Lecture 14 Flashcards

1
Q

Give examples of applications of friction

A
  • Bearing surfaces
  • Brake pad & disc
  • Nuts & bolts
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2
Q

What is waviness?

A

All irregularities which are longer than the roughness sampling length

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3
Q

What is roughness?

A

Fine irregularities on the surface texture, superimposed on the waviness

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4
Q

Why does roughness usually occur?

A

Inherent action of production method

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5
Q

What are asperities?

A

Undulations (peaks/troughs) at a microscopic level

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6
Q

What is Ra?

A

Arithmetic average roughness

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7
Q

How is Ra calculated?

A

L
Ra = (1/L) $ lzl dx
0
$ is an integral

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8
Q

How does friction act in relation to motion?

A

Friction opposes motion

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9
Q

What are mu(s) and mu(k)?

A
mu(s) = coefficient of static friction 
mu(k) = coefficient of kinetic friction
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10
Q

How do mu(s) and mu(k) compare?

A

mu(k) < mu(s)

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11
Q

What is the 1st law of friction?

A

Friction force is proportional to the normal force

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12
Q

What is the 2nd law of friction?

A

Friction force is independent of apparent area of contact

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13
Q

What is the 3rd law of friction?

A

Friction force is independent of sliding velocity

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14
Q

When does the 3rd law of friction break down?

A

At very high sliding speeds - mu(k) falls with increasing velocity

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15
Q

What happens to asperities at low loads? (In the adhesion model)

A

They deform elastically

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16
Q

What happens to asperities at high loads? (In the adhesion model)

A

The tips are deformed plastically and form weld junctions

17
Q

What are welds called in the adhesion friction model?

A

Adhesion junctions

18
Q

What is the friction force in the adhesion friction model?

A

The force needed to fracture the adhesion junctions

19
Q

What is the actual contact area?

A

a

The total junction area of asperities

20
Q

How is the approximate value of mu = 0.5 obtained?

A

P = asigmay
Welds will fail when F (shear force) overcomes max shear stress in material
F = a
tau(max) = a*sigmay/2
So F = P/2 = 0.5P

21
Q

Why is mu large for hard metal contacts?

A

Small but strong welds form, so lots of energy is required to break them

22
Q

Why is mu large for soft metal contacts?

A

Weak welds but large area means lots of energy is required to break the number of welds present

23
Q

How is low friction achieved with metals?

A

One hard metal surface and one soft metal surface gives a weak weld of small area

24
Q

How are bearings often made?

A

A thin film of soft metal between two hard metals

25
Why do polymers make good low friction bearings?
The polymer chains orientate parallel to the sliding surface | They shear easily, so mu is low
26
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)
27
How might excessive contact stress or deformation cause component failure?
- Overload - Wear - Rolling Contact Fatigue - Seizure - Loss of Tolerance
28
What is seizure?
Component surfaces locally weld under high contact stress
29
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
30
What is the highest point on the pressure profile?
The centre
31
Where is the maximum shear stress beneath a circular point contact?
In the subsurface
32
What is the effect of Q < mu*P?
No sliding | Regions of stick and microslip
33
What is the effect of Q = mu*P?
Sliding will occur
34
What do points 1 and -1 mean for the x/a axis on the contact stress distribution?
They are the contact boundary
35
What is a?
Half the width of the contact length
36
What is the shape of the contact region for spheres?
A circle
37
What is the shape of the contact region for cylinders?
An oblong