Lecture 14 Flashcards

1
Q

friction–even the most polished surfaces are made up of hills and valleys on the atomic level; asperities (aspect ratios height to rel. width ratio); smaller aspect ratio = less friciton

A

ye

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2
Q

height of asperities of optically polished metal?

A

10e-7 to 10e-8 m

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3
Q

height of asperities of finely turned metal

A

5 e-6 m

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4
Q

height of asperities of surface polished with 600 grit carborandum

A

10e-7 m

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5
Q

height of asperities of articular cartilage

A

2 to 20 x 10e-8 m

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6
Q

in general, friction between like and unlike materials is LESS than that betweeon similar materials

A

ye

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7
Q

what is one way to lower friction?

A

lubrication–separates the asperities

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8
Q

what are the types of lubrication?

A
  1. hydrodynamic
  2. boundary
  3. elastrohydrodynamic
  4. squeeze film
  5. mixed
  6. hydrostatic
  7. weeping
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9
Q

describe hydrodynamic lubrication

A

thickness of lubricant prevents the solid surfaces from touching;

layer thickness = η v/p

where η = viscosity
v = relative velocity of motion
p = applied pressure

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10
Q

describe boundary lubrication

A

film thickness is too low
η v/p is too low

surface to surface contact occurs–high friction

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11
Q

what is elastohdrodynamic lubrication

A

between hydrodynamic and boundary lubrication

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12
Q

what is squeeze film lubrication

A

lubricant which is sufficiently viscous to respond almost elastically to a sudden increase in pressure for a short period of time

–e.g. heel strike

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13
Q

describe mixed lubrication

A

occurs when lubricant is normally hydronamic in nature

can also coat the surface and provide some boundary lubrication at high pressure

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14
Q

describe hydrostatic lubrication

A

needed external pressure source to maintain the fluid lubricant film

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15
Q

describe weeping lubrication

A
  • lubricant is taken up in porous, deformable surfaces

- pressure builds up in liquid in the pores by load applied to the surface

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16
Q

what kind of lubrication do we find in normal joints?

A

boundary lubrication during initiation of motion

elastohydrodynamic during motion

squeeze film during short high load transients

17
Q

how are artifical prosthetic joints lubricated?

A

-boundary lubrication

but wear does occur; transfer film from a worn polymer may act as a lubrication, and synovial fluid contains hyalauronic acid which binds to metal

18
Q

what is synovial fluid?

A

ultrafiltrate of serum with long chain polysacch. called hyaluronic acid as a major component

19
Q

synovial fluid is thixotropic

A

ye

20
Q

trauma to a synovial joint decreases the percentage of hyaluronic acid and produces low viscosity but still thixotropic fluid

A

ye

21
Q

study slide 21

A

e

22
Q

what are the types of wear joints can undergo/

A

adhesive wear–atomic bonding takes place at the interface –>lift softer material and transfers it to a harder one

abrasive wear – asperities of hard surface gouge and plow softer surface

corrosive wear: protective oxide is softer than metal–>wears easily, subject to underlying metal corrosion

fatigue wear: surface undergoes cyclic loading and unloading–>cracks–>flaking

23
Q

study aslide 9

A

ye