5] New Technology of Repair of Bearing Shaft of Rotors / V.B. Tarel’nik, V.S. Matsinkovskiy, A.V. Belous, B. Antoshevskiy. –www/triz.sumy. ua (in Russian). [6] Ploshkin V.V. Structure Transformations at Electroerosion Treatment of Steels.- News of Institute of Higher Education. Ferrous Metallurgy, №11, 2005, p. 43-48. [7] Curing of fatigue damages of steels due to electric current pulses / L.B. Zuev, O.S. Sosnin, S.F. Podboronnikov, V.E. Gromov, S.N.Gorlova. – Journal of Technical Physics, 2000, T.70, Vol. 3, p. 24-26. [8] Pogrebnoy N.A., Volkov O.A., Siziy Yu.A., Ploshkin Gutsalenko Yu.G.: Electron microscopic investigation of white layer after thermofriction processing, Cutting and tool in technological systems, Kharkov, 2007, No 72, p. 126-131 (in Ukrainian). [9] Pogrebnoy N.A., Pogrebnoy Shevchenko S.M., Gutsalenko Yu.G., Ditinenko S.A.: Impulse strengthening of steel in processing of diamond-spark grinding, Cutting and tool in technological systems, Kharkov, 2005, No 68, p. 323-327 (in Russian).


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SPARK – IGNITED ENGINE MODEL FOR LAMBDA CONTROL

drd.ing.ec. Aurelian NICOLA, Petroşani

Abstract: In stoichiometric spark-ignited (SI) engine operation, emission levels heavily depend on how accurate the air-fuel ratio can be kept constant (at 1). Due to measurement and computational tolerances, sufficiently accurate stoichiometric operation requires a closed loop control. In this paper is described the problem of the spark-ignited engine model for lambda control. Keywords: lambda control, platinum, rhodium, pipe. 1.Introduction

In SI engines, the air-fuel ration λ is either very lean at load or stoichiometric at medium and high load. A stoichiometric radio of λ = 1 should lean to an ideal combustion. Due to turbulence and local inhomogeneity of the gas mixture, real combustions actually produces HC, CO and NOx at the same time. By means of a catalytic converter, these raw emission can be effectively reduced. A change of the average ∆λ = 0.1 % would already double the emission rates. Therefore, it is important to have an accurate closed loop lambda control to guarantee an average air-fuel ratio within a window smaller than 0.1 % around λ = 1. When engine speed and torque change, lambda deviation of 2 ÷ 3 % over a short period of time is allowed. If the average accuracy can be held, such deviations go into both directions. Within the volume of the catalytic converter excursions of the air-fuel ratio in one direction are compensated by those in the opposite direction. At the exhaust pipe tail, short time lambda deviations of a few percent do not deteriorate the emissions after the catalytic converter. The block-diagram of the lambda controlled SI engine is shown in Figure 1. The amount of injected fuel is controlled by the engine control unit which gets its feedback from the lambda sensor in the exhaust pipe as well as the mass air flow signal in the inlet pipe. Additional variables like engine speed and engine temperature are used in the control scheme.

2.Catalytic Converter

The catalytic aftertreatment reduces the emission considerably (supposing a correct lambda control at λ = 1). Due to turbulences and flame propagation, the air-fuel mixture is still uncompletely burned. Noxious gases like HC, CO and NOx are converted to CO2, H2O and N2 by the catalytic converter. The converter is integrated into the exhaust pipe. It consists of a ceramic or metal carrier substrate covered by a wash coat with an extremely large surface which is again covered with a thin layer of platinum and rhodium. The ratio of platinum to rhodium is approximately 2 to 1. Depending on the engine size about 1 ÷ 3 g of precious metals are used. They both support the chemical reactions: Platin supports more the oxidation of CO and HC and Rhodium supports more the reduction of the nitrogen oxides NOx. Reduction and oxidation processes are simultaneously running in the catalytic converter. The convertion ratio is defined as relative change of the gas concentration before and after the catalytic process:

c r = ( c in – c out ) / c in (1)


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Fig.1. Block diagram of a lambda controlled SI engine.

The convertion ratio has typical values of c r > 90% and is influenced by the air-fuel ratio and the converter volume. Deviations of ∆λ < 3% can be compensated for a short period of time. At stationary engine operation, the conversion ratio is high, even if the converter would be already party damaged. During transients, excursions in the air-fuel ratio occur, leading to high emissions. During the warm-up phase the engine and the exhaust pipe, temperatures are too low for chemical reactions and the conversion ratio is poor. The catalytic converter has to reach temperatures beyond 3000C to be effective. There are several possibilities to accelerate the engine warm-up:

• A fast heating of the exhaust pipe can be obtained by an ignition angle retard of e.g. 10 0 < ∆α i < 20 0. The combustion is shifted to a phase of the thermodynamic cycle, where the exhaust valves are already opened; • An additional start-up catalytic converter is mounted very close to the engine where the exhaust gases get hotter soon. After the warm-up period, this converter is bypassed; • Fresh air is added to the exhaust gases by a secondary air-pump. The engine runs with a rich mixture ( λ < 1 ). The additional combustion process in the exhaust pipe heats up the catalytic converter; • The catalytic converter is electrically heated. In order to reduce the required heating power, the heater is concentrated in the region of the converter where the exothermic reactions first starts.

3.Oxigen Sensor

A lambda sensor is used to measure the concentration of oxygen O2 in the exhaust pipe. The sensor is mounted in the collective exhaust pipe where the individual exhaust pipes from the cylinders end in. In engines with 6 or more cylinders two lambda sensors are used. Zirconium Dioxide Sensor consists of a solid ceramic electrolyte (zirconium dioxide) which conducts oxygen ions at temperatures above 250 0C. Another sensor is based on strontium titanate. Strontium titanate is a ceramic semiconductor material. Its conductivity in strontium titanate is less influenced by surface effects at high temperatures than in other materials. The dependence of the probe resistance from the temperature decreases at higher temperatures leaving the dependence on lambda only. The strontium titanate sensor has a planar structure. An advance of the planar device is short response time of a few milliseconds after lambda deviations. Because of its operation at temperatures around 800 0C it can be fitted closer to the engine.


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4.Engine model for lambda control Figure 2 shows a suitable model of the engine for lambda control, where: CYL – is number of cylinders; k – is the respective cylinder 1,K, CYL; 1 / n – is the time needed for one crankshaft revolution; T exh – is the time delay between exhaust valve and lambda sensor;

Fig.2. Engine model for lambda control.

Fig. 3. Inverted lambda – characteristic with

limiting range ±∆λL.

5.Lambda control circuit

The characteristic between the output voltage and the air-fuel ratio λ is not-linear. After operation for many years this characteristic slightly ages. Therefore the most stable measuring range of the characteristic is taken for the control. Figure 3 shows, that it is located in the steep linear range of the characteristic. The sensitivity factor in this rage is KL. Outside the measurement range the characteristic is cut-off. The center of the measurement range λ 0 is not at desired reference value λ ref but is determined exclusively by the stability of the characteristic. 6.Conclusions

Lambda control is a very high priority for engine design because a change of the average would already increase the emission rates. Therefore, it is important to have an accurate closed loop lambda control to guarantee an average air-fuel ratio within a window smaller. References: 1. B. Böning, Improvement of Fuel Economy by Systematic Computer-Aided-Control Optimization, Proceedings of ISATA, Torino, 1980. 2. U. Kienche, A View of Automotive Control Systems, IEEE-Control Systems, Volume 8, Number 4, 1988. 3. U. Kienche, L. Nielsen, Automotive Control Systems, Springer-Verlag, Berlin Heidelberg, 2000.


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ELECTRIC PERMITTIVITY OF A CLASS OF HYBRID COMPOSITES

Lect. Drd. Adrian CÎRCIUMARU, [email protected] Prof. Univ. Dr. Ing. Iulian-Gabriel BÎRSAN, [email protected] Prof. Univ. Dr. Ing. Gabriel ANDREI, [email protected] Dunărea de Jos University, Polymeric Composites Laboratory,

47th Domnească Str, Galaţi 800008, România

Abstract: The electric properties of the polymeric composites materials are insulators like ones and that is why their spreading is blocked some how. Starting with the idea that filling the matrix with various powders it is possible to change the electric behaviour some layered materials were formed having as reinforcement 15 layers of kevlar and carbon fibre simple type fabric. CNT, Ferrite and Talc were used as fillers and the Clay as modification agent for epoxy resin. The electric permittivity was evaluated using standard recommended method. Keywords: Kevlar and Carbon Fibre Simple Type Fabric, Epoxy, CNT, Ferrite, Clay 1.Introduction

While the major aim of composites is replacing the metals in aircraft, spacecraft and marine applications the electromagnetic properties of such materials are very important. As it is well known metals show high electric conductivities while the composites (excepting metal matrix, metal reinforced or metal-metal ones) show poor electric conductivity. Both thermoset and thermoplastic polymers are also relatively sensitive to the action of aggressive media and show high values of water absorption. The most common solution to improve the polymers’ properties is to fill them with various solid systems (such as small metallic spheres) or powders obtaining filled composites, according to [1]. The mechanical properties of composites are improved in the case of reinforcements (reinforced composites or fibrous composites). There are many studies regarding the design of the reinforcement in various geometries (2D, 3D, 4D) in order to obtain dedicated composite structures [2] and regarding the mechanical analysis of laminate composites [3]. In the case of laminate composites, the use of reinforcements induces a high level of anisotropy while the use of fillers has the opposite effect.

