Fractal model of thermal contact conductance of two spherical joint surfaces considering friction coefficient

Purpose The purpose of this study is to propose a fractal model of thermal contact conductance (TCC) of two spherical joint surfaces, considering friction coefficient based on the three-dimensional fractal theory. Design/methodology/approach The effects of friction coefficient, fractal parameters, radius of curvature and contact type on TCC were analyzed using numerical simulation. Findings The results indicate that the TCC decreases with the increase of friction coefficient and fractal roughness and increases with the increase of fractal dimension and radius of curvature; the contact type of two spherical joint surfaces has an important influence on the TCC, and the TCC of external contact is smaller than that of internal contact under the same contact load. Originality/value A fractal model of TCC of two spherical joint surfaces considering friction coefficient is proposed in this paper. Achievements of this work provide some theoretical basis for the research of TCC of bearings and other curved surfaces.

Technical solution for evaluation of contact stiffness and strength of the joint surfaces of injector

От работоспособности форсунки зависит качество работы двигателя, его долговечность, соответствие экологическим нормам и требованиям, а от контактной прочности, жесткости и работоспособности ее сопрягаемых поверхностей (различных соединений, которые можно отнести к условно-неподвижным) – нормальная эксплуатация рассматриваемой конструкции в целом. На базе существующей технологии производства форсунок и с использованием физико-математической модели упругопластического контактирования, созданной авторами, предложено техническое решение для оценки контактной жесткости и прочности стыковых поверхностей форсунки для распыления топлива. Авторами установлено, что работа двигателя в целом зависит от характеристик контактирующих поверхностей форсунки, а именно параметров микрогеометрии шероховатого слоя, фактической площади соприкосновения, податливости, физико-механических свойств материалов контактной пары, условий нагружения. На основе проведенных авторами исследований и комплексного анализа при заданных геометрических размерах конструкции форсунки двигателя внутреннего сгорания и условий работы ДВС разработано техническое решение, позволяющее повысить контактную жесткость стыковых сопряжений форсунки для распыления топлива и равномерного его распределения в камере сгорания дизеля в 1,2–1,5 раза. Таким образом, предлагаемое авторами решение показало свою эффективность, и его можно рекомендовать для применения в ДВС, в том числе в судовых двигателях.

Quantification of Fracture Roughness by Change Probabilities and Hurst Exponents

The objective of the current study is to utilize an innovative method called “change probabilities” for describing fracture roughness. In order to detect and visualize anisotropy of rock joint surfaces, the roughness of one-dimensional profiles taken in different directions is quantified. The central quantifiers, change probabilities, are based on counting monotonic changes in discretizations of a profile. These probabilities, which usually vary with the scale, can be reinterpreted as scale-dependent Hurst exponents. For a large class of Gaussian stochastic processes, change probabilities are shown to be directly related to the classical Hurst exponent, which generalizes a relationship known for fractional Brownian motion. While related to this classical roughness measure, the proposed method is more generally applicable, therefore increasing the flexibility of modeling and investigating surface profiles. In particular, it allows a quick and efficient visualization and detection of roughness anisotropy and scale dependence of roughness.

Facetting value of helical thread surface received by enveloping

В морской технике применяются различные резьбовые соединения. Детали судовых механизмов подвергаются в процессе работы вибрациям и переменным нагрузкам, которые приводят их к разрушениям, потере герметичности и к внеплановым остановкам судовых машин.. Для повышения эксплуатационных свойств и увеличения срока службы резьбовых соединений проводят анализ качества сопрягаемых поверхностей, в частности, параметров шероховатости. В предлагаемой статье исследуется величина огранки винтовой поверхности резьбы образующаяся при осуществлении нового метода изготовления резьбы огибанием. Процесс обработки металла, к которому относится новый метод нарезания, заключается в касании лезвия инструмента и обрабатываемого материала прерывистым касанием. Нарезание резьбы осуществляется, специальной червячной фрезой. При прерывистом резании обрабатываемая поверхность оказывается, как бы составленной из отдельных криволинейных площадок (граней), Линия стыка граней имеет определенную высоту по нормали к поверхности, при нарезании резьбы - к винтовой поверхности. Высота стыка граней определяет качественный показатель шероховатости поверхности, называемый волнистостью. При нарезании резьбы червячной фрезой высота стыка граней (волнистость винтовой поверхности) зависит от числа зубьев фрезы. Задача исследования состоит в том, чтобы выяснить, каким должно быть число зубьев фрезы для получение заданной шероховатости винтовой поверхности резьбы и как это влияет на качество поверхности резьбы. Так же в статье рассмотрено графическое описание процесса получения огранки резьбы и пример расчета числа гребенок фрезы. There are many different thread joints used on marine facilities. While in operation, components of the mechanisms undergo vibrations with repeated stresses that leads to their failures, loss of tightness, and unscheduled shutdowns of the equipment. The analyses of joint surfaces quality, e.g. roughness parameters, are conducted to enhance serviceability of the thread joints and to prolong their lifespan. The article studies facetting value of a helical thread surface while applying a new method of thread production – enveloping. The new method of cutting that is a process of metal treatment involves non-continuous contacting an instrument blade and the material being cut. The cutting of the thread is done with a special hob cutter. Due to the non-continuous cutting, a treated surface becomes as if consisting of curved pads or faces. The faces junction line has a specific normal line level towards the surface and towards the helical surface during the cutting. The height of the junction defines qualitative indicator of the surface roughness known as undulation. When cutting the thread with a hob cutter the height of the faces junction - undulation of a helical surface – depends on a cutter teeth number. The research objective is to specify the cutter teeth number needed to achieve the required roughness of a thread helical surface as well as to determine how it influences on the quality of the thread surface. The paper also provides several drawings illustrating the process of obtaining the thread facetting and the example of calculation of a cutter chaser number.

