Journal of Mechanical Engineering 54 (2008)1

Editorial

Dear readers, at the end of 2007, the Publishing Board changed in accordance with the rules of the Strojniški vestnik – Journal of Mechanical Engineering. Both faculties of mechanical engineering aimed at including in the new Publishing Board the associates who have contributed significantly - or still do so - to the written word in the field of mechanical engineering. The invited colleagues responded to the invitation with great enthusiasm. During personal interviews, they presented their views on the development of the Journal that represents the hub of knowledge in the field of scientific papers in Slovenia. It is our goal to further open the door for knowledge exchange on the path to international recognition of the magazine, started a decade ago. The common European area now makes it possible. The Publishing Board gathered several times in order to pave the way for the Editorial Board, the chief editor and his deputy in particular. Changes of the contents and design concept are now in front of you. Judge them and write your opinion...

Cover plate

The flow distribution of air and water among the parallel vertical tubes of manifold system has been studied over a wide range of air and water flow rates. Examples: slug flow and semi-annular flow regimes at the first T-junction recorded at 10.000 fps, respectively. Geometry: half circle cross section, 1.22 mm header hydraulic diameter, 0.61 mm side-arm hydraulic diameter. Courtesy of the Laboratory for Fluid Dynamics and Thermodynamics, Faculty of Mechanical Engineering.

 

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Analysis of Hydraulic Characteristics of Guard-Gate for Hydropower Plant

Corresponding author - Zlatko Rek

Authors - Anton Bergant, Miha Röthl, Primož Rodič, Iztok Žun

Abstract

A guard-gate can be installed at the inlet of the pressure tunnel, at the downstream end of the surge tank or in the draft tube of the water turbine. A hydraulic shape of the gate and characteristics of the hydropower plant flow-passage sytem govern the magnitude of pressure forces acting on the gate structure. Flow conditions at the downstream end of the gate may require adequate air admission. Numerical analysis of hydraulic characteristics has been performed for a vertical leaf gate at different gate openings. The analysis has been performed with Computational Fluid Dynamics (CFD) code using finite volume method. Computational results are compared with results of measurements carried out in a model test rig.

Keywords - hydropower plant, guard-gates, hydraulic characteristics, control volume methods

Paper's information

Strojniški vestnik - Journal of Mechanical Engineering 54(2008)1, 3-10 
UDC 627.84

Analysis of Hydraulic Characteristics of Guard-Gate for Hydropower Plant (pdf)

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Vibration Analysis to Determine the Condition of Gear Units

Corresponding author - Aleš Belšak

Avtorji - Jože Flašker

Abstract

The use of the most up-to-date production technologies and a high level of production stability without any unscheduled outages are of utmost importance; they are affected primarily by monitoring the condition and by adequate maintenance of mechanical systems. Life cycle design of machines and devices is nowadays gaining ground rather quickly; users want that machines and devices operate with a high level of accuracy and reliability and with as few outages as possible. Thus, by monitoring the condition, not only the presence of changes but also predictions related to the type and size of damage or error jeopardising the high quality of operation during the remaining life cycle of a machine is established.

Keywords - gear units, fault detection, vibration analysis, probability analysis, operation reliability

Paper's information
Strojniški vestnik - Journal of Mechanical Engineering 54(2008)1, 11-24 
UDC 621.83.052

Vibration Analysis to Determine the Condition of Gear Units (pdf)

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An Attempt to Detect Various Types of Stress-Corrosion Cracking on Austenitic Stainless Steels by Simultaneous Measurements of Acoustic Emission and Electrochemical Noise