Combining both alternatives of polymers’ properties improvement it is possible to obtain better materials but the problem is to predict the final properties which generally are unknown. It is generally accepted that the most important feature of the composites is the reinforcement/matrix or filler/matrix ratio (weight or volume) and this parameter is used in order to describe the macroscopic properties of finite material [4]. 2.Materials In the case of this study the starting point was to create an epoxy based composite with better electromagnetic, thermal and mechanical properties. Our final option about reinforcement is a simple type fabric realized by alternating kevalr and carbon fabric untwisted tows as warp and fill. A model for the mechanical properties of a fabric reinforced lamina is presented in [5] and is taking into account the fabric characteristics. The main reason for our option is about the excellent electromagnetic properties of carbon fibre and excellent mechanical properties of kevlar fibres but also by the fact that the epoxy is in adherent to carbon fibre but adheres to kevlar ones. At the beginning we noticed that there appears small gaseous intrusions in the bulk of epoxy resin and that using powders as fillers their particles are clusterizing especially in the case of CNT and Ferrite that is why based on article published in MRS Bulletin [6] we had tried to use clay in order solve the problems.


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Mixing the CNT, talc, and ferrite powders with clay powder before filling the epoxy we had obtained better results regarding the uniform distribution of fillers’ particles as can be seen in Fig. 1. and Fig. 2. It can be easily noticed that using the clay powder the number and the dimensions of gaseous intrusions and also the number and dimensions of CNT clusters are diminished.

Pure epoxy Talc filled epoxy CNT filled epoxy

Fig. 1. The aspect of filled epoxy samples without clay

Clay filled epoxy Clay and Talc filled epoxy Clay and CNT filled epoxy

Fig. 2. The aspect of filled epoxy samples with clay The reinforced samples were formed using a technique described in [7] and consist in 15 sheets of fabric reinforcement immersed in pure or filled epoxy for the first part of the study when the bulk properties were investigated. The samples were formed using clay modified epoxy and all the three fillers (at the same concentration) but the filled modified epoxy layers have different positions in the sample. So an alternate architecture was created such as each type of sample could be described as 3F1-3F2-3F3-3F2-3F1 from the resin layers point of view. This type of architecture is intended to be a solution in order to control the electromagnetic properties of polymeric composites but could be useful regarding thermal and mechanical properties. The structure of reinforcement layers is shown in Fig. 3. 3.Measurements

Measurements were performed in order to determine the electric permittivity for all the samples. The experimental arrangement consists in a measurement cell and a digital RLC-meter and it is respecting the recommendations in [8].

When a powder is used as filler the dimensions and the shape of the particles are important. Also because of the reticulate structure of the polymer and because the presence of filler’s particles it is expected that electric permittivity to be frequency sensitive. In the case of reinforced filled polymers (hybrid composites) the problem is more difficult due to the presence of long fibres and in the case of this study due to their quasi-arrangement. Using the same method the electric permittivity of powders was measured using a glass box filled with ferrite, clay, CNT, talc powders. The results are showed in the Table 1 in which we put also the determined values for the air permittivity.


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Table 1 Electric permittivity [F/m]

Frequency Air Clay Talc Ferrite CNT 0,1 kHz 1,255E-11 -1,526E-10 7,315E-11 -7,406E-11 -7,033E-11

1 kHz 1,136E-11 -2,613E-10 4,122E-11 -9,173E-11 -6,981E-11 10 kHz 1,046E-11 6,251E-10 2,827E-11 -2,579E-10 -6,991E-11

100 kHz 1,036E-11 1,437E-10 2,214E-11 -1,251E-08 -7,023E-11 The results of permittivity evaluation are presented in Fig. 3., Fig. 4. and Fig. 5. In all the mentioned figures is shown the electric permittivity of a material in which the filler is the same as the one in external layers of complex composites in order to ensure a reference.

Electric permittivity of composites with CNT filled epoxy in external layers [F/m]

1,00000E-10

1,00000E-09

1,00000E-08

1,00000E-07

1,00000E-06

1,00000E-050,1 kHz 1,0 kHz 10 kHz 100 kHz

Frequency

Ele

ctri

c pe

rmitt

ivity

[F/m

]

C CFTFC CTFTC

Fig. 3. The structure of reinforcement layer Fig. 4. Electric permittivity of composites with CNT filled epoxy in the external layers

It is obvious, analyzing Fig. 4., that the electric permittivity is frequency sensitive but in the CNT filled epoxy composite it is almost constant while in the two complex composites its variations respect almost the same tendency. In the case of Ferrite the behaviour is opposite high variation for the ferrite filled epoxy composite and small variations for the complex ones. In the case of Talc filled epoxy composites the behaviour is changed and can be explained (for the TFCFT) as a major influence of Ferrite filled layers.


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Electric permittivity of composites with Ferrite filled epoxy in external layers [F/m]

1,00000E-10

1,00000E-08

1,00000E-06

1,00000E-04

1,00000E-02

1,00000E+000,1 kHz 1,0 kHz 10 kHz 100 kHz

Frequency

Ele

ctri

c pe

rmitt

ivity

[F/m

]

F FTCTF FCTCF

Electric permittivity of composites with Talc filled epoxy in external layers [F/m]

1,00000E-09

1,00000E-08

1,00000E-07

1,00000E-06

1,00000E-05

1,00000E-04

1,00000E-030,1 kHz 1,0 kHz 10 kHz 100 kHz

Frequency

Ele

ctri

c pe

rmitt

ivity

[F/m

]

T TCFCT TFCFT

Fig. 5. Electric permittivity of composites with Ferrite filled epoxy in the external layers

Fig. 6. Electric permittivity of composites with Talc filled epoxy in the external layers

4.Conclusions Based on the above results it is our concern that using different fillers it is possible to design the electromagnetic properties of composite materials but it is necessary to study also the effects of various concentrations of fillers and the thermal and mechanical properties. Also taking into account the permittivity’s variations it would be interesting to study the magnetic permeability in order to identify the ways in which a filled polymeric material can act as a metamaterial. Bibliografie [1]. Jones, R. M., Mechanics of composite materials, Taylor & Francis, 1999, p. 2. [2]. Vinson, J. R., Sierakowski, R. L., The behavior of structures composed of composite

materials, Martinus Nijhoff Publishers, Dordrecht, 1986. [3]. Kalamarkov, A. L., Kolopakov, A. G., Analysis, Design and Optimization of

Composite Structures, John Wiley&Sons, 1997. [4]. Callister, W. D., Materials Science and Engineering, John Wiley & Sons, 1994. [5]. Morozov, E. V., Mechanics and Analysis of Fabric Composites and Structures,

AUTEX Research Journal, vol. 4, No. 2, june2004, p. 60. [6]. Hunter, D. L., Kamena, K. W., Paul, D. R., Processing and Properties of Polymers

Modified by Clays, MRS Bulletin, vol. 32, Aprilie 2007, p. 323. [7]. Cîrciumaru, A., Andrei, G., Bîrsan, I.-G., Dima, D., Electric and electromagnetic

Properties of Fiber Fabric Based Filled Epoxy Composites, The Annals of “Dunărea de Jos” University of Galaţi, Fascicle IX, Faculty of Metallurgy and Materials Science, XXV (XXX), May 2007, no. 1, pp. 97-102.

[8]. Webster, J. G. (ed), Measurements, Instrumentations, and Sensors, CRC Press, 1999; Misra, D. K., Permittivity measurement, 46.


