Çeşitli katkılı ve katkısız polimer yatakların sürtünme ve aşınma karakteristiklerinin deneysel tayini
Başlık çevirisi mevcut değil.
- Tez No: 75017
- Danışmanlar: PROF. DR. MUSTAFA GEDİKTAŞ
- Tez Türü: Doktora
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1998
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Makine Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 157
Özet
Yapılan çalışmada çeşitli polimer malzemelerden imal edilmiş kuru sürtünmeli radyal kaymalı yatakların çelik mil ile eş çalışmasında ve kuru sürtünme halinde sürtünme ve aşınma özellikleri deneysel olarak incelenmiştir. Bu amaçla bir deney tesisatı geliştirilmiştir. Tezde öncelikle kullanılan malzemeler hakkında kısaca bilgi verilmiş, daha sonra viskoelastisite, sürtünme ve aşınma teorileri ve kuru sürtünmeli kaymalı yatakların dizayn parametreleri ve bunların hesabından bahsedilmiştir. Deney tesisatında değiştirilebilen parametreler olarak kayma hızı, yüzey basıncı öngörülmüştür. Sistemde ölçülebilen fiziksel büyüklükler ise polimer-çelik temasındaki sürtünme katsayısı, polimer yatak burcunda meydana gelen aşınma miktarı (radyal olarak) ve sıcaklık artışıdır. Ayrıca sisteme monte edilen mesafe ölçerler ile sürekli olarak mil merkezinin yatak burcuna göre konumunu da izlemek mümkündür. Sistemde normal kuvvet ağırlık asma sureti ile mekanik olarak sağlanmaktadır. Sürtünme kuvveti ise motor ile mil arasındaki moment transdüserinden elde edilen sinyallerin yardımı ile sürtünme momentinden tayin edilmektedir. Çalışmada yatak malzemesi olarak polioksimetilen (POM), döküm poliamid (PA 6), çok yüksek molekül ağırlıklı polietilen, politetrafloretilen (PTFE) + %15 cam elyafı, PTFE + %25 cam elyafı, PTFE + %25 karbon, PTFE + %35 karbon, PTFE + %40 bronz ve PTFE + %60 bronz kullanılmıştır. Polimer yatak burçları 50 mm iç çapında, 60 mm dış çapında ve 50 mm genişliğindedir. Yatak burçları literatürde öngörülen toleranslarda işlenmiştir. Deneyler sonucunda çeşitli polimer yatakların kayma hızı yüzey basıncı, polimer malzemenin cinsi, katkı malzemesinin cinsi, katkı malzemesinin oranına bağlı olarak sürtünme katsayısı ve radyal aşınma miktarları ndaki değişimler elde edilmiştir.
Özet (Çeviri)
In this study, the friction and wear characteristics of polymer journal bearings have been studied. At first, the polymer or plastic materials used in the study were examined briefly. Plastics are a large group of materials consisting of carbon and oxygen, hydrogen, nitrogen and other organic and inorganic elements. A plastic is a solid in its finished state, but at some stage in its manufacture, it is liquid and capable of being formed into various shapes. Forming is most usually done thorough the application, either singly or together, of heat and pressure. Many different families of plastics are in commercial use today, and each may have dozens of subtypes and variations. Since plastics are not found in nature, they must be made synthetically from basic chemical raw materials called monomers. A monomer (one chemical unit) such as ethylene is reacted with other monomer molecules into a very long chain of repeating ethylene units, becoming the polymer polyethylene. In a similar manner, polystyrene is formed from styrene monomer; polypropylene from propylene monomer; and other thermoplastic polymers, from their respective monomers. These polymers are always composed of atoms of carbon in combination with other elements. Polymer chemists utilize only eight of the more than 100 known elements to create thousands of different plastics. A successful design in plastics is almost always a compromise among highest performance, attractive appearance, efficient production, and lowest cost. Achieving the best compromise requires satisfying the mechanical requirements of the part, utilizing the most economical resin or compound that will perform satisfactorily, and choosing a manufacturing process compatible with the part design and material choice. Setting realistic requirements for each of these areas is of most utmost importance. Choosing the most economical plastic for a part by comparing the cost per pound of various plastics is a mistake. Some plastics weigh twice as much per cubic centimeter as others and so would require twice as much to fill a given cavity and cost twice as much to ship. A more meaningful comparison is cost per cubic centimeter. But since most expensive plastics are far stronger than the cheaper ones, cost/strength values should be analyzed as well. Paying more per pound orper cubic centimeter is often more economical if less material can be used to achieve the same performance. Probably no plastic will provide 100% of requirements for the desired performance, appearance, processability, and price. Selecting the best qualified material is not a simple task of comparing numbers on published data sheets. Polymers differ to a great extent from metals from the viewpoint of tribology and elasticity. Viscoelastic properties of a polymer determines its tribologic behavior under load. The term viscoelasticity is commonly applied to materials which are neither ideal solids nor liquids, but in fact possess characteristics which are typical of both. The classical theory of elasticity deals with the mechanical properties of perfectly elastic solids, where in accordance with Hooke's Law stress is proportional to strain- but independent