Corresponding author - Jaka Kovač

Authors - Žiga Bajt, Mirjam Leban, Andraž Legat

Abstract

This paper discusses the potential use of acoustic emission (AE) and electrochemical noise (EN) for the detection and characterization of different types of stress-corrosion cracking (SCC) in austenitic stainless steels. The discussion is based on the performance of constant load experiments. Three different measurements were performed simultaneously during the experiment: acoustic emission, electrochemical noise, and elongation of the test specimen. The stainless steel was sensitized to increase its susceptibility to SCC. Two different electrolytes and two different load levels were used to obtain different types of SCC. Intergranular stress-corrosion cracking (IGSCC) was investigated in an aqueous solution of sodium thiosulphate at a load level below the yield point, whereas transgranular stress-corrosion cracking (TGSCC) was investigated in an aqueous solution of sodium thiocyanate at a load level beyond the yield point. No simultaneous AE activity increases, EN transients or elongation discontinuities were observed in the case of IGSCC. Except for a few EN transients, the signals were smoother than in the case of TGSCC, which could reflect the more continuous nature of IGSCC processes. This is also probably the reason that no simultaneous events were observed in this case. On the other hand, a significant increase in the DC part of the measured electrochemical current, corresponding in time with increases in elongation and, to some extent, also increases in AE activity, was observed in the second part of experiment. It was assumed that the current shift in the anodic direction was caused by an increase in the dissolving surface area due to the growth of cracks. On the other hand, on the basis of the simultaneous use of the three techniques, several single TGSCC events were detected. Detection was accomplished by the observation of simultaneous increases in AE activity, EN transients, and discontinuities (jumps) in elongation. On the basis of the time resolution of the measurements, it was concluded that the detected electrochemical events were the result of corresponding mechanical events. It was also concluded that the simultaneous use of EN and AE is a promising approach for the detection and characterization of SCC, but that further research and improvements of the measuring system are needed.

Keywords - stress corrosion cracking, electrochemical noise, acoustic emission, austenitic stainless steel

Paper's information
Strojniški vestnik - Journal of Mechanical Engineering 54(2008)1, 25-36 
UDC 620.19

An Attempt to Detect Various Types of Stress-Corrosion Cracking on Austenitic Stainless Steels by Simultaneous Measurements of Acoustic Emission and Electrochemical Noise (pdf)

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Production Scheduling Model in Aluminium Foundry

Corresponding author - Gordana Matičević

Authors - Niko Majdandžić, Tadija Lovrić

Abstract

There are numerous methods and strategies for production management which are successfully implemented in the metal industry and in the automobile and machine tools industry in particular. However, little research has been published regarding scheduling foundry operations. Therefore, the main goal of this paper is to develop a new mathematical model for scheduling foundry operations based on the MRP II (Manufacturing Resource Planning), JIT (Just in Time) and OPT (Optimized Production Technology) concepts. The research strategy includes a review of available literature and integration of the developed mathematical model into a foundry for testing the model with real data. The proposed model is successfully implemented into the ERP (Enterprise Resource Planning) system of Aluminium Ltd. in Mostar. The conclusions offered in this paper are based on the results of the tests carried out in that single foundry so that further work is required to validate the findings at other foundries and other manufacturing areas.

Keywords - aluminium production, scheduling, mathematical models, tardiness minimization

Paper's information
Strojniški vestnik - Journal of Mechanical Engineering 54(2008)1, 37-48 
UDC 658.5:669.712

Production Scheduling Model in Aluminium Foundry (pdf)

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Optimum Selection of Information Terminals for Production Monitoring in Manufacturing Industries

Corresponding author - Đani Jurićić

Authors - Jani Kleindienst

Abstract

The purpose of automated acquisition of production data in manufacturing industries is aimed to provide accurate data about the performance of the scheduled tasks and the production process. Hence, easier and more effective production control can be achieved. The information acquired includes the duration of each operation, the number of manufactured items of products, the amount of scrap as well as the duration of downtimes and their root-causes. A way to acquire relevant data makes use of special-purpose information terminals. The problem that arises here is how to select the optimum number of the terminals in order to minimize the overall losses. The approach presented below relies on optimization of a stochastic criterion function, which combines the terminal costs and costs related to the waiting times during busy sessions. The solution suggested is based on using the distribution of events, recorded during the past production session. A case study dealing with optimum selection of terminals in a real production process is presented in detail.