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VISUAL BASIC APPLICATIONS FOR COMPUTER AIDED DESIGN

Assoc. Prof. Ph.D. Eng. Mat. Ion PANĂ, Petroleum and Gas University from Ploieşti Abstract: Visual Basic for Applications (VBA) is a programming language that is supported by many Windows applications. VBA provides simple way to customize AutoCAD. The Bologna program introduced into Romanian universities offers many master courses. For the students who are working in the design areas the learning of VBA is useful for their activities. The paper offers some examples from currently teaching activity. Keywords: graphical interface, modeling program, drawing, AutoCAD. 1.Introduction

Visual Basic for Applications (VBA) is a programming language that is supported by many Windows applications. We can create programs with dialog boxes for AutoCAD using VBA. Using ActiveX which lets us access other Windows applications, we can write programs that integrate AutoCAD with other applications [1,2]. Visual Basic for Applications is a variation of Visual Basic. Visual Basic is not related to any specific application. Visual Basic code is compiled into an executable file that stands alone, unrelated to any specific document. VBA is connected to its application and document in which we created the code. VBA in AutoCAD works slowly different from VBA in most other applications in that VBA projects are stored in a separate file, with .dvb extension, but can be also stored within drawing file. VBA provides simple way to customize AutoCAD.

2.Applications

Following applications intend to illustrate the main benefits which we have using VBA in AcadCAD projects. The main steps used are: the construction of a graphical interface and the

achievement of necessary code. The result consists into a graphic shape. First application concerns in the drawing of a volute. This curve is used to construct the volute chamber of a centrifugal pump and it presents the aspect from figure 1. Using polar coordinates, the calculus relations are:

r ri a ϕ= + ⋅ (1) cosx r ϕ= ⋅ (2) siny r ϕ= ⋅ (3)

where: r is radius of current point of the volute; a – volute parameter; ri – inner radius of volute chamber; φ – current angle; x– abscise coordinate; y– ordinate coordinate.

We project a single form for the construction of the volute, figure 2, which contains tree labels: parameter a, inner radius ri and number of points used into representation (Precizie). We have got also two command buttons: Desen that executes the volute and Ieşire which ends application. We use tree text boxes to introduce the parameters: a, ri, precision. We can import the curve into a tri-dimensional modeling program (Solid Edge, Solid Works, Inventor) and using this curve to generate a surface and following the body of the volute chamber, figure 3. The code for this application is showed in table1.

-150 -100 -50 0 50 100 150 200-150

-100

-50

0

50

100

150

coordinate x [mm]

coor

dina

te y

[mm

]

volute chamber

Axis of Volute Chamber

Bundary Edge

Input of Volute Chamber

Fig.1. Geometrical shape of the volute.


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a

b

c

Fig.2.The using of a user form to draw a volute. Fig.3. The using of volute to construct a volute chamber, in Solid Edge.

Table 1. Application 1 – The construction of the volute of a centrifugal pump Dim a, ri As Double Dim nr, i As Integer Const vect = 100 Dim r(0 To vect) As Double Dim x(0 To vect) As Double Dim y(0 To vect) As Double Dim fi(0 To vect) As Double Const pi = 3.1415 Dim linie As AcadLine Dim punct1(0 To 2) As Double Dim punct2(0 To 2) As Double

Private Sub UserForm_Initialize()a = 5: ri = 100:nr = 12UserForm1.Caption = "voluta" txta.Text = a: txtRi.Text = ri: txtNr.Text = nr End Sub Private Sub cmdDesen_Click() For i = 0 To nr fi(i) = i * 360 / nr * pi / 180 r(i) = ri + a * fi(i) x(i) = r(i) * Cos(fi(i)) y(i) = r(i) * Sin(fi(i)) Next i

For i = 0 To nr - 1 punct1(0) = x(i): punct1(1) = y(i): punct1(2) = 0 punct2(0) = x(i + 1): punct2(1) = y(i + 1): punct2(2) = 0 Set linie = ThisDrawing.ModelSpace .AddLine(punct1, punct2) Next i ThisDrawing.Application. ZoomAll End Sub

Private Sub cmdIesire_Click()End End Sub Private Sub txta_Change() a = txta.Text End Sub Private Sub txtNr_Change() nr = txtNr.Text End Sub Private Sub txtRi_Change() ri = txtRi.Text End Sub

The second application suggests the generating of geometrical shapes based on calculus from the project. The application has two forms, introduced by a module. In the first form we calculate the pressure losses into a pipe. Text boxes are used to introduce: length of pipe L; value of flow into the pipe Q; liquid density ρ. The combo box diametru includes the standard values of inside diameter of pipe. Command button presiunea is associated with a procedure which permits the calculus of pressure losses. The second form of the project makes the followings: set the material for the pipe, material combo box; calculates the value of width of pipe, command button calcul grosime and executes the drawing of a section from pipe, button DESEN. The relations used are:


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Table 2. Application 2 – Example of project generating a graphical form ‘ MODULE Public presiunea As Double Public diametru As Double Public grosimea As Double Sub calcul() UserForm1.Show End Sub ‘ USERFORM1 Dim Lungimea As Double Dim Q As Double Dim v As Double Const pi = 3.141592 Dim aria As Double Const g = 9.81 Dim densitatea As Double Dim vascozitatea As Double Private Sub cmbDiametru_Change() Select Case cmbDiametru.ListIndex Case 0 diametru = 120 Case 1 diametru = 140 Case 2 diametru = 160 Case 3 diametru = 200 Case Else diametru = 240 MsgBox "valoarea lui D=" & Format(diametru, "#####.00"), vbInformation, "Diametrul" End Select diametru = diametru / 1000 End Sub Private Sub cmdCaderiPresiune_Click() MsgBox "diametru=" & diametru * 1000# & " mm", vbExclamation MsgBox "Lungimea=" & Format(Lungimea, "####.00") & " m", vbExclamation MsgBox "Debitul= " & Format(Q, "####.000") & " m^3/h", vbExclamation viteza = (4 * Q / pi / diametru / diametru) ^ 2 MsgBox "viteza= " & Format(viteza, "#####.000") & " m/s", vbExclamation viscozitatea = InputBox("introduceti

vascozitatea=", "VASCOZITATEA", 0.000001) viscozitatea = CDbl(viscozitatea) lambda = lambda1(viteza, diametru, viscozitatea) MsgBox "lambda= " & Format(lambda, "####.00000"), vbExclamation presiunea = viteza * viteza * Lungimea / 2 * lambda * densitatea MsgBox "presiunea= " & Format(presiunea, "########.00") & " Pa", vbExclamation UserForm1.Hide UserForm2.Show End Sub Function lambda1(viteza, diametru, viscozitatea) Dim re As Double re = viteza * diametru / viscozitatea If re < 2300 Then lambda1 = 64 / re Else lambda1 = 0.3164 / re ^ 0.25 End If End Function Private Sub txtDensitatea_Change() densitatea = txtDensitatea.Text End Sub Private Sub txtL_Change() Lungimea = txtL.Text End Sub Private Sub txtQ_Change() Q = txtQ.Text Q = Q / 3600 End Sub Private Sub UserForm_ Initialize() UserForm1.Caption = "CALCULUL HIDRAULIC" cmbDiametru.AddItem ("120") cmbDiametru.AddItem

("140") cmbDiametru.AddItem ("160") cmbDiametru.AddItem ("200") cmbDiametru.ListIndex = 0 diametru = CDbl(cmbDiametru. Value) / 1000# Lungimea = 100 Q = 10 txtL.Text = Lungimea txtQ.Text = Q densitatea = 850 txtDensitatea.Text = densitatea End Sub ‘ USERFORM2 Dim Ra As Double Dim punct1(0 To 2) As Double Dim raza As Double Dim cerc As AcadCircle Private Sub cmbMaterial_ Change() Select Case cmbMaterial. ListIndex Case 0 Ra = 17.5 Case 1 Ra = 22.5 Case 2 Ra = 27.5 Case Else Ra = 10 End Select End Sub Private Sub cmdCalcul_Click() grosimea = presiunea * diametru / 2 / Ra MsgBox "grosimea= " & Format(grosimea * 1000, "#####.00") & " mm", vbInformation punct1(0) = InputBox("x centru ", 100, 100, 100) punct1(1) = InputBox("y centru ", 100, 100, 100) punct1(2) = 0 End Sub Private Sub cmdDesen_Click() 'elementele hasurii Dim hasura As AcadHatch Dim modul As String

