• ## Spatial gearing: Kinematic characteristics of the instantaneous contact

pg(s) 57-61

In this work, case of rotations transformation between arbitrary crossed shafts (axes of rotations) by means of high kinematic joints, which elements configure active tooth surfaces of hyperboloid gear mechanisms is treated. |Analytical dependencies, defining the law of rotations transformation are illustrated. This law of transformation in this concrete case is a constant function of the relations of the angular velocities of the movable links of the spatial three-link gear mechanism. The shown functions are applicable both to the synthesis of the studied transmissions, and for the determining and control of the kinematic errors of these transmissions, caused by manufacturing and assembly errors.

• ## Finite elements method modelling of rolling bearings

pg(s) 62-65

This study presents to determine the contact stress in rolling bearings by using analytical and numerical method. Analytical solution is obtained by using Hertizan contact theory. Obtained analytical solution by this theory require comparison with the numerical calculations to obtain more accurate results for contact problems. Because of that the same problems are also examined by using finite element method. The geometry of the model being studied gives different type of contact configurations such as a point or line of contact. In cylindrical roller bearing the contact form is line contact and for the ball bearing the contact characteristic is point contact. High stress occurs on both of these two contact areas. Contact stress causes elastic or plastic deformation and the contact area will change depending on the magnitude of the contact stress. Therefore, it is really important to calculate more accurate stress at the contact area.

• ## Analysis of workover causes for wells with installed downhole sucker-rod pumps

pg(s) 66-69

Primary oil production represents the process of production only by exploiting the natural reservoir energy (reservoir pressure), i.e. eruption phase. This mode of production lasts until the reservoir pressure drops, so that it is no longer capable of pushing the oil to the surface. Oil wells which can’t erupt, must apply one of the methods of artificial lifting. The artificial lift system of oil production by using downhole sucker-rod pumps with piston rods is the oldest system for lifting fluids from wellbore to the surface. It is also the most widespread artificial lift system in practice in the world (over 80%). The basic principle of the operation of a downhole sucker-rod pumps is based on the transfer of the drive energy from the surface to the pump by piston rods. In principle, downhole sucker-rod pumps with piston rods can be applied in oil wells from depth of 500 to 2500 m while enabling production ranging from 1 to 100 and more m3/d of liquid. This system is suitable for wells with a smaller gas factor and consolidated production layers. The advantage of downhole sucker-rod pumps is their efficiency and economy because initial capital investments per well are lower. The disadvantages of downhole sucker-rod pumps are higher maintenance costs, i.e. more frequent workovers. This paper analysis the most common causes for well workover that produce oil with downhole sucker-rod pumps and recommendations in order to reduce their number.

• ## Numerical analysis of real open cycle gas turbine

pg(s) 70-73

The paper presents a thermodynamic analysis of gas turbine with real open cycle. Gas turbine operates in combined heat and power (CHP) system. Analysis is provided by using measured operating parameters of operating mediums (air and combustion gases) in all required operating points. Cumulative real turbine developed power amounts 78611.63 kW. In the whole gas turbine process, the highest losses occur in combustion chambers during the heat supply process and amounts 13689.24 kW. Turbine power losses are equal to 7976.22 kW, while the turbo-compressor power losses amounts 4774.24 kW. While taking into account all analyzed gas turbine components, the highest efficiency of 90.79% has turbine, followed by combustion chambers which efficiency is equal to 89.01%. Turbo-compressor efficiency amounts 88.59% and the whole gas turbine cycle has efficiency equal to 33.15%.

• ## Determination of geyser events in a thermosyphon working with graphene oxide nanofluid

pg(s) 74-77

Two-phase closed thermosyphons are efficient passive devices with potential for using in many heat transfer applications. One of the boiling regimes that may occur is the geyser boiling. It is a repetitive irregular process of pushing liquid without its previous evaporation in the direction of condenser. Although it does not affect time-averaged thermal performance of the device, it causes additional mechanical load and shortens the life-time of the device. Unfortunately, geysering is not well investigated, thus no precise definition exists. This paper focuses on the process of data reduction that leads to geyser boiling detection. It may be applied for various working fluids and operating conditions. Two parameters are crucial for recognizing geyser events from the background noise (pressure variations followed by the geyser): the minimum amplitude of pressure increase and waiting period between ensuing events. We compared two working fluids: water and graphene oxide nanofluid. In general, with increase of heat flux the frequency of geysers increases and their amplitude decreases.

