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The molecular attraction between particles that tends to cause unlike surfaces to stick together
Adhesion
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The molecular attraction between particles that tends to hold a substance or a body together.
Cohesion
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A layer of lubricant.
Film
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Accelerated surface damage occurring at the interface of contacting materials subjected to small oscillating displacement.
Fretting
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The friction that exists between moving bodies (or between one moving body and a stationary surface). The opposite of static.
Kinetic friction
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Erosion of components caused by impingement of solid particles or water droplets that reduces the life of machinery.
Particle or droplet erosion
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The friction that exists between a body at rest and the surface upon which it rests.
Static friction
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A plastic flow phenomenon that typically occurs in rolling element bearings.
Thermal softening
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The study that deals with the design, friction, wear, and lubrication of interacting surfaces in relative motion.
Tribology
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The removal of material from one or more solid surfaces that are in contact.
Wear
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DESCRIBE the environmental hazards associated with lubricants.
Many lubricants are hazardous to the environment and are carcinogenic (can cause cancer).
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Define tribology.
Tribology is defined as the study that deals with the design, friction, wear, and lubrication of interacting surfaces in relative motion.
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Describe sliding, rolling, and fluid friction.
The friction that exists between moving bodies (or between one moving body and a stationary surface) is called kinetic friction
Sliding friction exists when the surface of one solid body is moved across the surface of another solid body.
Rolling friction exists when a curved body (i.e.,cylinder or sphere) rolls upon a flat or curved surface.
Fluid friction is the resistance to motion exhibited by a fluid.
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Describe the desirable properties of lubrication.
Cohesion is the property that holds the lubricant together and enables it to resist breakdown under pressure
Adhesion is the property of a lubricant that causes it to stick to the parts being lubricated.
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Two modes in which friction and wear are affected by properties of the surfaces in contact as well as the lubrication.
Boundary lubrication and thin-film lubrication
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Occurs in a system due to the shape and relative motion of the surfaces in contact.
Hydrodynamic lubrication
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The result of a lubricant being supplied at a pressure that is high enough to separate the surfaces.
Hydrostatic
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The lubricant is not bonded to the surface; therefore, the lubricant will not separate the surfaces.
Thin-film lubrication
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A chemically bonded lubricant, which may or may not separate the surfaces, covers each surface and viscosity of the lubricant is not a factor for determining wear
boundary lubrication
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When one tooth of a spur gear meshes with the tooth of a mating gear, the initial contact is sliding contact and results in the wear and tear of
Tips and Roots of the teeth
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Lubricants may be broadly classified as
liquid, semi-solid, or solid.
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Two common solid lubricants are
graphite and molybdenum disulfide
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Recommended for gear drives when a higher load capacity lubricant is required.
Extreme pressure (EP) additives
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Compounded oils are available with many different additives. The most commonly available is a molybdenum disulfide compound that has been successfully used in
gear applications
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Used when a component cannot be lubricated on a timely basis or the system or
component cannot be accessed during operation
Greases
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Greases can have four parts
fluid base, thickener, additives, and fillers
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Combine the properties of two or more specialized greases to function over a broader range of conditions and applications.
Multipurpose greases
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Four classes of fluids generally used in hydraulic systems that operate in such an
environment are:
- phosphate esters,
- water-glycol fluids,
- invert emulsions, and
- conventional emulsions.
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The major cause of lubrication-related failures is the
incorrect amount of lubrication being applied to components.
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When too little lubricant is added, surface damage will result. The damage will rapidly cause failures that
can be difficult to distinguish from primary fatigue failures and results in small flakes
Spalling
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Occurs when two materials contact each other and the surface of one material is harder
than the surface of the other materials.
Abrasive wear
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Burrs at the sliding interface between two mating parts melts and welds the surfaces
together
Adhesive wear
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Defined as accelerated surface damage occurring at the interface of contacting materials subjected to small oscillatory displacement.
Fretting wear
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Erosion of components caused by impingement of solid particles or water droplets can
cause reduced life of machinery.
Particle & Droplet Erosion
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Current that seeks ground through the bearing can be generated from stray magnetic fields in the machinery or can be caused by welding on some part of the machine with the ground attached so that the circuit is required to pass through the bearing.
Spark Erosion
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Lubricants in gear units have two functions
separate the tooth and bearing surfaces, and cool the surfaces
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The most common and foolproof method of gear lubrication.
Splash lubrication
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Pressurized lubrication and is used on almost all high-speed gear drives, on spiral bevel drives, and on low-speed drives when splash lubrication cannot be used due to gear arrangement.
Force-fed lubrication
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Grease has several advantages as compared to oil:
- Easy to add additional grease
- Offers better adhesion capabilities
- Provides protection against moisture and contaminants
- Easily retained in the bearing housing
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The single most important property of a lubricant and may be influenced by temperature, pressure, and fluid motion (shear).
Viscosity
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That temperature at which oil will just flow under prescribed conditions.
Pour point
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The temperature at which the oil produces sufficient flammable vapor to ignite when
the vapor is brought into momentary contact with a flame.
Flash Point
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The higher temperature at which the oil vapors will continue to burn when ignited
Fire Point
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