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2024-11-12 01:21 本頁(yè)面
【正文】 are to work determine the type of lubricant to be used and the system selected for distributing the lubricant On 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 mon 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 lawnmower engines to lubricate the crankshaft connecting rod and parts of the piston A lubrication system monly 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 quantity There 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 devices Although one of the main purposes of lubrication is reduce friction any substanceliquid solid or gaseouscapable of controlling friction and wear between sliding surfaces can be classed as a lubricant Varieties of lubrication Unlubricated sliding Metals that have been carefully treated to remove all foreign materials seize and weld to one another when slid together In the absence of such a high degree of cleanliness adsorbed gases water vapor oxides and contaminants reduce frictio9n and the tendency to seize but usually result in severe wear this is called unlubricated or dry sliding Fluidfilm lubrication Interposing a fluid film that pletely separates the sliding surfaces results in fluidfilm lubrication The fluid may be introduced intentionally as the oil in the main bearing of an automobile or unintentionally as in the case of water between a smooth tuber tire and a wet pavement Although the fluid is usually a liquid such as oil water and a wide range of other materials it may also be a gas The gas most monly employed is air Boundary lubrication A condition that lies between unlubricated sliding and fluidfilm lubrication is referred to as boundary lubrication also defined as that condition of lubrication in which the friction between surfaces is determined by the properties of the surfaces and properties of the lubricant other than viscosity Boundary lubrication enpasses a significant portion of lubrication phenomena and monly occurs during the starting and stopping off machines Solid lubrication Solid such as graphite and molybdenum disulfide are widely used when normal lubricants do not possess sufficient resistance to load or temperature extremes But lubricants need not take only such familiar forms as fats powders and gases even some metals monly serve as sliding surfaces in some sophisticated machines Function of lubricants Although a lubricant primarily controls friction and ordinarily does perform numerous other functions which vary with the application and usually are interrelated Friction control The amount and character of the lubricant made available to sliding surfaces have a profound effect upon the friction that is encountered For example disregarding such related factors as heat and wear but considering friction alone between the same surfaces with on lubricant Under fluidfilm conditions friction is encountered In a great range of viscosities and thus can satisfy a broad spectrum of functional requirements Under boundary lubrication conditions the effect of viscosity on friction bees less significant than the chemical nature of the lubricant Wear control wear occu
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