of the rate of strain. On the other hand, the theory of hydrodynamics deals with the properties of perfectly viscous fluids, for which in accordance with Newton's Law, stress is directly proportional to rate of strain- but independent on the strain itself. These categories are, of course, idealizations. Any real solid shows deviations from Hooke's Law and any real liquid does not obey Newton's Law directly. There are two important types of deviations. First, the strain (for a solid) or rate of strain (for a liquid) may not be directly proportional to stress, but may depend on stress in a more complicated manner. Such effects are familiar when the elastic limit exceeded for a solid. Secondly, the stress may depend on both strain and rate of strain together, as well as higher derivatives of strain. Both of these deviations are classified as viscoelastic. They are distinct in that in the first case the ratio of stress to strain depends on the magnitude of the stress itself, whereas in the second case this ratio depends only on time (linear viscoelastic behavior). The mechanical properties of a linear viscoelastic material can be duplicated by models consisting of some suitable combinations of springs which obey Hooke's Law and viscous dashpots which follow Newton's Law. Viscoelastic behavior is therefore conveniently represented by mechanical models of the spring- and- dashpot type. In the study, some of these models such as Maxwell, Voigt, Modified Maxwell, Modified Voigt, Maxwell-Voigt are presented. After that the creep and relaxation behavior of polymers are introduced briefly. In the fourth chapter, a brief introduction on the friction and wear for both metals and elastomers was given. Two main factors contribute to the friction generated between unlubricated surfaces in relative motion. The first, and usually more important, factor is the adhesion which occurs at the regions of real contact, and the second may be described as a deformation term. Thus we may write for the friction coefficient (j.=|j.a+hd where; ^i total friction coefficient and suffixes A and D denotes adhesion and deformation respectively. By a careful selection of experimental conditions, it is possible to separate the adhesion and deformation terms. Thus, by choosing optically smooth surfaces the roughness features are virtually eliminated so that the contribution of thedeformation coefficient is negligible. The total measured friction force is then due to the adhesion alone. Alternatively, the use of a lubricant between rough surfaces in relative motion virtually eliminates the adhesion term and the measured friction force can be attributed solely to the deformation component. In the normal case of dry sliding between rough surfaces, the coefficient of adhesional friction is generally at least twice as large as the deformation contribution. The contact area between an elastomer and a rough support surface is characterized by draping of the elastomer about individual asperities in the rigid base. The adhesion component of the friction force may be attributed to a molecular bonding of exposed surface atoms in both members, according to a stretch, break and a relaxation cycle of events. The deformation term is due to a delayed recovery of the elastomer after indentation by a particular asperity, and gives rise to what is generally called the hysteresis component of friction. Adhesion is distinctly a surface effect, whereas hysteresis is a bulk phenomenon dependent on the viscoelastic properties of elastomer. Furthermore a brief explanation on the wear mechanisms of polymeric materials is presented. Three distinct mechanisms of wear can be identified when an elastomer slides on a rigid surface, and they depend on the surface roughness. 1. Abrasive wear: A sharp texture in the base surface causes abrasion and tearing of sliding elastomer. Micro-cutting and longitudinal scratches are observed on the abraded elastomer surface. In the study the shaft was machined to 0,2 urn for average surface roughness to avoid any abrasion. 2. Fatigue wear. If the base has blunt rather than sharp projections, the surface of elastomer undergoes cyclic deformation and failure eventually occurs as a result of fatigue. 3. Roll formation: On smooth surfaces, a new mechanism of wear specific to highly elastic materials causes roll formation at the sliding interface and eventual tearing of the rolled fragment. At the end of this chapter the most common wear mechanism, transfer film, for polymeric materials is introduced. Sliding bearings are one of the most important machine elements. Over decades this application has been dominated by metallic materials. It was closely connected with lubrication. Very often, sliding bearings are used under working conditions where no hydrodynamic lubrication takes place. Therefore, on the one hand, good sliding properties are demanded. To improve these emergency sliding properties, certain types of metallic compound have been developed. However, to fulfill the demand of maintenance-free operation these materials are no longer suitable. In special cases, metals have been replaced by syntethic carbon or sintered materials with incorporated solid lubricants. In recent years, however, dry sliding bearings made of polymeric materials have been successfully used. Polymers offer intrinsic properties which make them very