Lecture 14

Question 1

Give examples of applications of friction

Answer 1

• Bearing surfaces
• Brake pad & disc
• Nuts & bolts

Question 2

What is waviness?

Answer 2

All irregularities which are longer than the roughness sampling length

Question 3

What is roughness?

Answer 3

Fine irregularities on the surface texture, superimposed on the waviness

Question 4

Why does roughness usually occur?

Answer 4

Inherent action of production method

Question 5

What are asperities?

Answer 5

Undulations (peaks/troughs) at a microscopic level

Question 6

What is Ra?

Answer 6

Arithmetic average roughness

Question 7

How is Ra calculated?

Answer 7

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

Question 8

How does friction act in relation to motion?

Answer 8

Friction opposes motion

Question 9

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

Answer 9

mu(s) = coefficient of static friction 
mu(k) = coefficient of kinetic friction

Question 10

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

Answer 10

mu(k) < mu(s)

Question 11

What is the 1st law of friction?

Answer 11

Friction force is proportional to the normal force

Question 12

What is the 2nd law of friction?

Answer 12

Friction force is independent of apparent area of contact

Question 13

What is the 3rd law of friction?

Answer 13

Friction force is independent of sliding velocity

Question 14

When does the 3rd law of friction break down?

Answer 14

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

Question 15

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

Answer 15

They deform elastically

Question 16

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

Answer 16

The tips are deformed plastically and form weld junctions

Question 17

What are welds called in the adhesion friction model?

Answer 17

Adhesion junctions

Question 18

What is the friction force in the adhesion friction model?

Answer 18

The force needed to fracture the adhesion junctions

Question 19

What is the actual contact area?

Answer 19

a

The total junction area of asperities

Question 20

How is the approximate value of mu = 0.5 obtained?

Answer 20

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

Question 21

Why is mu large for hard metal contacts?

Answer 21

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

Question 22

Why is mu large for soft metal contacts?

Answer 22

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

Question 23

How is low friction achieved with metals?

Answer 23

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

Question 24

How are bearings often made?

Answer 24

A thin film of soft metal between two hard metals

Question 25

Why do polymers make good low friction bearings?

Answer 25

The polymer chains orientate parallel to the sliding surface

They shear easily, so mu is low

Question 26

What are the drawbacks to using polymers in bearings?

Answer 26

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)

Question 27

How might excessive contact stress or deformation cause component failure?

Answer 27

• Overload
• Wear
• Rolling Contact Fatigue
• Seizure
• Loss of Tolerance

Question 28

What is seizure?

Answer 28

Component surfaces locally weld under high contact stress

Question 29

What assumptions are used in Hertz Theory?

Answer 29

-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

Question 30

What is the highest point on the pressure profile?

Answer 30

The centre

Question 31

Where is the maximum shear stress beneath a circular point contact?

Answer 31

In the subsurface

Question 32

What is the effect of Q < mu*P?

Answer 32

No sliding

Regions of stick and microslip

Question 33

What is the effect of Q = mu*P?

Answer 33

Sliding will occur

Question 34

What do points 1 and -1 mean for the x/a axis on the contact stress distribution?

Answer 34

They are the contact boundary

Question 35

What is a?

Answer 35

Half the width of the contact length

Question 36

What is the shape of the contact region for spheres?

Answer 36

A circle

Question 37

What is the shape of the contact region for cylinders?

Answer 37

An oblong