Lecture 15 Flashcards
What is the definition of tribology?
The study of friction, wear, lubrication, and design of bearings; science of interacting surfaces in relative motion
When is high friction wanted?
To brake or avoid slipping over, eg railway wheels on leaves
Why would low friction be wanted?
For energy consumption reduction, eg in a car engine
How is surface wear and friction reduced?
By lubrication with oil or coating surfaces
What is wear?
Loss of material from surfaces rubbing together
What is wear from rolling or sliding contact (w)?
The volume lost from the wearing surface per unit sliding distance
What does wear rate depend on in a dry, unlubricated situation?
- Normal load
- Relative sliding speed
- Temperature
- Thermal/mechanical/chemic properties in contact
- Third bodies (eg grit)
What are the types of mechanical wear process?
Adhesive
Abrasive
Fatigue-caused
What is the non-mechanical wear process covered?
Chemical/corrosive wear
What are the features of wear?
Worn surface
Debris
What is the mechanism of adhesive wear?
- Work hardening occurs at weld
- As surfaces slide, material will fracture below the weld on one face and remain attached to the other
Why do similar metals wear rapidly?
Materials weld more easily
Check this one
What happens when a soft and hard material are in sliding contact?
There is much more wear on the soft material
How can adhesive wear be reduced?
- Minimise size of torn particles
- Minimise contact area
- Use lubrication to avoid all contact
What is the extreme case of adhesive wear?
Seizure
High pressures cause junctions and actual area to grow to the size of the apparent contact area
Materials are essentially welded together
What is two-body abrasive wear?
Damage to a component surface due to motion relative to that surface of harder asperities
What is three-body abrasive wear?
Wear due to hard particles trapped between the surfaces
Where might particles come from in three-body abrasive wear?
The outside environment
Wear debris
How can abrasive wear be reduced?
Reduce load
Increase hardness
Use lubrication
What is the process of wear caused by fatigue?
- Stress at/near surface varies with time
- Fatigue occurs
- Slow crack growth
- Surface is weakened
- Large pieces detach
What are the names for large pieces detaching?
Delamination
Spalling
How do cracks grow with traction?
From the surface
What is traction?
Rolling/sliding
What does oxidation-related wear depend on?
How much the wearing material oxidises
Availability of oxygen
What is the Archard wear equation?
w = K(P/H) K = wear coefficient P = normal load H = surface hardness
What is k?
k = K/H k = wear volume per unit of sliding distance per unit of normal loads
What is the primary purpose of a lubricant?
To separate contact surfaces, thereby reducing friction and wear
What are the additional purposes of a lubricant?
Cooling medium
Protection from corrosion
What are the 5 main classes of lubricant?
Oils Greases Emulsions Solid lubricants Gases
What effect does temperature have on lubricants?
It can give viscosity variation
What is hydrodynamic lubrication?
Lubricant is dragged into a wedge between components
The lubrication pressure increase supports the load
What are the applications of hydrodynamic lubrication?
- Journal bearings
- Machine slideways
- Piston ring/liner
What is hydrostatic lubrication?
Lubricant is pumped into the interface to separate surfaces
It doesn’t require relative movement
What are the applications of hydrostatic lubrication?
- Machine spindles
- Bearing start-up, reversing or stopping
What is elastohydrodynamic lubrication?
Same as hydrodynamic, but high local pressure causes an increase in viscosity and elastic deformation
What are the applications of elastohydrodynamic lubrication?
Rolling element bearings
Gears, cams and tappets
What is boundary lubrication?
Surfaces are not fully separated
Thin chemical layers reduce the tendency of the asperities to adhere
What are the applications of boundary lubrication?
Metal cutting
Bearing start-up or shutdown
What is solid lubrication?
Low shear-strength solid separates surfaces
Shears more easily than the component materials
What are the applications of solid lubrication?
- ‘dry’ bearings
- graphite, PTFE, MoS(2)
