16 Wear and Lubrication

16ห้องสมุดไป่ตู้Wear and Lubrication
• Lubrication is a process by which the friction and wear between two solid surfaces in relative motion is reduced significantly by the interposing a lubricant between them. These are four types of fluid lubrication that are generally of interest in engineering practice (Fig.16.4): 1. In thick-film lubrication, the two surfaces are completely separated from each other by a continuous fluid film. The fluid film can be developed either hydrostatically or, more often by the wedge effect of the sliding surfaces in the presence of a viscous fluid at the interface. 2. As the load between the two surfaces increases or as the sliding speed and viscosity of the fluid decrease, the lubricant film becomes thinner (thin-film lubrication). This condition raises the friction at the sliding interfaces and results in slight wear.
16 Wear and Lubrication
• Adhesive Wear. When two surfaces are loaded against each other, the whole of the contact load is carried on very small area of the asperity contacts. The real contact pressure at these asperities is very high, and adhesion takes place between them. If a tangential force is applied to the model shown in Fig.16.1, shearing can take place either (a) at the interface or (b) below or above the interface, causing adhesive wear. Because of factors such as strain hardening at the asperity contact, the adhesive bonds often are stronger than the base metals. Thus, during sliding, fracture usually occurs in the weaker or softer component, and a wear fragment is generated. Although this fragment is attached to the harder component (upper surface in Fig. 16. 1c), it eventually becomes detached during further rubbing at the interface and develops into loose wear particle.
• Wear can be defined as the progressive loss of material from the operating surface of a body occurring as a result of relative motion at the surface. The problem of wear arises wherever there are load and motion between surface, and is therefore important in engineering practice, often being the major types of wear are describe next:
16 Wear and Lubrication
• Abrasive Wear. This type of wear is caused by a hard, rough surface (or a surface containing hard, protruding particles) sliding across another surface. As a result, microchips are produced, thereby leaving grooves or scratched on the softer surface (Fig.16.2). In fact, processes such as filing and grinding act in this manner. The difference is that, in these operations, the process parameters are controlled to produce the desired shapes and surfaces through wear; whereas, abrasive wear generally is unintended and unwanted. • Corrosive Wear. Also known as oxidation or chemical wear, this type of wear is caused by chemical or electrochemical reactions between the surfaces and the environment. The fine corrosive products on the surface constitute the wear particles in this type of wear. When the corrosive layer is destroyed or removed through sliding, another layer begins to form, and the process of removal and corrosive layer formation is repeated.
机械工程专业英语 English In Mechanical Engineering
Prepared by Xiaoping Pang From College of Mechanical Engineering Chongqing University
16 Wear and Lubrication
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16 Wear and Lubrication
• 3. In mixed lubrication, although the contacting surfaces are separated by a thin lubricant film, asperity contact also may take place. The total applied load is thought to be carried by asperity contacts and partly by hydrodynamic action. 4. In boundary lubrication, the load is supported by contacting surfaces covered with a boundary film of lubricant (Fig.16.4d). This is a thin (molecular) lubricant layer that is attracted physically to the metal surfaces, thus preventing direct metal-to-metal contact of the two bodies and reducing wear.
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16 Wear and Lubrication
• Fatigue Wear. Fatigue wear is caused when the surface of a material is subjected to cyclic loading; one example of this is the rolling contact in bearings. The wear particles usually are formed through spalling or pitting. Another type of fatigue wear is by thermal fatigue. Cracks on the surface are generated by thermal stresses from thermal cycling, such as when a cool forging die repeatedly contacts hot workpieces. These cracks then join, and the surface begins to spall, producing fatigue wear. • Although wear general alters a part’s surface topography and may result in severe surface damage, it also can have a beneficial effect. The running-in period for various machines and engines produces this type of wear by removing the peaks from asperities (Fig.16.3). Thus, under controlled conditions, wear may be regarded as a type of smoothing or polishing process.