Dim modul_tip As Long Dim asoc As Boolean Dim margine1(0 To 0) As AcadEntity Dim margine2(0 To 0) As AcadEntity raza = 1000# * (diametru / 2 + grosimea) 'Set cerc = ThisDrawing.ModelSpace. AddCircle(punct1, raza) Set margine1(0) = ThisDrawing.ModelSpace. AddCircle(punct1, raza) raza = 1000# * diametru / 2 'Set cerc = ThisDrawing.ModelSpace. AddCircle(punct1, raza) Set margine2(0) = ThisDrawing.ModelSpace. ddCircle(punct1, raza) ' stabilire elemente hasura modul = "ANSI31" modul_tip = 0 asoc = True Set hasura = ThisDrawing.ModelSpace. AddHatch(modul_tip, modul, asoc) 'alege un punct de pe ecran 'p1 = ThisDrawing.Utility.GetPoint(, "Clic:") 'For i = 0 To 2: pt1(i) = p1(i): Next i 'r1 = diametru / 2: r2 = r1 + s hasura.AppendOuterLoop (margine2) hasura.AppendInnerLoop (margine1) hasura.Evaluate ThisDrawing.Application. ZoomAll End Sub Private Sub cmdEnd_Click() End End Sub Private Sub UserForm_Initialize() UserForm2.Caption = "SECTIUNE CONDUCTA" cmbMaterial.AddItem ("OLC 35") cmbMaterial.AddItem ("OLC 45") cmbMaterial.AddItem ("OLC 55") cmbMaterial.ListIndex = 0 End Sub


14

2

2vp L

Dρλ= (4)

Re vDυ

= (5)

0,25

64 ,Re 2300Re

0,3164 ,Re 2300Re

λ

⎧


15

IMPROVING MAINTENANCE IN ENERGETIC UTILITIES OF MECHANICAL PLANTS

Prof. Iosif ANDRAS, University of Petrosani

Eng. Ghita STANOI, Arcelor Mittal Romania - Roman Eng. Marius Eremia VLAICU POPA, Industrial Parc Sadu

Eng. Pompilia MERISESCU, Energetic Complex Turceni

Abstract: The paper deals with some measures carried out in order to improve the maintenance in the energetic utilities in MITTAL STEEL mechanical plants, based on predictive maintenance and cost benefit analysis. Keywords: maintenance, costly downtime, breakdowns, vibration.

1. Introduction

The activity objects of the energetic utility section are: * Production of industrial compressed air * Production of industrial steam * Production of industrial oxygen * Captation, treatment and recycling of industrial water * Transforming and distribution of electrical energy * Distribution of natural gas

According to the repairing plan elaborated on the base of the repairing standards for the preventively planned system, to achieve the activity volume are requested a three time more manpower that are in this moment. Financial, the value of repairs is three times bigger that the allocated budget.

It was analyzed the possibility to achieve these maintenance activities and it was taken the decision to renounce to the preventively planned repairs and to pass on the predictive maintenance system.

2. Predictive maintenance system

Technical state of equipment can be appreciated on the base of operating sign as: vibrations, noise, and temperature, variation of temperature and of pressure in cooling circuit.

From all of it, the vibration level and noise level are the main parameters to evaluate the technical state of equipment.

Preventive maintenance assume that periodical are performed some measurements of vibrations and noises and these value are compared with standardized value.

Predictive maintenance represents a qualitative step to a modern maintenance system because offer all necessary information to:

- track in time the fault appearance - localization of fault - diagnose the fault - determination of operating time in safety conditions First organizational measure to implement preventive and predictive maintenance

system on S.C. MITTAL STEEL ROMAN S.A. is to constitute the working teams. These have to include mechanics and electro-mechanics according to the number of equipments that must be monitored and must to have a leader, a mechanic or an electro-mechanic engineer.

Chiefs of production sectors must to propose and to perform the list of equipments that must to be permanent or periodical observed.


16

Selection criterion for these equipments could be: A) Operating safety: 1. unique equipments on flow, without any operational back up 2. equipments with increased risk of explosion, fire and to eliminate some

toxic products. 3. important equipments recent mounted or repaired 4. automatic equipments, without surveillance manpower B) Financial criterion: 1. if an equipments is stopped the whole flow is stopped and appear big production

looses 2. equipments with a high value of maintenance expenses (spare parts, manpower,

materials and intervention devices) C) Maintenance criterion: 1. equipment with a high rate of breakdowns 2. hard to reach equipments for intervention 3. equipments that can not be stopped for current interventions 4. complex equipments with difficult mounting, repairing and operation technologies. In order to constitute the data bank, the maintenance and repairs manpower must to

provide technical data of equipments from their activity area. They have to provide following information's: number of turns, the power of motor, bearings type, coupling type and other information requested by the soft of measuring devices. Also, we have to obtained internal and international standards that establish accessible levels for the measured parameters.

According to the number of equipments that must to be monitored and of their complexity, we have to purchase a measuring system or device with following functions:

- measuring of vibration level - measuring of bearings wear - frequency analyze of vibrations (even fault diagnosis) - measuring of lubrication quality - measuring of temperature - the soft to administrate the data bank and measurements - dynamic balancing on position of rotor's

Even the purchase price is pretty big, expenses amortization will be fast, obtained through savings avoiding accidental stoppage, decreasing of consumed spare parts and standing time into repair. 3. The Benefits of Predictive Maintenance System at S.C. MITTAL STEEL ROMAN SA

1. Minimizes or eliminates costly downtime - increases profitable uptime. 2. Minimizes or eliminates catastrophic machinery failures - damage from catastrophic

failure is usually much more extensive than otherwise would have been. 3. Reduces maintenance costs. 4. Reduces unscheduled maintenance - repairs can be made at times that least affect

production. 5. Reduces spare parts inventories - many parts can be purchased just in time for repairs

to be made during scheduled machinery shutdowns.. 6. Optimizes machinery performance - machinery always operates within

specifications. 7. Reduces excessive electric power consumption caused by inefficient machinery

performance - saves money on energy requirements. 8. Reduces need for standby equipment or additional floor space to cover excessive

downtime - less capital investment required for equipment or plant.


17

9. Increases plant capacity. 10. Reduces depreciation of capital investment caused by poor machinery maintenance

- well maintained machinery lasts longer and performs better. 11. Reduces unnecessary machinery repairs - machines are repaired only when their

performance is less than optimal. 12. Minimizes or eliminates the possibility that machinery repairs were the wrong

repairs. 13. Reduces the number of dissatisfied customers or lost customers due to poor quality

- with less than optimal machine performance, quality always suffers. 14. Disposition the personnel for another production activities 15. Reduces scrap caused by poorly performing machinery. 16. Eliminate the supplementary time and the costs for aspectual recovery and the

conditions needed restarting and operating in optimal conditions. 17. Reduces overtime required to make up for lost production due to

broken down or poorly performing machinery. 18. Reduces the possibility of accepting recently purchased new or used machinery with

defects - the invoice is not paid until the defects are corrected. 19. Increases machinery safety - injuries are often caused by poorly performing

machinery. 20. Reduces safety penalties levied against a company for unsafe machinery. 21. Reduces insurance rates because well maintained machinery

increases safety. 22. Reduces the time required to make machinery repairs - advance notice of machinery

condition permits more efficient organization of the repair process. 23. Increases the speed that machinery can be operated, if desirable. 24. Increases the ease of operation of machinery. In figure no.l is presented the comparative situation of intervention number in those two

maintenance systems and in figure no.2. is presented the comparative situation of maintenance specific expenses.

Fig 1. Comparative situation : evolution of the number of interventions


18

Fig 2. Comparative situation : maintenance specific costs in two working systems.

Bibliografie

[1]. Atti, G. Cheia succesului în afaceri 500+. Editura Bravox, Braşov, 1992. [2]. Negoescu, Gh. Risc şi certitudine în economia contemporană. Editura Alter-Ego

Cristian, Galaţi, 1995. [3]. Piţurescu, I. Incubatoare de afaceri, Parcuri ştiinţifice şi Tehnologice. Editura

Promotel, 2005. [4]. Spica, S. Directory - Science Park and Innovation Center Association. WEINDLER

Buchverlag, Berlin, 2001.


19

QUALITY ASSURANCE BY DESIGN OF MINING EQUIPMENT

Prof. Iosif ANDRAS, Universitatea din Petrosani Prof. Nicolae ILIAŞ,Universitatea din Petrosani

Ass. Prof. Andrei ANDRAS, Universitatea din Petrosani Eng. Attila KOVACS, INSEMEX Petrosani

Eng.Victor BADARCA, E.M. Uricani Eng. Florin NICOLAE, Universitatea din Petrosani

Abstract: The present paper examines practical application of the principles of constructive design in order to increase the quality of powered roof support used in coal mines.

Keywords: design, separation, corrosion, fail-safe, quality .