• ## Determination of geometric parameters of gradient structures formed in optical glass by the electron beam method

pg(s) 78-82

The results of experimental studies of the geometry of gradient structures formed in optical glass by the method of electron-beam modification of its surface are presented. The expediency of using the atomic force microscopy method for determining the geometrical parameters of microlayers formed in the surfaces of optical materials by the method of their electron-beam modification is substantiated. A new method for determining the basic geometric parameters (thickness of the gradient layer, topology of the interface “gradient layer – the basis of the material”, surface microrelief, etc.) gradient structures, based on the method of atomic force microscopy is proposed. The proposed method is based on the principle of the complex application of various operating modes of an atomic-force microscope (the results of the instrument operation are considered, both in contact and in non-contact modes) in one measurement cycle. According to the proposed method, the dependencies between the change in the refractive index and the modes of electron-beam modification of the optical glass surface, as well as the dependence between the electron-beam effect modes and the geometric parameters of the gradient structure on the glass (thickness of the gradient structure, surface microrelief and interface between the layers and the base material ). The possibility of predicting the development of hidden microdefects at the interface “gradient layer – the basis of the material” is shown. Proved high reliability and adequacy of the proposed method by comparing the results obtained with the results of determining the geometric parameters obtained by other alternative methods.

• ## Diagnostics of thermal pipes with symmetric structure thermal impact method

pg(s) 83-85

On the basis of completed studies, including computer modeling of the morphology of the temperature field of heat pipes and thermal measurements in the framework of the experiment, a method was developed for diagnosing the quality of heat pipes with a symmetrical structure.

• ## An experimental study on cutting forces and surface roughness in MQL milling of aluminum 6061

pg(s) 86-89

The minimum quantity lubrication (MQL) and dry machining operations are two types of environmentally friendly processes that have potentials to replace with conventional cooling methods. In recent years, the studies have been performed on the environmentfriendly lubrication methods such as MQL method in machining operations. These studies have also focused on the usage of vegetable cutting fluids instead of mineral based oils due to increasing awareness to the environment and human health. In this experimental study, Aluminum 6061 was machined under dry and MQL conditions and the cutting force components and surface roughness values were measured. The experiments were carried out by CNC milling machine tool at three different cutting speeds and constant depth of cut and feed. The vegetable cutting fluid was pulverized to the rake face by using single nozzle and double nozzles at two flow rates during MQL milling. Cutting force components and surface roughness values were compared for minimum quantity lubrication and dry milling operations of Aluminum 6061.

• ## Technogenic raw materials for the production of magnesium and silicon-containing compounds

pg(s) 90-92

This work considers technogenic waste from chrysotile asbestos production as a source of magnesium and silicon-containing compounds. The products obtained by hydrochloric acid dissection of chrysotile asbestos wastes were studied by chemical, scanning electron microscopy, thermogravimetric analysis.

• ## Flank wear of surface textured tool in dry turning of AISI 4140 steel

pg(s) 93-96

Surface texturing on a cutting tool is a process which can provide reduction in friction, improved performance of a contact interface and a better tool life. In this paper, influence of surface texturing on tool wear of both textured and non-textured tools were compared. Different textures were applied on coated carbide tools by Femtosecond laser to enchance the wear resistance. Dry cutting experiments on AISI 4140 (32 HRC) were carried out with conventional cutting inserts having parallel and perpendicular textured sufaces to the cutting edge on the flank face. Finally, results showed that surface texturing of cuttting insert was found to be beneficial to the decrease of the flank wear.

• ## Geopolymers based on Bulgarian raw materials – preliminary studies

pg(s) 97-100

The geopolymers are novel class of inorganic polymer materials consist of chains, sheets or networks made of covalently bonded mineral molecules. The main precursors of the geopolymers are reactive alumosilicate material and hardener solution. Various suitable raw materials are located in Bulgaria. The present paper shows successful examples of geopolymers based on Bulgarian raw materials – fly ash, metakaolin, natural zeolite and iron-rich waste from copper industry. The properties and structure of the prepared geopolymers greatly depend on the composition and type of the raw material, as well as concentration and type of the activator solution. The results show the potential of geopolymers based on Bulgarian raw materials as building materials.

• ## Effects of bismuth on the matrix structure of ductile iron castings

pg(s) 101-103

This paper deals with the effects of the addition of 0.00031, 0.00064, 0.001 and 0.0042 wt.% Bi on the matrix structure of ductile iron castings consisting of 7 sections of different thicknesses (3, 12, 25, 38, 50, 75 and 100 mm) and contain low content of Si (2.11 wt.%) and pearlite promoting element (0.018 wt.% Cu, 0.0055 wt.% Sn, 0.00041 wt.% Sb, 0.098 wt.% Mn). The Bi contents of 0.00031, 0.00064 and 0.001 wt.% were not significantly affected the share of ferrite and pearlite in the section thicknesses of 12, 25, 38, 50, 75 and 100 mm compared to the casting which does not contain Bi. In all these sections the share of pearlite was increased and the share of ferrite was decreased by the addition of 0.0042 wt.% Bi. All of the above-mentioned Bi contents were resulted in the formation of iron carbides in the section thickness of 3 mm. The share of carbides increases with increasing Bi content.