Machinery - Repository of the Faculty of Mechanical Engineering, University of Belgrade
Not a member yet
8397 research outputs found
Sort by
DEVELOPMENT OF EXOSKELETON FOR HANDHELD POWERTOOLS
No full textHandheld power tools, such as electric drills and electric screwdrivers, are widely used today. They are applied in households, the furniture industry, the automotive industry, and even in construction and interior design. Major companies that produce handheld power tools (e.g., BOSCH, MAKITA, BLACK&DECKER, STIHL, HILTI) invest significant material and financial resources in improving existing products and developing new ones to outperform the competition in the international market and achieve greater profit. One way to overcome existing problems in the use of handheld power tools is to develop and design external systems for their easier use and control – exoskeletons. This is precisely the goal of the prototype developed at the Faculty of Mechanical Engineering, University of Belgrade, within the proof-of-concept project funded by the Innovation Fund of the Republic of Serbia. It has been observed that almost all handheld power tools available on the market require the use of both hands during operation, demand significant physical strength from the user, and generate substantial vibrations that are transmitted to the user, making the use of handheld power tools even more challenging. To overcome these problems, the primary function of the developed exoskeleton has been broken down into elementary functions, for which appropriate actuators have been defined, followed by the development of several alternative solutions, from which the best was further developed into a functional prototype with multiple benefits for the user. The developed exoskeleton transfers the loads typically handled by the user's second hand to the forearm of the first hand using a lever system, preventing wrist overload while maintaining precise control and handling of the tool. This allows the user's second hand to remain free for other tasks, such as holding the workpiece. Additional force in the tool's direction is provided by the exoskeleton being supported on the user's upper arm, utilizing body mass along with the biceps and other arm muscles during operation. Vibrations from the tool are minimized by incorporating a vibro-elastic element in the exoskeleton, significantly reducing vibration transfer to the user. Additionally, the handheld power tool can be used in various positions – above the head, downwards (towards the floor), at shoulder, or waist level. Also, during the development of the exoskeleton, special attention has been paid to the safety of the user, so the handheld power tool is connected to the exoskeleton by a detachable joint, allowing the tool to be quickly discarded if necessary. It is important to note that during the development of the exoskeleton, all guidelines of the German association VDI were applied, significantly accelerating the product development process, along with the use of available rapid prototyping and 3D printing technologies. Finally, all solutions presented have been submitted for patent protection, with registration currently underway (patent application no. 2024/0025)
Comparative analysis of ABS materials mechanical properties
No full textAdditive Manufacturing (AM) has shown rapid development during the last decades. More commonly known as 3D printing, this technology offers significant benefits for the fabrication of physical models with complex geometries. This technology started as a valuable tool for prototyping purposes but is considered for general manufacturing due to the production cycle shortening and less (if none) material waste. Out of seven possible AM technologies, this research covers three of them, namely: Fused Deposition Modeling (FDM), Stereolithography (SLA), and Digital Light Processing (DLP). Although, ABS thermoplastic material is a thoroughly researched AM material, a new resin material also called ABS is an interesting object for research. Hence, the aim of this paper is to acquire the tensile properties of this material and to compare them to the “original” ABS thermoplastic material
Effect of Aging on Tensile and Chemical Properties of Polylactic Acid and Polylactic Acid-Like Polymer Materials for Additive Manufacturing
No full textAdditive manufacturing, with its fast development and application of polymeric materials, led to the wide utilization of polylactic acid (PLA) materials. As a biodegradable and biocompatible aliphatic polyester, produced from renewable sources, PLA is widely used in different sectors, from industry to medicine and science. The aim of this research is to determine the differences between two
