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
Why do polymers make good low friction bearings?
The polymer chains orientate parallel to the sliding surface
They shear easily, so mu is low
What are the drawbacks to using polymers in bearings?
Molecules peel off easily, so there is heavy wear
Creep allows junction growth in the static state, so mu(s) becomes very large in comparison to mu(k)
How might excessive contact stress or deformation cause component failure?
- Overload
- Wear
- Rolling Contact Fatigue
- Seizure
- Loss of Tolerance
What is seizure?
Component surfaces locally weld under high contact stress
What assumptions are used in Hertz Theory?
-The size of the contact area is small compared with the size of the curved bodies
-Both contacting surfaces are smooth and frictionless
-The deformation is elastic
-The gap between the undeformed surfaces can be approximated by
h = Ax^2 + By^2
What is the highest point on the pressure profile?
The centre
Where is the maximum shear stress beneath a circular point contact?
In the subsurface
What is the effect of Q < mu*P?
No sliding
Regions of stick and microslip
What is the effect of Q = mu*P?
Sliding will occur
What do points 1 and -1 mean for the x/a axis on the contact stress distribution?
They are the contact boundary
What is a?
Half the width of the contact length
What is the shape of the contact region for spheres?
A circle
What is the shape of the contact region for cylinders?
An oblong