1.Constructive Design

Quality assurance of a product should be assumed in any stage of its lifecycle. In the design stage, application of constructive principles provides an effective instrument of quality assurance for power supports (stepping sections, articulated beams, hydraulic and cylindrical power props). Constructive design is that part of the design process where technical and economic structure of the product is calculated without ambiguity and completely, starting from the principle solution.

Constructive design requires a large number of correction stages as well, where analysis and synthesis alternate, and which makes quality elements turn into quantity elements, that is, general constructive design into detailed one. A particularity of constructive design is the assessment and searching procedure, supported by complying with directives including principal characteristics of constructive design.

2. Basic principles of Constructive Design

A brief review of principles and rules to be kept in mind in constructive design follows, since its correct application can provide a basis in quality assurance in the design stage.

2.1.Basic rule of constructive design

“Clearly, simply and reliably” is a primordial instruction for the designer and arises from the general objective.

“Clarity” means that the designer should state his action and behavior in a clear manner to accomplish the technical function of the product.

“Simplicity” resides in the combination of certain clearly defined shapes in the accomplishment of constructive shapes and provides lower manufacturing costs.

“Reliability” involves necessity of stating life duration, reliability, safety and environment protection (safety for man and environment).

Application of this rule, corroborated with the directives stated in the table, will lead to a series of questions and answers, in the sense that no important factor had been overlooked and the newly designed product will be truly a quality product. 2.2.Principles of constructive design These principles involve specific product strategies and are not all applicable in general to all products. In this sense, only principles applicable to power supports are analyzed.


20

2.2.1 Principle of task separation Practical selection and giving functional elements for functions to be accomplished lea to the following questions: - which sub-functions can be accomplished together, only using a single element? - Which sub-functions must be accomplished using separate functional elements. The disadvantage of applying the principle lies in higher costs and difficulty of

comparing economic efficiency and safety. 2.2.2. Principle of self-protection Meaning that the system should support itself, so that its functions would be

accomplished more efficiently and to avoid damages in overload situations. The general effect required in this principle comes from a basic effect and an auxiliary one. This project can be self-protective or self-damaging, as the case may be.

2.2.3.Principles of force and energy transmission Energy transfer, by transmission of forces and moments which lead to deformations, is

characteristic to any technical product (system). In case of power supports, the value and duration of active forces, and especially reaction ones, involve additional constructive measures provided in design stage.

2.2.4.Constructive design and force flow Forces and moments transmitted through component sections can be perceived as a force

flow. Special requirements result for a constructive solution including the concept of force flow. Force flow should always be closed (action = reaction).

Principle of equal resistance. Equal distribution of resistance is achieved by an adequate choice of materials and shape.

Principle of minimum force transmission path. Forces and moments should be transmitted with minimum costs for materials.

Principle of harmonized deformation. The components should be designed so that a proportional adaptation to stress should take place, providing an equal deformation and a minimum relative deformation.

Principle of force compensation suggests compensation elements for medium forces and symmetrical distribution for relatively high forces.

2.2.5. Principles of safety and reliability Literature distinguishes between direct, indirect and inferred safety principles. Direct

safety principles inherently and intrinsically ensure that no danger situation might occur. Indirect safety involve protective systems and devices. Inferred safety principles only alert on the danger or show danger areas, without solving any safety issues.

Safe-Life principle mean that all components and their links would resist during the pre-established time of use in all possible and probable conditions, without defects or destruction.

Fail-Safe principle means controlled deterioration (fall and/or break), avoiding serious consequences.

2.2.6.Principle of multiple or redundant structure It means an increased safety, since one faulty system element does not an intrinsic

danger, and system elements in series or in parallel take over the disturbed function, or at least part thereof.


21

2.3.Specifications for constructive design Specifications for constructive design come from general and special constraints for a

product, but also from nature’s and probability laws regarding machine parts design. Design for resistance The principles of material resistance theories and force transmission principles should be

considered. In power supports, where most of the reference points are subject to relatively high strain

and for a long time, this is a significant specification. The choice of quality materials, rigorous sizing and applying safety coefficients are essential.

Stand trials are mandatory both for new projects and for each prototype. Design for controlled deformation In modern power supports, their evolve kinematics allow a gradual, controlled

deformation of support structure, so that material deformation of subassemblies should not reach values that might generate defects or destruction thereof, all these being possible along with all the range of projected working heights.

Stability design avoiding resonance Stability involves both rigidity and turnover risk, and buckling and crushing risks, but

also includes possibility of stable operation of the machinery. Design for thermal dilatation In case of normal use of power supports, thermal dilatation is not an issue, their

operation being in a relatively limited temperature range, specific to mining, and during operation they do not generate a significant temperature increase (such as electrical or thermal motors).

Design for corrosion Corrosion can sooner be reduced than avoided, since it is about implacable chemical and

electro-chemical phenomena. Practical and economical reasons prevent the use of non-corrosive materials or the use of equipment in non-corrosive environment.

Constructive design should still propose adequate measures in this sense. Design for wear Specific kinematics of power supports and the impossibility of checking on the condition

of surfaces (such as inside cylinders, bolts) during work or general servicing, make the designer take adequate measures against adhesive and abrasive wear.

Design for ergonomics and safety of user Safety of users. Romanian laws for health and safety at work make the designer to work

in a responsible way. The safety of people and environment is an essential requirement for any technical equipment. Subjective aspects should also be considered, such as lack of understanding regarding proper use, fatigue, so that in the event of an incorrect handling the designed equipment should not generate accidents.

Ergonomics. The designer should consider easy handling, illumination and ventilation of workspaces, possibility of equipment functioning parameters surveillance, noise, dust, etc.

Design for aesthetics Refers to permanent analysis regarding shape, color and graphics. Design for ease of manufacturing and inspection Refers to component structure to allow simple manufacturing, quality control and

inspection. In this sense the designer should be familiarized with the technical level and

technological possibilities of the manufacturer, he should adapt the project to the latter’s possibilities of making the product so that the required qualities would not be impaired.


22

Design for adapted assembling Assembling surfaces and order of parts will be considered in constructive design.

Solutions are applied that would need minimum simple assembling operations, parallel assembling of modules, automation possibilities for operations.

Design for capacity of operation and maintenance Constructive maintenance should allow for operation and maintenance. Maintenance

includes up keeping, inspection and repair. Commissioning and use will be reliable and simple.

Operation results should be clearly perceived, so that inspectors could see during inspections if the equipment operates correctly, according to the requirements, or a critical situation came up that requires repair.

Design for easy recycling Any product has a well-determined lifecycle. Design should have in mind economical

recycling when the product ends its use. References

[1] ILIAŞ, N., KOVACS, A. – Creşterea calităţii susţinerilor mecanizate de abataj prin aplicarea principiilor proiectării constructive. Asigurarea şi promovarea calităţii. Nr. 2/2002 INID Bucureşti [2] KOVÁCS , A. – Contribuţii la îmbunătăţirea susţinerii mecanizate în funcţie de utilajele din complex şi de interacţiunea cu rocile înconjurătoare. Teză de doctorat – 2006 [3] ANDRAS, A. – Studii privind perfecţionarea metodologică de proiectare a echipamentelor pentru industria extractivă – Teză de doctorat – 2006


23

ASPECTE EXPERIMENTALE ALE REZISTENŢEI MECANICE A LEMNULUI ÎNTĂRIT CU MATERIALE COMPOZITE

Asistent univ. drd. ing. Cătălina IANĂŞI, Universitatea “Constantin Brâncuşi”,

Facultatea de Inginerie, Catedra de Mecanică Aplicată Profesor dr. ing. Dan ILINCIOIU, Universitatea din Craiova, Facultatea de Mecanică

Abstract: This paper presents a quantification model of the resistance of an wood element, reinforced or not, with a composite material. Is conceived a mechanical testing laboratory procedure which will be used for the experimentation of some strengthening solutions of wood with composite materials from carbon fiber. Keywords: resistance, wood , strengthening, carbon, EDZ20.

1. Introducere Elementele unei construcţii sunt de mare diversitate. Din punct de vedere al rezistenţei se remarcă, însă numai stâlpii şi grinzile.

Grinda este solicitată la încovoiere şi forfecare; solicitarea principală este cea de încovoiere, conform schemei din fig.1. Tensiunile ce se produc în grindă sunt arătate în fig.2. Momentul încovoietor produce tensiuni normale σ ce variază liniar pe înălţimea secţiunii, valorile maxime producându-se în zonele extreme (sus-jos), conform fig.2. Forţa tăietoare produce tensiuni tangenţiale τ ce variază pe secţiune conform fig.2, fiind maxime în zona mijlocie a acesteia.

