tailieunhanh - Bearing Design in Machinery Episode 2 Part 6

Tham khảo tài liệu 'bearing design in machinery episode 2 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | injection molding where large quantities are manufactured in mass production resulting in low unit manufacturing cost. Plastic bearings can be used with or without liquid lubrication. If possible liquid lubrication should be applied because it reduces friction and wear and plays an important role in cooling the bearing. Whenever liquid lubrication is not applied solid lubricants can be blended into the base plastics to reduce friction often referred to as improving the lubricity of plastics. Polymer is synthetically made of a monomer that is a basic unit of chemical composition such as ethylene or tetrafluorethlene. The monomer molecules always have atoms of carbon in combination with other atoms. For example ethylene is composed of carbon and hydrogen and in tetrafluorethlene the hydrogen is replaced by fluorine. The polymers are made by polymerization that is each monomer reacts with many other similar monomers to form a very long-chain molecule of repeating monomer units see Fig. 11-2 . The polymers become stronger as the molecular weight increases. For example low-molecu- Monomer Polymer Ethylene Long Chain Molecule Polyethylene H H H H H H c c c c c c H H H H H H Monomer Polymer Tetrafluoroethylene Long Chain Molecule Polytetrafluoroethylene F F F F F F c c c c c c F F F F F F Fig. 11-2 Examples of monomers and their multiunit polymers. Copyright 2003 by Marcel Dekker Inc. All Rights Reserved. lar-weight polyethylene which has at least 100 units of CH2 is a relatively soft material. Increasing the number of units makes the material stronger and tougher. The longest chain is ultrahigh-molecular-weight polyethylene UHMWPE . It has up to half a million units of CH2 and it is the toughest polyethylene. This material has an important application as a bearing material in artificial replacement joints such as hip joints. Over the last few decades there has been an increasing requirement for low-cost bearings for various mass-produced machinery and appliances. This .