forms of the PLA material, i.e., fused deposition modeling (FDM) printed filament and digital light
processing (DLP) printed resin, followed by aging due to environmental and hygiene maintenance
conditions for a period of two months. Specimens underwent 3D scanning, tensile testing, and
Fourier transform infrared (FTIR) spectrometry to obtain insights into the material changes that
occurred. Two-way Analysis of Variance (ANOVA) statistical analysis was subsequently carried out
to determine the statistical significance of the determined changes. Significant impairment can be
observed in the dimensional accuracies between both materials, whether they are non-aged or aged. The mechanical properties fluctuated for aged FDM specimens: 15% for ultimate tensile stress, 15% for elongation at yield, and 12% for elastic modulus. Regarding the DLP aged specimens, the UTS decreased by 61%, elongation at yield by around 61%, and elastic modulus by 62%. According to the FTIR spectral analysis, the PLA materials degraded, especially in the case of resin specimens. Aging also showed a significant influence on the elastic modulus, ultimate tensile stress, elongation at yield, elongation at break, and toughness of both materials, which was statistically shown by means of a two-way ANOVA test. The data collected in this research give a better understanding of the underlying aging mechanism of PLA materials
APPLICATION OF INDUSTRIAL AIR CLEANERS IN PRODUCTION HALLS
No full textAir cleaners are devices that an increasing number of people use to improve Indoor Air Quality (IAQ) in their homes and workplaces, especially in areas where the outdoor air is often excessively polluted. However, such devices, from small to large air flow capacities, are also widely used in the industries of developed countries to control and reduce the impact of the pollutants’ emission. This study deals with the analysis of experimental data regarding IAQ in a production hall with CNC machines in conditions of high concentration of emulsion vapors, focusing on the applica tion of mobile industrial air cleaners, as independent and supplementary components of HVAC systems. Experimental data is obtained through measurements, conducted by two different methods using two different measuring instruments.
Analyzed results show that the application of mobile industrial air cleaners can lead to significant increase of the num ber of production hall working hours, during which, according to U.S. Environmental Protection Agency (EPA), indoor air quality, according to PM2.5 (µg/m3), coincides with the “Moderate” air quality category, while it may also lead to occurrence of working hours during which indoor air quality, according to PM2.5 (µg/m3), coincides with the “Good”
indoor air quality category. Furthermore, the application of mobile industrial air cleaners leads to the decrease of the indoor air concentration of particulate matter of all sizes in the range of PM0.3 to PM10. However, analysis also shows that the devices’ effectiveness in the PM concentration reduction is heavily dependent on the mobile air cleaners mutual position, as well as the time that has passed since their actuation
Effect of Acetylsalicylic Acid on Biological Properties of Novel Cement Based on Calcium Phosphate Doped with Ions of Strontium, Copper, and Zinc
No full textThis study aimed to compare the biological properties of newly synthesized cements
based on calcium phosphate with a commercially used cement, mineral trioxide aggregate (MTA).
Strontium (Sr)-, Copper (Cu)-, and Zinc (Zn)-doped hydroxyapatite (miHAp) powder was obtained
through hydrothermal synthesis and characterized by scanning electron microscopy (SEM), X-ray
diffraction (XRD), and energy dispersive X-ray spectrometry (EDX). Calcium phosphate cement
(CPC) was produced by mixing miHAp powder with a 20 wt.% citric acid solution, followed by the
assessment of its compressive strength, setting time, and in vitro bioactivity. Acetylsalicylic acid
(ASA) was added to the CPC, resulting in CPCA. Biological tests were conducted on CPC, CPCA,
and MTA. The biocompatibility of the cement extracts was evaluated in vitro using human dental
pulp stem cells (hDPSCs) and in vivo using a zebrafish model. Antibiofilm and antimicrobial effect
(quantified by CFUs/mL) were assessed against Streptococcus mutans and Lactobacillus rhamnosus.
None of the tested materials showed toxicity, while CPCA even increased hDPSCs proliferation.
CPCA showed a better safety profile than MTA and CPC, and no toxic or immunomodulatory
effects on the zebrafish model. CPCA exhibited similar antibiofilm effects against S. mutans and L.