Estimation of differences in selected indices of vibroacoustic signals between healthy and osteoarthritic patellofemoral joints as a potential non-invasive diagnostic tool

Osteoarthritis (OA) is currently the most generic form of joint disease. It is a complex process in which degenerative changes occur in the articular cartilage [AC], subchondral bone, and synovial membrane and can lead to permanent joint failure. The primary and most commonly used method of diagnosing degenerative changes is classic radiography. Magnetic resonance imaging (MRI) may be used to assess the extent of damage to joint surfaces, but this method is limited by the availability of specialised equipment and the excessive cost of the examination. Arthroscopy, an invasive procedure, is considered the “gold standard” in joint diagnosis. The occurrence of degenerative changes is closely related to the friction and lubrication processes within the joint. The main causes of osteoarthritis are a change or lack of synovial fluid, deformation of the joint bones, local damage to the articular cartilage, and a change in the mechanical properties of the articular cartilage due to water loss from the damaged superficial layer. An alternative, non-invasive method that allows for a delicate assessment of the condition of moving joints is vibroarthrography (VAG). The analysis of vibroacoustic signals generated by moving joint surfaces has an immense potential in the non-invasive assessment of the degree of damage to articular cartilage, meniscus and ligaments and the general diagnosis of degenerative diseases. The purpose of this study is to analyse and statistically compare the basic characteristics of vibroacoustic signals recorded with a CM-01B contact microphone placed on the patella for motion in the 90°–0°–90° range in a closed kinetic chain (CKC) in a control group (HC) and a group of patients diagnosed with osteoarthritis (OA), qualified for the knee alloplasty.

The Assessment of the Condition of Knee Joint Surfaces with Acoustic Emission Analysis

The knee joint, being the largest joint in the human body, is responsible for a great percentage of leg movements. The diagnosis of the state of knee joints is usually based on X-ray scan, ultrasound imaging, computerized tomography (CT), magnetic resonance imaging (MRI), or arthroscopy. In this study, we aimed to create an inexpensive, portable device for recording the sound produced by the knee joint, and a dedicated application for its analysis. During the study, we examined fourteen volunteers of different ages, including those who had a knee injury. The device effectively enables the recording of the sounds produced by the knee joint, and the spectral analysis used in the application proved its reliability in evaluating the knee joint condition.

Theoretical and Experimental Study on Normal Contact Stiffness of Plane Joint Surfaces with Surface Texturing

Effects of surface texturing on the normal contact stiffness of joint surfaces had been investigated by experiments in many previous researches; however, there are relatively few theoretical models in this regard. The rough surface with surface texturing can be divided into two parts: the textured zone and the remaining zone, and their theoretical models are established respectively in this research considering surface morphology and material properties. For the textured zone, micro textures are modeled theoretically based on the three-dimensional topographic data obtained via a VK-X250 type laser profilometer from KEYENCE. For the remaining zone, the model of normal contact stiffness is established based on the fractal theory for the surface topography description and elastoplastic deformation of surface asperities, and the structure function method is used to calculate the fractal dimension of rough surface profiles. In the experiment, the normal contact stiffness of specimens is obtained under different normal loads, and the test results are compared with the theoretical predictions. The result shows that the predictions of proposed theoretical model are in good agreement with the experimental data. For the joint surfaces with Sa > 2.69 μm, the normal contact stiffness can be effectively increased through proper surface texturing.

Correlation of knee society score and oxford knee score in patients with moderate to severe osteoarthritis of the knee

Osteoarthritis (OA) of the knee is a degenerative, non-inflammatory joint condition marked by articular cartilage disintegration and the growth of new bone (osteophytes) at the joint surfaces and borders. It impairs one's ability to function and makes one disabled. It is the most common rheumatic disease. Because the Knee is a weight-bearing joint and a crutch joint, it is the most usually afflicted joint by Osteoarthritis.A Correlational study was conducted to find out the correlation between Knee Society Score (KSS) and Oxford Knee Score (OKS) on 142 Osteoarthritis Knees. Purposive sampling was used to collect data from the Orthopaedics OPD at AIIMS Jodhpur from October to December 2018.On evaluation, the mean age of the patients was 60.19±1.01. Bilateral Knee Osteoarthritis affected about 42% of the patients. Approximately half of the patients were obese. Only 34% of patients had compliance in physiotherapy. Analgesics and massage treatment are used by about 76% of the patients to relieve knee pain. In the Knee Society Score, the majority of the patients (82.4%) had a bad knee condition, with a mean score of 49.07±1.06. In OKS, about half of the patients (46.5%) had Moderate to Severe Knee Osteoarthritis, with a mean score of 22.69±7.09. The correlation coefficient between the Knee Society Score and the Oxford Knee Score is 0.660. As a consequence of the analysis, both scales have almost the same outcome, indicating that they are moderately associated. The Knee Society Score is highly associated with occupation and physiotherapy, whereas the Oxford Knee Score is significantly related to the patients' age alone. According to the study, there is a moderate correlation between the Knee Society Score (KSS) and the Oxford Knee Score (OKS). These scores should be included during the diagnosis of Knee Osteoarthritis for improved patient care.