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Lubrication & Tribology (2) (Design Considerations (Enclosure…
Lubrication & Tribology (2)
Design Considerations
Enclosure
Requires sealing
Static
and dynamic
Collecting contamination
and contamination out
Allowing:
Inspection
Drainage
and filling
Heat rejection
Keeping lubricant in
Distribution
To all contact surfaces (bearings, gears, cams, seals…)
Passive (submerged or dipping into sump)
Dynamic (spraying, splashing ,dripping…)
Active (by pump, distribution channels, filter)
Clearances
Power
Temperature rise / cooling
Maintenance
and filling
Also disassembly
Drainage
Seal replacement
Inspection
Additives
Three Types:
Materials which get absorbed onto surfaces
Friction modifiers:
e.g. natural oils, fatty acids
low shear resistance solids
e.g. graphite, PTFE, MoS2
Additives which enhance the properties of the liquid
e.g. anti foam
Anti oxidants
Material which reacts chemically with surfaces
Anti-wear additives:
e.g. containing phosphorous - reduce wear rates
Extreme pressure (EP) additives:
e.g. containing sulphur – improve load carrying capacity
Protect surface but
do not necessarily reduce friction
Wear Mechanisms
Five Types:
Surface fatigue
Repeated alternating mechanical stress
Formation and
propagation of cracks under the surface
Consequences are:
Cracks
Pitting
Tribo-chemical wear
Worst at higher temperature
Frequently caused by oxidation
Sliding contact can lead to chemical reaction
Consequences include:
Reaction products can cause jamming
Corrosion
Abrasion wear
Removal of material due to
Sharp-edged particles
Surface roughness (asperity peaks)
Hard edges
Consequences are:
grooves
Micro-chips
scratches
Dimensional changes
Corrosion
or electro chemical reaction
due to contact with
e.g electrolyte
or humid gases
Arises from chemical
Consequences include:
Rust
Cracks
pitting
Adhesion wear
E.g when running dry
Friction pairs form intimate adhesive bond
Very likely where surfaces are of similar composition
or have particular affinity for each other
Consequences are:
Cold welding
Scuffing
Scoring
Pits
Seizing
Edge build-up
Filters
Why:
otherwise the lubricant becomes an abrasive
Remove contaminants in suspension
or the additives loose their effectiveness
But:
energy for pump work and
regular cleaning / replacement
Requires special forced flow system,