rhamnosus to MTA
DIVERSITY OF FOULING ORGANISMS ON A CARGO SHIP SAILING THE SAVA RIVER, SERBIA
No full textThe study investigates the constituents of biofilm formed on a hull of a cargo ship in the
Sava River, Serbia. Thickness of biofilm play main role in energy efficiency of ship during
exploitation. A biofilm sample was collected from the ship's surface on March 2023 from
the site of the “Vahali” Shipyard in Mačvanska Mitrovica using a brushing method. For
mycological analysis a 100 μL aliquot made from the biofilm sample was inoculated onto
a standard PDA medium under aseptic conditions. The average bacterial count in the
sample was 315.33 CFU per Petri dish/100 μL of the sample, while the average fungal
count was 11 CFU per Petri dish/100 μL of the sample. Based on colony characteristics
and microscopic features of reproductive structures, filamentous fungi Cladosporium
cladosporioides, Mucor megalocarpus and Penicillium spp. were identified. Additionally,
yeast Candida sp. was observed alongside filamentous fungi. Microscopic analysis of the
ship's biofilm revealed densely interwoven, branched hyphae predominantly enveloping
threads of green algae from the Cladophora genus. Besides mycelia, the presence of
micro- and macroconidia, as well as chlamydospores of Fusarium species, was noted in
the biofilm.
Light microscope observations of algal material was made using a Zeiss AxioImagerM.1
microscope. Seven algal species from four divisions were identified through microscopic
analysis: Aphanothece sp., Audouinella chalybea, Cladophora glomerata, Cocconeis
pediculus, Gomphonema parvulum, Navicula sp., and Rhoicosphaenia abbreviata.
Quantitatively, the most dominant biofilm species was Cladophora glometara, completely
covered with diatoms Cocconeis pediculus and Rhoicosphaenia abbreviata
Impact of Nanocellulose Loading on the Crystal Structure, Morphology and Properties of PVDF/Magnetite@NC/BaTiO3 Multi-component Hybrid Ceramic/Polymer Composite Material
No full textThe hybrid multifunctional magnetic organic/inorganic composite materials, with addition of optimal filler type and quantities are attractive due to wide range of potential application, from various pressure sensors, through smart packaging, to tissue engineering and medicine. The structural, morphological and magnetic properties of polyvinylidene fluoride/nanocellulose/magnetite@BaTiO3 hybrid films were investigated. The presented study revealed significant impact of nanocellulose (NC) content on formation of the polymorphs of PVDF, responsible for ferro-, piezo- and pyroelectric properties. The structural characterization, XRD and Raman measurements confirmed enhancement of the β and γ phases when the loading of NC higher then 4 wt% in multi-component hybrid films. The saturation magnetization value gradually raises with increasing amount of NC and reaches its maximum value of 41.2 emu/g at content of 4 wt% NC. Further, addition of NC decreases saturation magnetization value regardless of constant amount of magnetite, indicating optimal content of NC substrate for co-precipitation of Fe3O4 onto NC matrix.Peer-reviewed version of the article:
[https://machinery.mas.bg.ac.rs/handle/123456789/7919
APPLYING RODRIGUES' FORMULA FOR KINEMATIC MODELING OF VIBRATORY CONVEYORS
No full textVibratory technology is involved in various industries, such as food, metallurgical, arms
industry, etc. [1]. Major implementation can be found in material conveyance in the form of
vibratory conveyers. Depending on the continuity of the contact of the conveyed particles and
vibratory trough’s surface, these machines can set material into motion in two ways – hopping
and no-hopping motion. Conventional vibratory conveyers (Fig. 1) have an actuator positioned
at a certain angle to the ground. This angle defines the force vector that sets the material into
forward motion.Main goal of this research is to explore a potential design with two orthogonally positioned
actuators that can change the direction of the resultant vector. Variation of frequencies, phase
differences and amplitude ratios enable different vibratory regimes implemented not only in
conveyance, but at fine dosing, shaking and stirring, sifting etc.
Contrary to conventional technological processes that use hopping motion, where a new
machine is introduced for various operations, this way only the vibratory trough should be
replaced, and the control and actuating system generally remains the same. Additionally, with
dual excitation, the no-hopping motion can be achieved in simpler control algorithms. This form of motion is suitable for industries that demand sensitive handling with material (e.g.
chemical or military industry).
For this purpose, a mathematical model of a standard vibratory conveyer is defined with
segregated upward and sideways displacements of the base. This research explores the
kinematics of such vibratory machines from a robotics aspect (Fig. 1). More precisely, the
conveyer system (vibratory trough with stationary base) is being analyzed and remodeled as an
open serial kinematic chain. The interdependencies of the virtual segments are derived using
Rodrigues’ formula, that’s thoroughly defined in [3].
