Lecture 14 Flashcards
Give examples of applications of friction
- Bearing surfaces
- Brake pad & disc
- Nuts & bolts
What is waviness?
All irregularities which are longer than the roughness sampling length
What is roughness?
Fine irregularities on the surface texture, superimposed on the waviness
Why does roughness usually occur?
Inherent action of production method
What are asperities?
Undulations (peaks/troughs) at a microscopic level
What is Ra?
Arithmetic average roughness
How is Ra calculated?
L
Ra = (1/L) $ lzl dx
0
$ is an integral
How does friction act in relation to motion?
Friction opposes motion
What are mu(s) and mu(k)?
mu(s) = coefficient of static friction mu(k) = coefficient of kinetic friction
How do mu(s) and mu(k) compare?
mu(k) < mu(s)
What is the 1st law of friction?
Friction force is proportional to the normal force
What is the 2nd law of friction?
Friction force is independent of apparent area of contact
What is the 3rd law of friction?
Friction force is independent of sliding velocity
When does the 3rd law of friction break down?
At very high sliding speeds - mu(k) falls with increasing velocity
What happens to asperities at low loads? (In the adhesion model)
They deform elastically
What happens to asperities at high loads? (In the adhesion model)
The tips are deformed plastically and form weld junctions
What are welds called in the adhesion friction model?
Adhesion junctions
What is the friction force in the adhesion friction model?
The force needed to fracture the adhesion junctions
What is the actual contact area?
a
The total junction area of asperities
How is the approximate value of mu = 0.5 obtained?
P = asigmay
Welds will fail when F (shear force) overcomes max shear stress in material
F = atau(max) = a*sigmay/2
So F = P/2 = 0.5P
Why is mu large for hard metal contacts?
Small but strong welds form, so lots of energy is required to break them
Why is mu large for soft metal contacts?
Weak welds but large area means lots of energy is required to break the number of welds present
How is low friction achieved with metals?
One hard metal surface and one soft metal surface gives a weak weld of small area
How are bearings often made?
A thin film of soft metal between two hard metals