【DO!BOOK・ページリンク】
bearing   334 / 392

BOOKをみる

10秒後にBOOKのページに移動します


On Design There is no specific method for sellection and designing of self-lubricating bearings which is a very important element in any machinary. However, selection of highly credible bearing is possible if fundamental requirements for bearing design are studied based on design process. Here, fundamentals of tribology and fundamentals of design items will be discussed in order to improve bearing design. ■Contact of two solid surfaces Foundation of Tribology ■Lubrication Mechanism When lubrication film exists between two relatively moving surfaces, friction conditions of two contact surfaces can be classified in three steps as below: 1. Hydrodynamic lubrication 2. Boundary lubrication 3. Solid lubrication Hydrodynamic Lubrication Lubrication film between the two contact surfaces is thick enough and two contact surfaces are separated completely by viscous oil film. At this moment, frictional force of two contact surfaces is determined by viscous resistance of lubricant and it can take a very small value (coefficient of friction ; 1 × 10−3 to 5 × 10−4). In hydrodyanmic lubrication, when shaft is rotated, the oil around the shaft also rotates due to the viscoity of the lubricant oil and generates oil pressure at the loaded area. This phenomenon is called wedge effect. Oil pressure P generated within lubricating oil film is affected by change of temperature and viscosity of lubricating oil, surface roughness and clearance "h" and the rotational speed of the shaft "v." Fig.2 shows the model of hydrodynamic lubrication. Plastic Load W deformation Elastic deformation Fig.1 Real contacts of two solid surfaces Fig.2 Distribution of oil film pressure under hydrodynamic lubrication Oil film pressure P Wedge effect V h Load W Lubricant oil Contact of two solid surfaces No matter how smooth the surface may appear, there are roughness and winding on the surface of the solid body. When two surfaces come into contact, they are actually the contacts between the rough projections of the two surfaces. Therefore, of the total area of the two surfaces the area that is actually in contact with each other is limited. As such even if the load is very small, there is strong pressure on the actual points of the contacts. These very limited minute areas of the contacts take charge of friction between the two solid bodies, surface damage and interaction of two solid surfaces. As the load increases, projected areas of the contact surfaces go through plastic deformation and consequently the area that bears the load is expanded. As the hardness increases, the area is decreased. These points that actually contact and support the load are called "Real Contact Points." The total sum of the contact points is called "Real Contact Area." The contact supporting area that is determined by geometric appearance is called "Apparent Contact Area." Fig.1 shows a model of the contact. 333Selection Guide Product Air Bearings Metallic Bearing Multi-layer Bearing Plastic Bearing Information Technical Corporate Profi le Information Slide Shifter