Interfacing surfaces in relative motions, including friction, wear and erosion
Lubrication Engineer
Friction
Resisting force between surfaces with relative movement
- Energy loss
- Increased power consumption
- Temperature Increase
- Noise increase
- Wear of component with dimensional change
Types
- Static
- Kinetic
- Rolling
Wear
Material loss out of body as consequence of tribological stress
- Abrasive wear: Surface peaks of harder body making scratches in softer body
- 2 body abrasion: Micro cutting of body with hard roughness peaks of opposing body
- 3 Body Abrasion: Micro cutting of both bodies with hard particles within intermediate layer
- Abrasive wear Leads to material losses in form of troughs which affects Lubrication film & Contaminates the lube oil.
- Adhesive wear ?
- Results in Seizure
- Caused by Poor lubrication or Insufficient oil film
- Characterized by local micro-welding points between peaks of both surfaces, with damage following separation.
Wear & Wear Mechanisms
- Corrosive: Chemical reaction with surrounding environment.
- Pitting: Surface cavitation caused by rolling contact
- Cavitation: Formation and collapse of bubbles on surface due to rapid pressure changes
Function of Lubricant
- Friction Control → Separates moving surfaces
- Wear Control → Reduces abrasive wear
- Corrosion Control → Protects Surfaces from Corrosive substances
- Temperature Control → Absorbs and Transfer Heat
- Contamination Control → Transports Particles and other contaminants of Filters\Separators
- Power Tran-mission → Transmits force and Motion
Lubrication Regimes
- Hydrodynamic: Full film lubrication or Surface Separation
- Pressure generated by Sliding velocity of layers
- Adequate oil supply
- Oil Viscosity
- Found in Journal Bearings.
- Thin Film or Mixed
- Film thickness ~ Surface Asperities
- Elasto-Hydrodynamic
- Found in Gears, anti-friction bearings, cams
- Elastic Deformation of surfaces
- Large increase in Pressure viscosity
- Boundary
- Film thickness < Surface Asperities
Lubrication in Bearings Stages
Startup (No oil Pressure), Boundary Lubrication
Intermediate: Low Oil pressure (Mixed Lubrication)
Running: Full oil pressure (Hydrodynamic Lubrication)
Structure of Lubricating oil
Base stock (Mineral\Synthetic\Vegetable) Provides
- Viscosity for controlling friction and wear
- Corrosion and Oxidation resistance
- Heat transfer
- Contamination suspension
- Additive suspension
Additives (Organic & Inorganic)
- Enhances existing Base stock properties
- Surpasses undesirable base stock props
- New properties
Base Oil Technologies
Mineral oils and Hydrocarbons
- Paraffins
- Branched paraffins
- Naphthene
- Aromatics
Chain Length : C25- C40
Refining Process
- Solvent Refining
- Hydro Cracking
API Base stock Categories
Viscosity index increases as group increases
- Group 1 - Solvent Refined Mineral oils
- Group 2 - Hydro Processed mineral oil
- Group 2+ - Hydro Cracked mineral oils
- Group 3 - Severe Hydrocracked Synthetic oils
- Group 4 - Poly-alpha olefins (PAO’s)
- Group 5 - Other Synthetics
Properties to keep in mind
- Viscosity
- Moisture
- TBN Total Base num : Measure of ability to neutralize acid
- TAN Total Acid Number: measure of acidity
- Flash point, Pour point (lowest point at which oil is observed to flow)
- Thermal operating range
- Fire resistance
- Oxidation stability
- Lubricity
- Shear stability
- Detergency
- Seal compatibility
- Contaminant sensitivity
- INFRA Red analysis (Water, Oxidation, Nitration)
- Foam test
- Particle Counting
- Water Metals
Components of Grease
- Base Oil \ Mineral Oil \ Synthetic Oil
- Thickeners \ Soaps
- Additives \ Solid Lubricants \ Corrosion Inhibitors \ Adhesion Improvers
Properties
Role of Additives
- Extreme Pressure
- Mitigate effects of high load boundary lubrication
- Reacts with surface under heat to form ductile films
- Sulphur-phosphorous compounds, graphite, Borate or MoS2
- Anti Wear
- Reduce wear and friction from sliding contact
- Surface active materials form low shear strength between sliding surfaces
- ZDDP, TCP
- Corrosion Inhibitors
- Reduce rust formation on iron surface
- Additive is metal -phillic and hydrophobic
- Long chain fatty acids, Naphthalene sulphorates, Phosphoric acid derivatives
- Anti-Oxidants or Oxidation Inhibitors
- Inhibit reaction with oxygen
- Additive oxidizes in preference to oil seeking out free radicals and peroxides
- Hindered Phenols, Zinc di thiophosphates, Aromatic amines
- Foam Inhibitors
- Prevents formation of foam and aerated oils
- Weakness bubbles allows them to break rapidly
- Organic polymers, Methyl silicone
- Viscosity Index Improvers
- Additive expands at higher temperature, slowing down thinning rate of oil
- Ethylene propylene copolymers, polymethacrylates
- Dispersants
- Detergents
Lube Oil Properties
- Viscosity grading (ISO VG) → cSt vs Temp
- Kinematic Viscosity
- Dynamic or Absolute Viscosity = KV * Density
Viscosity Index
- Mineral oils ≥ 80
- Hydrotreated or Hydrocracked ≥ 95
- Poly-alpha-olefins ≥ 120
- Silicones ≥ 300
Oxidation of Oils
Catalysts of oxidation
- Increased Viscosity, acids, oil darkening
Free radical mechanism via alkyl peroxy radicals
Performance Testing of Oils (ASTM)
- Rotating pressure vessel oxidation (RBOT)
- Grease oxidation stability
- Turbine oil oxidation test TOST
- Rust Test
- Copper Strip Corrosion Test
- Grease Rolling Bearing Rust Test
- Demusibility
Testing
- Viscosity
- Decrease
- Thermal Cracking
- Shearing
- Mixture with lower viscosity oil, Solvents
- Increase
- Mix with high viscosity oil
- Oxidation, Nitration
- Decrease
- Water Contamination
- Crackle test
- Karl Fisher Test
- TAN
- TBN
Oil analysis
- Spectrography
- Ferrography