interesting for sliding application, e.g. low coefficient of friction, chemical resistance, ability for dry sliding, maintenance-free operation and high damping properties for low noise emission. Today, in particular, in reinforced and internally lubricated modifications, polymers are therefore used in those bearing applications where the above-mentioned properties are especially required. The problem of these materials is the restriction to comparatively low operation temperatures owing to the loss of strength properties because creep occurs under a load. The low thermal conductivity of polymer-based materials causes heat accumulation in the sliding region of such materials. In such cases a metallic support supplies a high load capacity and thermal conductivity to the component. To avoid thermal softening, frictional heating has to be limited by applying only low sliding velocities. Therefore, in the case of a composite bearing without metallic support, the restriction of operating conditions is most important. Today, thermoplastic composites are used up to operating temperatures of 150°C although the lifetime are reduced owing to creep and increased wear. In recent years, the problems of polymer-containing materials, i.e. their limited applicability at higher sliding speeds or elevated ambient temperatures, and the lack of reliable scientific data for design, hindered development in this area. On the contrary, there is an increasing demand for use in application fields that have previously been restricted to metallic or ceramic materials. This is because the latter materials exhibit disadvantages such as higher weight and noise emission or the need for compulsory lubrication. The recently developed high temperature thermoplastics promise great advances in this field by extending the application range of polymeric sliding bearings. Related to contact parameters of dry sliding bearings are; the arc of contact between the shaft and the bush, which is called angle of contact, the distribution of contact pressures and the maximum contact pressure. In the study two different method for the calculation of contact parameters are presented. First one is the classic Hertz formula for internal contact of two cylinders under load. The following assumptions were made for calculating contact parameters in the case of polymeric bush and metallic shaft: 1. The bearing is deformed elastically, and its material exhibits the isotropy of the elastic properties, 2. The shaft and pedestal body are undeformable components, 3. Tangential stresses are absent along the contact arc because the coefficient of friction is low, 4. The load is distributed uniformly through the journal length. The second method is the Korovchinsky's elastoreologic equation. This equation consider the presence of shearing stresses in the contact region and gives pressure distribution. After that, a calculation for the pressure distribution and maximum pressure is presented by using Hertz formula for the bearings used in the study. In the fifth chapter, the test apparatus established for the investigation of tribological properties of dry rubbing sliding bearings under operating conditions is introduced. The system is driven by a D.C. Motor having adjustable speed range of 0...3000 rpm. A torque transducer is placed between D.C. Motor and bearing race in order to measure the friction torque occurring at contact region. The hardened steel shaft is mounted to the pedestal body by means of two roller bearing which have practically no radial clearance. Thus, during experiments, the axis of steel shaft will stay horizontal.Bearing race which the polymer bearing bush is mounted is attached to lever arm holder by means of a set of springs. Normal load is applied to bearing system by means of a lever mechanism having dead weights. Furthermore, there are three non-contact displacement transducer fixed to the bearing race in order to obtain rate of radial wear during experiments. The torque transducer and inductive displacement transducer connected to a 5 kHz carrier frequency multichannel amplifier to observe the results. Two capacitive displacement transducer are connected to two indicator developed for these sensors. The polymer materials used for bearing bush have been tested in aqueous environment in a commonly known pad on disc apparatus. Polymer materials used in experiments are ultrahigh molecular weight polyethylene, polyoxymethylene (commercial name delrin), cast polyamid 6 (commercial name nylon 6), polytetrafluoroethylene + %15 glass fiber, polytetrafluoroethylene + %25 glass fiber, polytetrafluoroethylene + %25 carbon, polytetrafluoroethylene + %35 carbon, polytetrafluoroethylene + % 40 bronze and polytetrafluoroethylene + 60 bronze by weight. These polymers were machined as a bearing bush 50 mm in inner diameter, 60 mm in outer diameter and 50 mm in length. The steel shaft also was machined to 0,2 |im average surface roughness to avoid any abrasive wear during experiments. In the last section the diagrams obtained as a result of experiments are presented. First, the variation of friction coefficient with respect to sliding speed and normal load are given. After that, the wear rate of the same polymer bearing bushes are introduced. At the end of this section comments about the results are made.
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