Dacă se compară valorile maximale ale tensiunilor (σ şi τ) se constată că tensiunea normală maximă σm este mult mai mare decât tensiunea tangenţială maximă τm, deci este suficient să ţinem seama în mod covârşitor de tensiunile normale σ.

F

0,5F

0,5F•l

M

0,5F

T

Fig.1. Schema de solicitare a unei grinzi încărcată cu forţă concentrată

Fig.2. Schema tensiunilor produse în secţiunea grinzii

-σm

+σm

τm


24

Distribuţia tensiunilor σ ne arată că zona mijlocie a grinzii (pe secţiune) este puţin solicitată (în centrul secţiunii σ=0), fapt ce ne îndreptăţeşte să spunem că materialul din această zonă nu este folosit economic.

Pentru utilizarea raţională a materialelor se propune folosirea unei secţiuni neomogene, din două materiale, pentru alcătuirea grinzii. În partea de mijloc, unde tensiunile sunt mici, se foloseşte un material de rezistenţă mică; în zonele extreme (sus-jos) vom avea un material cu rezistenţă mai mare. Modul de alcătuire a unei secţiuni neomogene pentru grindă este prezentat schematic în fig.3; materialul cu rezistenţă mai bună va avea modulul de elasticitate longitudinal mai mare E1>E2. 2. Soluţie constructivă Se propune folosirea unui material ieftin şi uşor de prelucrat care este lemnul. Ca material rigid, cu bună rezistenţă şi cost relativ scăzut, se va utiliza un compozit. Pentru ca investigaţia să conducă la rezultate relevante se vor studia mai multe soluţii constructive care sunt prezentate în figura 4. 3. Modelare matematică Pentru proba supusă la încovoiere, aşezată la capete şi acţionată la mijloc cu forţa F crescătoare până la valoarea maximă Fm care produce ruperea, vom calcula momentul încovoietor maxim Mm, modulul teoretic de rezistenţă la încovoiere pentru secţiune Wt şi tensiunea maximă de rupere σm, astfel:

lFM m25,0= ; 6

200hbWt = ;

t

mm W

M=σ . (1)

Notaţiile folosite au semnificaţia: l – distanţa între reazeme l=460 mm; bo – lăţimea reală a probei (mm); ho – înălţimea reală a secţiunii probei (mm). În cazul solicitărilor axiale vom calcula tensiunea maximă de rupere astfel:

Fig.3. Tensiunea normală σ în cazul solicitării la

încovoiere a grinzii neomogene

-σm E1 E2

E1>E2

+σm


25

A

Fmm =σ ; hbA ×= (2)

Fig.4. Soluţii constructive pentru încercări la încovoiere 1-lemn, secţiune bxh, 2-lamelă compozit bxδ, 3-şipcă bxh1,

4,5-ţesătură impregnată cu răşină, 6-lipitură cu răşină

b

h

δ

1

Varianta 0.1

b

2

Varianta 0.2

c

h1

h

b

δ

1 2

3

Varianta 0.3

d

h1

h

b

δ

1

2 3

3

Varianta 0.0

a

h

b

1

h

h1

1

3

5

Varianta 0.4

e

b

h1

5

3

1 h

Varianta 1.0

f

Varianta 2.1

h

b

h

1 b

6

Varianta 2.0

g

b

h

1 b

Varianta 2.2

i

b

h

b 1

5

Varianta 2.3

j

h1

h

3

b

b 5


26

Vom defini tensiunea de referinţă σr valoarea pentru care se rupe proba de referinţă (cea neîntărită) şi coeficientul de rezistenţă kσ :

r

mkσσ

σ = (3)

Se defineşte săgeata maximă de referinţă fr deformaţia înainte de rupere a probei de referinţă şi coeficientul de rigiditate kf :

r

mf f

fk = (4)

Se definesc parametrul de rezistenţă λ1 şi parametrul de rigiditate λ2 astfel:

;0

1 Amσλ = ;

02 A

fm=λ 000 hbA ×= (5)

Cei doi parametrii permit să se aprecieze eficienţa folosirii materialului, valoarea mai mare a acestora indicând o soluţie constructivă mai bună. 4. Rezultate experimentale S-a utilizat o maşină universală pentru încercări mecanice tip EDZ20. S-a folosit dispozitivul pentru încovoiere al maşinii, distanţa între rolele de rezemare fiind l=460 mm. Proba standard a fost o baghetă de lemn de fag uscat, cu secţiune dreptunghiulară de dimensiuni aproximative 24x49 mm. S-au folosit, ca materiale de întărire, produse ale firmei Sika-Elveţia :

- lamelă de compozit Sika Carbodur (din fibre de carbon). - ţesătură fibră SikaWrap Hex-230 (din fibre de carbon). - răşină Epoxy Sikadur-30

S-a solicitat la încovoiere bagheta de lemn, reţinându-se forţa F şi săgeata f la mijlocul barei. Rezultatele experimentale se găsesc în TABELUL 1.a şi 1.b. TABELUL 1.a. Rezultate experimentale

Săgeata f (mm) Var.0.0 Var.0.1 Var.0.2 Var.0.3 Var.0.4 Var.1.0 Forţa (daN) A B A A B A A B C A B

0,2 2,5 2,5 1 0,5 - 1,5 1 2 1,5 0,5 1 0,4 3,9 3,5 1,9 1,4 1 2 1,7 2,5 2,2 1 1,2 0,6 5 5 2,5 2 1,8 2,5 2,5 3 3 1,5 1,5 0,8 6,7 6,5 3,5 3 2,5 3 3,2 3,5 3,5 2 1,9 1,0 8,5 8 4,5 3,8 4 4 4 4,5 4 2,2 2,1 1,2 11,1 10,2 5,5 4,6 5,1 5 5,2 5 5 3 2,5 1,4 13 - 7,2 6 6,3 5,5 6,2 6 5,5 3,5 3 1,6 17 - 9,5 7,5 - 6 7,2 7 6,5 4 3,5 1,8 - - 15 - - 7 9 8 7,5 4,5 4 2,0 - - - - - 8 14 9,5 8,5 5,3 4,5 2,2 - - - - - 9,5 - 11,5 10 6,5 5 2,4 - - - - - 10,5 - - 12 7,3 6 2,6 - - - - - - - - 15 8,5 6,5 2,8 - - - - - - - - - 10,1 7,8 3,0 - - - - - - - - - 11,5 9 3,2 - - - - - - - - - 13,2 10,5 3,4 - - - - - - - - - 15,5 12 3,6 - - - - - - - - - - 13,5 3,8 - - - - - - - - - - 15 4,0 - - - - - - - - - - 17


27

TABELUL 1.b. Rezultate experimentale Săgeata f (mm)

Var.1.1 Var.2.0 Var.2.1 Var.2.2 Var.2.3 Forţa (daN) A A A B A A 0,2 - 3,5 2,5 1 - 3,5 0,4 - 5,6 3 2 - 4,2 0,6 - 7,5 4 2,5 - - 0,8 - 10,5 5 3,3 - 5 1,0 - 13,8 6 4 - 6 1,2 - 18 9 5 - 6,3 1,4 - - 15 5,5 - 7,2 1,6 - - 5 6,5 - 8 1,8 - - - 7,5 - 8,8 2,0 - - - 9 - 9,8

În urma prelucrării datelor experimentale au rezultat parametrii calculaţi din TABELUL 2.a şi 2.b. TABELUL 2.a. Parametrii experimentali calculaţi

Nr crt

Vari-anta

Epru-veta

Aria secţ. Ao

(mm2)

Modul Wt

(mm3)

Moment Mm

(N⋅mm)

Tensiune σt (MPa)

Săgeata max.fm (mm)

OBSERVAŢII

1 0.0 A 1225 10000 1,58⋅106 158 17 Proba de referinţă 2 B 1225 10000 1,35⋅106 135 10,2 3 0.1 A 1187 9988 2,09⋅106 209 15 4 0.2 A 1440 14400 1,8⋅106 125 7,5 5 B 1391 14030 1,58⋅106 112 18 6 0.3 A 1728 20740 2,8⋅106 136 10,5 7 0.4 A 1610 18780 2,25⋅106 120 14 8 B 1610 18780 2,48⋅106 132 11,5 Medie σt=136 MPa 9 C 1680 19600 3,04⋅106 155 15

10 1.0 A 2640 48400 5,08⋅106 105 13,5 σt=92 MPa; flambaj 11 B 2640 48400 3,83⋅106 79 12 flambaj 12 1.1 A 2640 10560 1,74⋅106 164 - 13 2.0 A 2450 10210 1,54⋅106 151 18 Probă de referinţă 14 2.1 A 20400 20400 1,7⋅106 83 15,5 Medie σt=96 MPa 15 B 20400 20400 2,25⋅106 110 9

16 2.2 A 18800 18800 3,02⋅106 161 - Medie σt=96 MPa, un strat ţesătură 17 B 19500 19500 2,6⋅106 135 - un strat ţesătură 18 C 19200 19200 2,82⋅106 147 - un strat ţesătură 19 D 19200 19200 2,2⋅106 117 - un strat ţesătură 20 E 19200 19200 3⋅106 156 - 3 straturi ţesătură 21 2.3 A 26900 26900 3,27⋅106 121 14 Medie σt=125 MPa 22 B 26900 26900 3,6⋅106 134 - 23 C 26900 26900 3,2⋅106 120 -

TABELUL 2.b. Parametrii experimentali calculaţi Nr. crt.

