轴承的摩擦与润滑 -机械类毕业设计外文翻译 毕业设计论文外文翻译毕业设计论文题目 工具盒盖注塑模具设计 外文题目 Friction Lubrication of Bearing 译文题目 轴承的摩擦与润滑 系 部 机 械 系 专业班级机械设计制造及其自动化学生姓名 指导教师 外文文献原文 Friction Lubrication of BearingIn many of the problem thus far the student has been asked to disregard or neglect friction Actually friction is present to some degree whenever two parts are in contact and move on each other The term friction refers to the resistance of two or more parts to movementFriction is harmful or valuable depending upon where it occurs friction is necessary for fastening devices such as screws and rivets which depend upon friction to hold the fastener and the parts together Belt drivers brakes and tires are additional applications where friction is necessaryThe friction of moving parts in a machine is harmful because it reduces the mechanical advantage of the device The heat produced by friction is lost energy because no work takes place Also greater power is required to overcome the increased friction Heat is destructive in that it causes expansion Expansion may cause a bearing or sliding surface to fit tighter If a great enough pressure builds up because made from low temperature materials may meltThere are three types of friction which must be overcome in moving parts 1 starting 2 sliding and 3 rolling Starting friction is the friction between two solids that tend to resist movement When two parts are at a state of rest the surface irregularities of both parts tend to interlock and form a wedging action To produce motion in these parts the wedge-shaped peaks and valleys of the stationary surfaces must be made to slide out and over each other The rougher the two surfaces the greater is starting friction resulting from their movement Since there is usually no fixed pattern between the peaks and valleys of two mating parts the irregularities do not interlock once the parts are in motion but slide over each other The friction of the two surfaces is known as sliding friction As shown in figure starting friction is always greater than sliding friction Rolling friction occurs when roller devces are subjected to tremendous stress which cause the parts to change shape or deform Under these conditions the material in front of a roller tends to pile up and forces the object to roll slightly uphill This changing of shape known as deformation causes a movement of molecules As a result heat is produced from the added energy required to keep the parts turning and overcome frictionThe friction caused by the wedging action of surface irregularities can be overcome partly by the precision machining of the surfaces However even these smooth surfaces may require the use of a substance between them to reduce the friction still more This substance is usually a lubricant which provides a fine thin oil film The film keeps the surfaces apart and prevents the cohesive forces of the surfaces from coming in close contact and producing heat Another way to reduce friction is to use different materials for the bearing surfaces and rotating parts This explains why bronze bearings soft alloys and copper and tin iolite bearings are used with both soft and hardened steel shaft The iolite bearing is porous Thus when the bearing is dipped in oil capillary action carries the oil through the spaces of the bearing This type of bearing carries its own lubricant to the points where the pressures are the greatestMoving parts are lubricated to reduce friction wear and heat The most commonly used lubricants are oils greases and graphite compounds Each lubricant serves a different purpose The conditions under which two moving surfaces are to work determine the type of lubricant to be used and the system selected for distributing the lubricantOn slow moving parts with a minimum of pressure an oil groove is usually sufficient to distribute the required quantity of lubricant to the surfaces moving on each other A second common method of lubrication is the splash system in which parts moving in a reservoir of lubricant pick up sufficient oil which is then distributed to all moving parts during each cycle This system is used in the crankcase of lawn-mower engines to lubricate the crankshaft connecting rod and parts of the pistonA lubrication system commonly used in industrial plants is the pressure system In this system a pump on a machine carries the lubricant to all of the bearing surfaces at a constant rate and quantityThere are numerous other systems of lubrication and a considerable number of lubricants available for any given set of operating conditions Modern industry pays greater attention to the use of the proper lubricants than at previous time because of the increased speeds pressures and operating demands placed on equipment and devicesAlthough one of the main purposes of lubrica