The analysis starts by defining the relative coordinates of the virtual robotic system. Since
the conveyor depicted in Fig 1. can be approximated to perform exclusively planar motion it
can be visualized as a robotic system with multiple segments interconnected by means of joints
– one for each degree of freedom (DOF). The proposed system will consist of a total of four
joints - two linear and one rotational joint to define the absolute movement of the stationary
base in the vertical plane and an additional linear joint to define displacement of the trough
relative to the base itself.
After the definition of the configuration of the robotic system, absolute radius vectors of
centers of inertia of corresponding segments are defined. Furthermore, expressions for angular
and translational velocities are obtained for each proposed segment which will be served for
setting in an automatic manner of the kinematic model of vibration conveyor
STABILITY OF FRACTIONAL-ORDER TIME-DELAY DYNAMICAL SYSTEMS: NEW RESULTS
No full textTime delay often appears in many engineering systems and it may lead to bifurcation,
chaos and even instability, [1]. Control design and stability issues of time-delay systems
(TDS) were widely studied due to the effect of delay phenomena on system dynamics, which
often leads to poor performance or even instability. In the biomechanics of humans, a well-known
fact is due to the human reaction time, which introduces the effect of a time delay
into a control problem. Particularly, it has been obtained that self-balancing models of
standing man can be presented in the form of neutral time-delay systems of the integer
order, where in solving the stabilization problem, feedback control is introduced that contains
a delay (TDTC) and include into account its position, speed, and acceleration [2]. Recently,
fractional calculus (FC) has attracted the increased attention of scientific society where
fractional operators are often used for complex dynamical systems,[3]. Also, fractional order
dynamical systems have drawn much attention from researchers and engineers over
the past few decades, [4,5], particularly for different kinds of stability. Here, we study the
problem of human postural balance by applying fractional-order TDFC where the
asymptotic stability closed loop of fractional-order neutral time delay systems is studied.
Also, some attention will be devoted to the finite-time stability (FTS)/stabilization problem
of nonlinear fractional-order (uncertain) time-delay systems,[6]. By use of the (generalized)
Gronwall inequality and its new extended form, new sufficient conditions for FTS of such
systems are obtained.
Finally, suitable numerical examples will be given to illustrate the effectiveness and
applicability of the proposed theoretical results
Continuous drive friction welded Al/Cu joints produced using short welding time, elevated rotational speed, and high welding pressures
No full textThe present study aimed to enhance the efficiency and efficacy of the Al/Cu joint production process implemented by the company VEMID Ltd., Jagodina, Serbia, by attaining sound joints within a very short welding time. For this purpose, the present study aimed at investigating the accuracy and the quality of the continuous drive friction welding (CDFW) process, as well as the optimum combination of CDFW parameters with highest joint efficiency in terms of investigated properties. The accuracy was estimated through an analysis of temperature–time curves recorded during CDFW using an infrared camera. The quality was evaluated through an investigation of the properties of Al/Cu joints produced using different friction (66.7, 88.9, and 133.3 MPa) and forging (88.9, 222.2, and 355.6 MPa) pressures and a constant total welding time (4 s) and rotational speed (2100 rpm). Thermal imaging with an infrared camera demonstrated that the actual total welding time was 15% longer compared to the nominal value. This was attributed to the slow pressure response of the pneumatic brake system. The relative changes in the maximum surface temperature (TMS) during the CDFW process corresponded to changes in welding pressures, indicating the potential of the thermal imaging method for monitoring and assessing this process. A preliminary investigation demonstrated that Al/Cu joints produced using welding pressures less than 88.9 MPa often displayed the presence of non-joined micro-regions at the Al/Cu interface and a significant thickness of interfacial Al2Cu (up to 1 µm). However, when friction pressure was set at 66.7 MPa, an increase in the forging pressure to 222.2 MPa eliminated the presence of non-joined micro-regions and reduced the thickness of Al2Cu to 0.5 µm on the average level. These Al/Cu joints achieved the highest joint efficiencies in terms of strength (100%) and ductility (61%). They exhibited an electrical conductivity higher than 92% of the theoretical value. A further increase in any welding pressure produced similar or deteriorated properties, accompanied by an increase in the consumption of raw materials and energy. Such turn of events was counterproductive to the original goal of increasing the efficiency and efficacy of the CDFW process