Lecture 15

Question 1

What is the definition of tribology?

Answer 1

The study of friction, wear, lubrication, and design of bearings; science of interacting surfaces in relative motion

Question 2

When is high friction wanted?

Answer 2

To brake or avoid slipping over, eg railway wheels on leaves

Question 3

Why would low friction be wanted?

Answer 3

For energy consumption reduction, eg in a car engine

Question 4

How is surface wear and friction reduced?

Answer 4

By lubrication with oil or coating surfaces

Question 5

What is wear?

Answer 5

Loss of material from surfaces rubbing together

Question 6

What is wear from rolling or sliding contact (w)?

Answer 6

The volume lost from the wearing surface per unit sliding distance

Question 7

What does wear rate depend on in a dry, unlubricated situation?

Answer 7

• Normal load
• Relative sliding speed
• Temperature
• Thermal/mechanical/chemic properties in contact
• Third bodies (eg grit)

Question 8

What are the types of mechanical wear process?

Answer 8

Adhesive
Abrasive
Fatigue-caused

Question 9

What is the non-mechanical wear process covered?

Answer 9

Chemical/corrosive wear

Question 10

What are the features of wear?

Answer 10

Worn surface

Debris

Question 11

What is the mechanism of adhesive wear?

Answer 11

• Work hardening occurs at weld

- As surfaces slide, material will fracture below the weld on one face and remain attached to the other

Question 12

Why do similar metals wear rapidly?

Answer 12

Materials weld more easily

Check this one

Question 13

What happens when a soft and hard material are in sliding contact?

Answer 13

There is much more wear on the soft material

Question 14

How can adhesive wear be reduced?

Answer 14

• Minimise size of torn particles
• Minimise contact area
• Use lubrication to avoid all contact

Question 15

What is the extreme case of adhesive wear?

Answer 15

Seizure
High pressures cause junctions and actual area to grow to the size of the apparent contact area
Materials are essentially welded together

Question 16

What is two-body abrasive wear?

Answer 16

Damage to a component surface due to motion relative to that surface of harder asperities

Question 17

What is three-body abrasive wear?

Answer 17

Wear due to hard particles trapped between the surfaces

Question 18

Where might particles come from in three-body abrasive wear?

Answer 18

The outside environment

Wear debris

Question 19

How can abrasive wear be reduced?

Answer 19

Reduce load
Increase hardness
Use lubrication

Question 20

What is the process of wear caused by fatigue?

Answer 20

1. Stress at/near surface varies with time
2. Fatigue occurs
3. Slow crack growth
4. Surface is weakened
5. Large pieces detach

Question 21

What are the names for large pieces detaching?

Answer 21

Delamination

Spalling

Question 22

How do cracks grow with traction?

Answer 22

From the surface

Question 23

What is traction?

Answer 23

Rolling/sliding

Question 24

What does oxidation-related wear depend on?

Answer 24

How much the wearing material oxidises

Availability of oxygen

Question 25

What is the Archard wear equation?

Answer 25

``` w = K(P/H) K = wear coefficient P = normal load H = surface hardness ```

Question 26

What is k?

Answer 26

``` k = K/H k = wear volume per unit of sliding distance per unit of normal loads ```

Question 27

What is the primary purpose of a lubricant?

Answer 27

To separate contact surfaces, thereby reducing friction and wear

Question 28

What are the additional purposes of a lubricant?

Answer 28

Cooling medium | Protection from corrosion

Question 29

What are the 5 main classes of lubricant?

Answer 29

``` Oils Greases Emulsions Solid lubricants Gases ```

Question 30

What effect does temperature have on lubricants?

Answer 30

It can give viscosity variation

Question 31

What is hydrodynamic lubrication?

Answer 31

Lubricant is dragged into a wedge between components | The lubrication pressure increase supports the load

Question 32

What are the applications of hydrodynamic lubrication?

Answer 32

- Journal bearings - Machine slideways - Piston ring/liner

Question 33

What is hydrostatic lubrication?

Answer 33

Lubricant is pumped into the interface to separate surfaces | It doesn't require relative movement

Question 34

What are the applications of hydrostatic lubrication?

Answer 34

- Machine spindles | - Bearing start-up, reversing or stopping

Question 35

What is elastohydrodynamic lubrication?

Answer 35

Same as hydrodynamic, but high local pressure causes an increase in viscosity and elastic deformation

Question 36

What are the applications of elastohydrodynamic lubrication?

Answer 36

Rolling element bearings | Gears, cams and tappets

Question 37

What is boundary lubrication?

Answer 37

Surfaces are not fully separated | Thin chemical layers reduce the tendency of the asperities to adhere

Question 38

What are the applications of boundary lubrication?

Answer 38

Metal cutting | Bearing start-up or shutdown

Question 39

What is solid lubrication?

Answer 39

Low shear-strength solid separates surfaces | Shears more easily than the component materials

Question 40

What are the applications of solid lubrication?

Answer 40

- 'dry' bearings | - graphite, PTFE, MoS(2)