Wear Mechanism
SHIVAJI CHOUDHURY
Wear
Wear is the undesirable removal of solids from a sliding or rolling component.
Wear analysis
In analyzing a wear problem in a machine, it is necessary to determine the kind of wear that occurred. Analysis requires microscopic examination of the worn area and a close look at the used lubricant. Wear is generally proportional to the applied load and the amount of sliding.
Types of wear :
1.Adhesive Wear 2.Abrasive Wear 3.Erosion Wear 4.Polishing Wear 5.Contact fatigue 6.Corrosive wear
7.Electro-Corrosive wear 8.Fretting wear 9.Electrical discharge wear 10.Cavitation damage 11.False brinelling
1.Adhesive Wear Adhesive Wear — the removal of material due to adhesion between surfaces. 1. – Mild adhesion — is the removal of surface films, such as oxides, at a low rate. 2. – Severe adhesion — the removal of metal due to tearing, breaking, and melting of metallic junctions. This leads to scuffing or galling of the surfaces and even seizure.
1.1.Adhesive Wear -Prevention
1. 2. 3. 4.
MECHANICAL Reduce load, speed and temperature Improve oil cooling Use compatible metals Apply surface coatings such as phosphating
LUBRICANT 1. Use more viscous oil to separate surfaces 2. Use "extreme pressure" (antiscuff) additives such as a sulfurphosphorous or borate compounds
2.Abrasive Wear • Abrasive Wear — the cutting of furrows on a surface by hard particles, (for example, sand particles between contact surfaces, or hard asperities on an opposing surface). Cause: Hard particles contaminating oil Hard metal with rough surface against soft. metal
2.1.Abrasive Wear-Prevention
MECHANICAL
1. Remove abrasive by improved air and oil filtering, clean oil handling practices, improved seals, flushing and frequent oil changes 2. Minimize shot peening, beading, or sand blasting of surfaces because abrasives cannot be completely removed 3. Increase hardness of metal surfaces
LUBRICANT 1. Use oil free of abrasive particles 2. Use more viscous oil
3.Erosion Wear
• Erosive Wear — the cutting of furrows on a surface by hard particles contained in a fluid traveling at high velocity.
3.1.Erosion Wear -Prevention
MECHANICAL
1. Remove abrasive by improved air and oil filtering, clean oil handling practices, improved seals, flushing and frequent oil changes 2. Reduce impact angle to less than 15 degrees 3. Increase hardness of metal surfaces
LUBRICANT 1. Use oil free of abrasive Particles 2. Use more viscous oil
4.Polishing Wear
• Polishing Wear — the continuous removal of surface films, laid down via a chemical reaction from an additive in oil or by very fine hard particles in the lubricant, and so on. Polishing wear is characterized by very shiny, very smooth, mirror like metal surfaces. Fine abrasives wear away the surface films as they form and reform.
4.1.Polishing Wear -Prevention MECHANICAL 1. none
LUBRICANT
1. Choose less chemically active additive 2. Remove corrosive
contaminant 3. Remove abrasive
5.Contact fatigue
• Contact Fatigue — the cracking, pitting, and spalling of a surface in sequence due to cyclic stresses in a contact. Contact fatigue is most common in rolling element bearings, gear teeth, and cams.
5.1.Contact fatigue- Prevention MECHANICAL 1. Reduce contact pressures and frequency of cyclic stress. 2. Use high quality vacuum melted steels. 3. Use less abusive surface finish
LUBRICANT 1. Use clean, dry oil. 2. Use more viscous oil 3. Use oil with higher-pressure viscosity coefficient
6.Corrosive wear
• Corrosive Wear — the removal of corrosion products from a surface by motion, such as the rubbing off of rust. cause :water contamination most common cause
6.1.Corrosive wear-Prevention MECHANICAL 1. Use more corrosion resistant metal (not stainless) 2. Reduce operating temperature 3. Eliminate corrosive material
LUBRICANT 1. Remove corrosive material such as too chemically active additive and contaminates 2. Use improved corrosion inhibitor 3. Use fresh oil
7.Electro-Corrosive wear
• Electro-Corrosive Wear — the removal of metal by dissolution in a corrosive liquid with the aid of electric currents. One source of currents is streaming potential from high velocity fluids. The oil serves as the electrolyte.
7.1.Electro-Corrosive wearprevention MECHANICAL 1. Decrease liquid velocity and velocity gradients 2. Use corrosionresistant metals 3. Eliminate stray currents
LUBRICANT 1. Decrease or increase electrical conductivity of lubricants or hydraulic fluids
8.Fretting wear
• Fretting Wear — localized wear of lubricated surfaces due to reciprocating sliding of extremely low amplitude because of vibration.
8.1.Fretting wear -Prevention
MECHANICAL
1. Reduce or stop vibration by tighter fit or higher load . 2. Improve lubrication between surfaces by rougher (then honed) surface finish
LUBRICANT
1. Use oil of lower viscosity 2. Relubricate frequently 3. Use oxidation inhibitors in oil
9.Electrical discharge wear
• Electrical Discharge Wear — the removal of molten metal from surfaces due to electrical sparks between them. High static voltages are sometimes generated by large rotating machinery and these are relieved by sparking to regions of lower potential.
9.1.Electrical discharge wearPrevention
MECHANICAL
1. Improve electrical insulation of bearings
2. Degauss magnetic rotating parts 3. Install brushes on shaft 4. Improve machine grounding
LUBRICANT 1. Use of oil of higher electrical conductivity
10.Cavitation damage
• Cavitation Damage — the removal of material due to cracking and pitting caused by high energy implosions of vacuous cavities in a cavitating liquid. Liquids cavitate when suddenly subjected to low pressures. Removal of metal by vapor cavity implosion in a cavitating liquid.
10.1.Cavitation damage-Prevention MECHANICAL 1. Use hard, tough metals, such as tool steel 2. Reduce vibration, flow velocities and pressures 3. Avoid restriction and obstructions to liquid flow
LUBRICANT 1. Avoid low vapor pressure, aerated, wet oils 2. Use noncorrosive oils
11.False Brinelling
• False Brinelling — localized wear in lubricated rolling element bearings due to slight rocking motion of rollers against raceways.
11.1.False Brinelling- Prevention MECHANICAL 1. Reduce or eliminate impact 2. Rotate bearings occasionally
LUBRICANT 1. Change viscosity
Microscopic Observation (Wear debris analysis)
(a) Rubbing wear, (b) cutting wear, (c) fatigue wear, (d) scuffing wear, (e) corrosive wear, (f) abrasive wear.
Bearing damage classifications: ISO 15243
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