Keywords - production systems, optimization, production monitoring, production control

Paper's information
Strojniški vestnik - Journal of Mechanical Engineering 54(2008)1, 49-55 
UDC 658.5.012.7

Production Scheduling Model in Aluminium Foundry (pdf)

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Control of the Cutting Forces in Turning by Entry Angle and Cutting Inserts Geometry

Corresponding author - Toma Udiljak

Authors - Stephan Škorić, Damir Ciglar

Abstract

Machinability is assessed by a set of criteria or machinability functions, the knowledge of which is needed in optimising the machining process. New cutting tool materials and new concepts of the machine tools provide new possibilities and cause quantitative changes in machinability functions. This research has studied the cutting forces function, as one of the crucial machinability functions. The research was done at longitudinal turning of steel 16MnCr5. Coated carbide inserts for roughing and finishing with different inclination angles were used, and tool clamping system has enabled the changes of entry angle. Obtained results confirm that entry angle and geometry of cutting insert significantly influence cutting forces especially thrust cutting force.

Keywords - turning, cutting forces, cutting tool inserts, inserts geometry

Paper's information
Strojniški vestnik - Journal of Mechanical Engineering 54(2008)1, 56-61 
UDC 621.9.04

Control of the Cutting Forces in Turning by Entry Angle and Cutting Inserts Geometry (pdf)

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Exponential Tracking Control of an Electro-Pneumatic Servo Motor

Corresponding author - Dragan V. Lazić

Abstract

According to the fundamental importance of the tracking theory on technical systems, the main goal of this paper is a further development of the theory and the application of the tracking, especially to the practical tracking concept. The new definition of the practical exponential tracking is shown. The practical tracking is defined by the output vector, which is different from the former definitions, given by the output error vector. The defined exponential tracking is elementwise. The new criterion of the practical exponential tracking is shown. Based of the new criteria, the control algorithm of practical exponential tracking is determined by using the self-adaptive principle. The structural characteristic of such a control system is the existence of two feedback sources: the global negative of the output value and the local positive of the control value. Such a structure ensures the synthesis of the control without the internal dynamics knowledge and without measurements of disturbance values. The plant under consideration is an electro-pneumatic servo motor. This system is often applied as the final control element of a controller in automatic control systems. The correction device for the mentioned plant will be a digital computer. The mentioned control forces the observed plant output to track the desired output value with prespecified accuracy. In this paper the simulation results produced by practical tracking control algorithm on an electro-pneumatic servosystem will be presented. The results show a high quality of the practical exponential tracking automatic control. The type of the control ensures the change of the output error value according to a prespecified exponential law.

Keywords - telectro-pneumatic servo systems, servo motors, non-linear systems, self adaption

Paper's information
Strojniški vestnik - Journal of Mechanical Engineering 54(2008)1, 62-67 
UDC 621.3.077.2

Exponential Tracking Control of an Electro-Pneumatic Servo Motor (pdf)

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Laser Supported Optical Control of High Pressure Aluminium Cast Products

Corresponding author - Janez Možina

Authors - Valter Gruden, Drago Bračun

Abstract

We present a new method for surface quality control of aluminium high pressure cast products. By this method it is aimed to improve efficiently the existing practice of castings being only visually checked for surface defects such as laminations, non-fills and cold shots. The method is based on laser triangulation principle. The measured cloud of points is analysed using software designed specifically for automatic detection of surface defects. The paper describes a measurement system, measuring procedure focussed on the detection of surface defects and the comparison of the results with a visual inspection.

Keywords - aluminium pressure casting, surface quality control, laser 3D-measurement, optical control

Paper's information
Strojniški vestnik - Journal of Mechanical Engineering 54(2008)1, 68-76 
UDC 658.562.4:669.71

Laser Supported Optical Control of High Pressure Aluminium Cast Products (pdf)