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MEC302 > Lecture 15 > Flashcards

Lecture 15 Flashcards

1
Q

What is the definition of tribology?

A

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

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2
Q

When is high friction wanted?

A

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

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3
Q

Why would low friction be wanted?

A

For energy consumption reduction, eg in a car engine

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4
Q

How is surface wear and friction reduced?

A

By lubrication with oil or coating surfaces

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5
Q

What is wear?

A

Loss of material from surfaces rubbing together

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6
Q

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

A

The volume lost from the wearing surface per unit sliding distance

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7
Q

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

A
  • Normal load
  • Relative sliding speed
  • Temperature
  • Thermal/mechanical/chemic properties in contact
  • Third bodies (eg grit)
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8
Q

What are the types of mechanical wear process?

A

Adhesive
Abrasive
Fatigue-caused

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9
Q

What is the non-mechanical wear process covered?

A

Chemical/corrosive wear

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10
Q

What are the features of wear?

A

Worn surface

Debris

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11
Q

What is the mechanism of adhesive wear?

A
  • Work hardening occurs at weld

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

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12
Q

Why do similar metals wear rapidly?

A

Materials weld more easily

Check this one

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13
Q

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

A

There is much more wear on the soft material

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14
Q

How can adhesive wear be reduced?

A
  • Minimise size of torn particles
  • Minimise contact area
  • Use lubrication to avoid all contact
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15
Q

What is the extreme case of adhesive wear?

A

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

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16
Q

What is two-body abrasive wear?

A

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

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17
Q

What is three-body abrasive wear?

A

Wear due to hard particles trapped between the surfaces

18
Q

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

A

The outside environment

Wear debris

19
Q

How can abrasive wear be reduced?

A

Reduce load
Increase hardness
Use lubrication

20
Q

What is the process of wear caused by fatigue?

A
  1. Stress at/near surface varies with time
  2. Fatigue occurs
  3. Slow crack growth
  4. Surface is weakened
  5. Large pieces detach
21
Q

What are the names for large pieces detaching?

A

Delamination

Spalling

22
Q

How do cracks grow with traction?

A

From the surface

23
Q

What is traction?

A

Rolling/sliding

24
Q

What does oxidation-related wear depend on?

A

How much the wearing material oxidises

Availability of oxygen

25
Q

What is the Archard wear equation?

A 
w = K(P/H)
K = wear coefficient 
P = normal load 
H = surface hardness
26
Q

What is k?

A 
k = K/H
k = wear volume per unit of sliding distance per unit of normal loads
27
Q

What is the primary purpose of a lubricant?

A

To separate contact surfaces, thereby reducing friction and wear

28
Q

What are the additional purposes of a lubricant?

A

Cooling medium

Protection from corrosion

29
Q

What are the 5 main classes of lubricant?

A 
Oils
Greases 
Emulsions 
Solid lubricants 
Gases
30
Q

What effect does temperature have on lubricants?

A

It can give viscosity variation

31
Q

What is hydrodynamic lubrication?

A

Lubricant is dragged into a wedge between components

The lubrication pressure increase supports the load

32
Q

What are the applications of hydrodynamic lubrication?

A
  • Journal bearings
  • Machine slideways
  • Piston ring/liner
33
Q

What is hydrostatic lubrication?

A

Lubricant is pumped into the interface to separate surfaces

It doesn’t require relative movement

34
Q

What are the applications of hydrostatic lubrication?

A
  • Machine spindles

- Bearing start-up, reversing or stopping

35
Q

What is elastohydrodynamic lubrication?

A

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

36
Q

What are the applications of elastohydrodynamic lubrication?

A

Rolling element bearings

Gears, cams and tappets

37
Q

What is boundary lubrication?

A

Surfaces are not fully separated

Thin chemical layers reduce the tendency of the asperities to adhere

38
Q

What are the applications of boundary lubrication?

A

Metal cutting

Bearing start-up or shutdown

39
Q

What is solid lubrication?

A

Low shear-strength solid separates surfaces

Shears more easily than the component materials

40
Q

What are the applications of solid lubrication?

A
  • ‘dry’ bearings

- graphite, PTFE, MoS(2)

MEC302 (13 decks)

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