Vari-anta

Epru-veta

Coeficient kσ

Coeficient kf

Parametru λ1

Parametru λ2

OBSERVAŢII

1 0.0 A 1 1 0,129 0,014 Proba de referinţă 2 B 1 1 0,11 0,0080 3 0.1 A 1,42 0,6 0,176 0,0086 4 0.2 A 0,85 0,441 0,087 0,0052


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5 B 0,77 1,06 0,081 0,0130 6 0.3 A 0,93 0,618 0,079 0,0060 7 0.4 A 0,819 0,824 0,075 0,0087 8 B 0,9 0,676 0,082 0,0071 σt mediu este 136 MPa 9 C 1,06 0,882 0,093 0,0089

10 1.0 A 0,715 0,794 0,04 0,0051 flambaj 11 B 0,54 0,706 0,03 0,0045 flambaj 12 1.1 A 1,12 - 0,062 - 13 2.0 A 1 1 0,062 0,0073 Probă de referinţă

14 2.1 A 0,55 0,86 0,034 0,0063 Medie kσ=0,64, kf=0,69, λ1=0,04

15 B 0,73 0,5 0,045 0,0037 λ2=0,005

16 2.2 A 1,06 - 0,070 - un strat ţesătură, medie kσ=0,94

17 B 0,89 - 0,057 - un strat ţesătură, medie λ1=0,06

18 C 0,97 - 0,061 - un strat ţesătură 19 D 0,77 - 0,049 - un strat ţesătură 20 E 1,028 - 0,065 - 3 straturi ţesătură 21 2.3 A 0,8 0,78 0,044 0,005 Medie kσ=0,82 22 B 0,88 - 0,048 - Medie λ1=0,045 23 C 0,79 - 0,043 - Încercările la compresiune au dat următoarele rezultate: • epruvetă de referinţă: σt=78 MPa, kσ=1, λ1=0,065; • epruvetă învelită cu un strat de ţesătură: σt=82 MPa, kσ=1,06, λ1=0,07; • epruvetă învelită cu două straturi de ţesătură: σt=70 MPa, kσ=0,9, λ1=0,59; • epruvetă vopsită cu răşină: σt=67 MPa, kσ=0,86, λ1=0,056. 5. Concluzii În urma experimentărilor se pot evidenţia următoarele concluzii importante: - lemnul este un material cu un anumit grad de neomogenitate, fapt ce face ca proprietăţile

sale mecanice să varieze într-o gamă prea largă; de aceea este cu atât mai necesară ameliorarea rezistenţei cu ajutorul adaosurilor compozite:

- proprietăţile lemnului depinzând de dispunerea fibrelor, care prezintă neomogenităţi, în experimente este necesar un număr mare de epruvete pentru a face o analiză statistică şi a determina valorile sigure ale rezistenţei şi rigidităţii:

- întăririle cu material compozit sunt cu atât mai eficiente cu cât se află mai departe de axa neutră a secţiunii; variantele cu întăriri plasate la mijlocul secţiunii grinzii nu au crescut rezistenţa, în unele cazuri având chiar efect negativ;

- materialul compozit folosit trebuie să fie de calitate, formarea corectă a matricei fiind

hotărâtoare; cazurile care au folosit pânză impregnată cu răşină direct pe probă nu au condus la creşterile scontate de rezistenţă din cauza compozitului de slabă calitate rezultat;

- folosirea compozitului de mare rigiditate, comparabil cu lemnul (lamele compozite), nu conduce la rezultate foarte bune întrucât se produce relativ repede dezlipirea de lemn; în această situaţie se impune pretensionarea lamelei de compozit şi solidarizarea mecanică a acesteia de grinda de lemn;

- pentru stâlpi nu se îmbunătăţeşte rezistenţa dacă se folosesc compozite de întărire; putem spune că aplicarea pe suprafaţa stâlpului de lemn a unui compozit formatat local (ţesătură impregnată cu răşină) scade rezistenţa acestuia, probabil din cauza unor degradări chimice a lemnului produsă de răşină;


29

- dacă se creşte excesiv înălţimea secţiunii, rigiditatea creşte foarte mult dar capacitatea portantă este limitată de fenomenul de flambaj; considerăm că materialul compozit a accentuat tendinţa de flambaj, fapt ce arată că trebuie evitată această soluţie;

- conform fig.5, capacitatea portantă maximă prezintă varianta 0.1; cea mai mică portantă o are varianta 1.0 şi 2.1;

- cea mai rezistentă soluţie (din punct de vedere al portanţei) se realizează în cazul variantei 0.1 iar la polul opus (din acest punct de vedere) sunt variantele 1.0 şi 2.1 (conform graficului din fig.6);

-din punct de vedere al eficienţei folosirii materialului pentru portanţă mare, cea mai bună soluţie este cea din varianta 0.1, la cealaltă extremă fiind varianta 1.0 şi 2.1 (conform fig.8);

0

50

100

150

200

250

1 4 7 10 13 16 19 22

Fig.5. Variaţia tensiunii maxime σt în cele 23 de epruvete

Fig.6. Variaţia coeficientului kσ în cele 23 de epruvete

00,20,40,60,8

11,21,41,6

1 4 7 10 13 16 19 22

Fig.7. Variaţia coeficientului kf în cele 23 de epruvete

0

0,2

0,4

0,6

0,8

1

1,2

1 4 7 10 13 16 19 22

Fig.8. Variaţia parametrului λ1 în cele 23 de epruvete

0

0,05

0,1

0,15

0,2

1 4 7 10 13 16 19 22


30

Fig.9. Variaţia parametrului λ2 în cele 23 de epruvete

00,0020,0040,0060,0080,01

0,0120,0140,016

1 4 7 10 13 16 19 22

- eficienţa cea mai mare, din punct de vedere al rigidităţii, se atinge în cazul variantei 0.2,

urmată de variantele 0.1 şi 0.4 (conform fig.9); Experimentările au arătat viabilitatea metodei de creştere a rezistenţei construcţiilor din

lemn cu ajutorul materialelor compozite. Soluţia este uşor de implementat iar costurile sunt mici. Eficienţa utilizării întăriturilor din compozit este deocamdată modestă, impunându-se continuarea şi aprofundarea studiilor. Bibliografie [1]. Dan Iincioiu, Rezistenţa materialelor, ediţia a II-a, Editura TOP TPT, Craiova, 2007. [2]. V. Roşca, D. Ilincioiu, M. Marin, I. Aştefanei, Încercări de Rezistenţa Materialelor, Ed.

Universitaria, Craiova, 2007. [3].Cătălina Ianăşi, Studii experimentale pe materiale compozite. Modelări aplicative,

Referat doctorat. Universitatea din Craiova, Craiova, 2008. [4]. V. Stoian, T.Nagy-György, D.Dan, ş.a, Materiale compozite pentru construcţii, Ed.

Politehnica, Timişoara, 2004. [5]. D.Drimer, I. Demetrescu , Fibre de carbon, Editura Tehnică, Bucureşti,1986. [6]. St.Ispas, Materiale compozite, Editura Tehnică, Bucureşti ,1987 [7]. F. Ştefănescu , G. Neagu, A. Mihai, Materiale compozite, Editura Didactică şi

Pedagogică, Bucureşti,1996. [8]. www.sika.com [9]. Complete structural strengthening systems with composite materials, Balkan and Black

Sea Partnership Seminar, november17th-18h 2003, Thessaloniki


31

UNELE ASPECTE PRIVIND ALEGEREA PRECIZIEI, GRUPEI ŞI FELULUI DE AJUSTAJ ÎN CONSTRUCŢIA DE MAŞINI

Prof.univ.dr.ing. Nicolae NIŢESCU, Universitatea din Petroşani

Prof.univ.dr.ing. Aron POANTĂ, Universitatea din Petroşani Şef de lucr.drd.ing. Dan DOJCSAR, Universitatea din Petroşani

Şef de lucr.dr.ing. Anne-Marie STOIAN, Universitatea din Petroşani Prof.ing. Marilena DOJCSAR, Liceul Teoretic Mihai Eminescu, Petroşani

Abstract:In the paper are presented some aspects in the regard to the precision choice, the fit`s group and optimal way of fit, used in machine manufacturing.The designing of clearance fit in base of basic shaft using the ,,key” relation and the made table with numerical examples represents the base of establishing alghorithms and programs for computing design. Keywords: designing, precision, alghorithms, numerical. Având o foarte mare importanţă din punct de vedere tehnic, funcţional şi economic, precizia de execuţie a pieselor şi ajustajele se stabilesc şi se aleg în concordanţă cu posibilităţile de realizare, cu economicitatea prelucrării şi asamblării, cu parametri fucţionali impuşi de condiţiile de funcţionare şi exploatare şi cu alţi factori. O primă modalitate de alegere a preciziei şi ajustajelor se bazează pe recomandările din tratate, articole, norme, instrucţiuni, standarde, etc. [7]. Când se pune problema în legătură cu stabilirea felului optim de ajustaj, alegerea preciziei şi a ajustajului în sine nu mai este chiar aşa de simplă. În cele ce urmează se prezintă o sinteză a cercetătorilor din domeniu prezentate exhaustiv în [1] şi punctual în [2], [3], [4] şi [5]. Clasele de precizie (calităţile ISO) la ajustaje, se aleg în funcţie de factorii [1]:

- calitatea asamblării – ce constă în proprietatea pieselor asamblate de a fi cât mai apropiate ca mărime de valoarea nominală precizată în desen;

- gradul de determinare funcţional al asamblării care este cu atât mai mare cu cât toleranţa caracteristicii de asamblare este mai mică şi invers.

Proiectantul, la alegerea clasei de precizie va proceda astfel: I. Din condiţiile funcţionale se impune o variaţie a caracteristicii de asamblare (joc sau strângere), proprie unei anumite asamblări, între limite aproximative, ca în exemplul:

( )mmJ 052,0...009,0= , de unde toleranţa jocului: mJJTj µ43952minmax =−=−= (1)

Din formula generală a toleranţei: iCxITxT ⋅== (2) se determină numărul unităţii de toleranţă Cxaj , pentru ajustajul cu joc considerat şi pentru dimensiunea nominală 35 mm ce aparţine intervalului ]40...30( mm cu mmm 35=φ :

2853,285069,143

001,045,0 3≅==

+==

mm

jjaj

Ti

TCx

φφ; (3)


32

pentru fiecare din piesele perechi ale ajustajului; mCxaj µ14

228

2== .

Din tabelul numărului unităţilor de toleranţă în funcţie de calităţile IT (tabelul 1) rezultă pentru ajustajul din exemplul analizat clasa 6 (care are (C6 = 10) şi clasa 7 (C7 = 16)).

Odată stabilite clasele de precizie 7, respectiv 6: 67Nφ (s-a ţinut seama de faptul că arborele

se poate prelucra mai precis la acelaşi cost). 28161076 ≅+=+ CC Aceste calităţi vor satisface valorile toleranţelor pieselor care urmează să asigure jocul impus de condiţiile funcţionale. Tabelul 1 Calităţi ITx 5 6 7 8 9 10 11 12 13 14 15 16 Numărul unităţilor de toleranţă Cx 7 10 16 25 40 64 100 160 250 400 640 1000 II. Cel de-al doilea criteriu este prezentat în detaliu în [1], [2], [3], [4] şi [5]. Alegerea şi proiectarea în baza arbore unitar, sunt prezentate în [4] pentru ajustaje cu joc, în [2] pentru cele intermediare şi în [5] pentru cele cu strângere. Chenarul I din figura 1 conţine toleranţele fundamentale IT01…IT18 cu preciziile 01; 0; 1; 2 …18, iar chenarul II conţine abaterile fundamentale ale arborilor, pentru cele 11 feluri de ajustaje cu joc, iar sub tabelul 2 (fig.1) este reprezentată amplasarea câmpurilor de toleranţă în funcţie de felul ajustajului şi semnificaţia valorilor cuprinse în chenarele I şi II. Din figura 1 se deduce: ITxJJ med += min (4) în legătură cu care se stabileşte ,,cheia” de utilizare a tabelului 2 din figura1:

↑

↓

↑

↓↓

+=

cifra

chenarIvaloare

litera

chenarIIvaloarees

calculatavaloare

med ITxJJ min (5)

În [4] este stabilită ,,cheia” de utilizare a tabelului construit pentru ajustaje în baza arbore unitar. Jocul mediu calculat este egal cu jocul minim (valoare din chenarul II al tabelului 2 sub formă de es ) care pe verticală corespunde unei litere, aceasta va fi litera de simbol al felului de ajustaj proiectat, plus un câmp de toleranţă ITx, care se găseşte în chenarul I al tabelului 2 şi căruia îi corespunde pe verticală o cifră. Aceasta va fi cifra de simbol care reprezintă precizia (calitatea) a ajustajului proiectat [1].

Observaţie: Condiţia ,,cheie”, după [1], este ca: ITxes ≈ sau ITxes > , cu condiţia ideală ca cele două valori să fie cât mai apropiate.


33


34

Dacă prescrierea unui ajustaj standardizat pentru care există calibre [2] este indicată la producţia de serie mică, în cazul producţiei de serie mare şi mai ales a celei de masă, dacă piesele se verifică cu calibre nestandardizate, atunci rentează construcţia unor calibre nestandardizate. Transformarea din sistemul de ajustaj cu alezaj unitar în sistemul cu arbore unitar în cazul în care cele două trepte de precizie nu sunt egale, piesa îşi păstrează treapta (clasa) de precizie. În scopul eliminării muncii de rutină (calculul algebric simplu, consultare tabele abateri, toleranţe fundamentale, etc), cât şi a micşorării substanţiale a timpului aferent acestor operaţii, problema proiectării ajustajelor se pretează foarte bine a fi rezolvată cu ajutorul calculatorului electronic [1], [6]. La proiectarea ajustajelor cu joc, datele de intrare sunt: dimensiunea nominală a ajustajului, turaţia la care funcţionează ajustajul, temperaturile celor două piese în timpul funcţionării şi coeficienţii de dilataţie termică liniară pentru materialele pieselor. După ce se alege din lista de ajustaje cel mai convenabil, vor fi determinate abaterile limită şi caracteristica de asamblare corectată la prelucrare, temperatură şi rugozitate. Bibliografie: [1]. Bagiu, L., - Toleranţe şi ajustaje, Editura Helicon, Timişoara, 1994. [2]. Niţescu, N., Stoian, A-M., Unele aspecte privind proiectarea ajustajelor intermediare. Lucrările ştiinţifice ale Simpozionului Internaţional Multidisciplinar ,, UNIVERSITATEA SIMPRO 2005”, Tehnologie, Mecanisme şi Organe de maşini, pag. 43-48, Editura Universitas, Petroşani, 2005. [3]. Niţescu, N., Stoian, A-M., Niţescu, Al., Some Aspects in Regard to the Designing of Fits. Annals of the University of Petroşani. Mechanical Engineering, vol. 7 (XXXIV), pag. 75-86, Universitas Publishing House, Petroşani – România, 2005. [4]. Niţescu, N., Stoian, A-M., Unele aspecte privind proiectarea ajustajelor cu joc. Lucrările ştiinţifice ale simpozionului Internaţional Multidisciplinar ,, UNIVERSITARIA SIMPRO 2006”, Tehnologie, Mecanisme şi Organe de maşini, Mecanică şi Rezistenţă, pag. 67-72, Editura Universitas, Petroşani, 2006. [5]. Niţescu, N., Stoian, A-M., Some Aspects in Regard to the Designing of Interference Fits. Annals of the University of Petroşani, Mechanical Engineering, vol. 9 (XXXVI), part. II, pag. 73-78, UNIVERSITAS Publishing House, Petroşani – România, 2007. [6]. Poantă, A., ş.a. Proiectarea asistată a ajustajelor cu joc. Simpozionul ,,Durabilitatea şi fiabilitatea sistemelor mecanice”, Universitatea ,,Constantin Brâncuşi” Târgu Jiu, 20-21 iunie 2008. [7]. Zamfir, V., Niţescu, N., - Toleranţe şi control dimension

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