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Biomedical Engineering > MAGEC Rods as a Piece of Mechanical Engineering > Flashcards

MAGEC Rods as a Piece of Mechanical Engineering Flashcards

1
Q

How much on average does a child’s spine grow every year?

A

13mm

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2
Q

Why is Ti6Al4V used in the MAGEC rods?

A

It is biocompatible, limiting the harm/toxins in the living tissue

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3
Q

Which types of MAGEC rods are analysed at Newcastle Uni?

A

Mod 1 and Mod 2

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4
Q

What percentage of Mod 1 and Mod 2 rods are analysed of all rods at Newcastle Uni?

A

20% of all examined are mod 1 and 80% mod 2

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5
Q

How can the Mod 1 and Mod 2 MAGEC rods be told apart?

A

The Mod 1 has two silver lines at the end of the extension part, and the mod 2 only has one

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6
Q

How many of the rods analysed have been 90mm long?

A

92%

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7
Q

How many of the rods analysed have been 70mm long?

A

8%

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8
Q

How many of the rods analysed have been 4.5mm long?

A

66%

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9
Q

How many of the rods analysed have been 5.5mm long?

A

34%

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10
Q

Which rod would most surgeons decide to implant if given the choice?

A

Most surgeons would prefer to implant the longer of the rods, the 90mm long one as they have more length to grow, hence why 92% of the implants are for the 90mm rod.

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11
Q

What may also be the reason behind the high amount of 90mm rods explanted?

A

This may be because surgeons do not want to implant the rods in young children.

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12
Q

Which of the rods will have a lower strength?

A

The rods with the smaller diameter, 4.5mm

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13
Q

What is the diameter of the drive pin in the MAGEC rod?

A

0.8mm

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14
Q

Why is the diameter of the drive pin so small?

A

This is because they are trying to minimise the size of the rods.

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15
Q

What is the definition of strength?

A

The ability of a material to withstand an applied load without failure or plastic deformation

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16
Q

Name some of the different types of stress?

A
  • normal stress
  • shear stress
  • bending stress
  • torsional stress
  • fatigue stress
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17
Q

How can we work out what the maximum load is that the drive pin can take?

A

We can simplify the drive pin to a 3 point bending of a simply supported beam with a centre load.

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18
Q

What are the equations needed to work out the maximum load on the drive pin?

A

Need to know equations for a 3 point bending of a simply supported beam. For a circular bar, z=0.78r^3
sigma=PL/4z .

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19
Q

How could the drive pin be altered to increase the maximum load?

A

The maximum load of the drive pin could be increased by altering the material with a higher failure stress. Could also try reducing the length or increasing the diameter.

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20
Q

Why were some of the explanted drive pins bent and snapped?

A

The forces in the drive pins had been above the yield point, so some had some plastic deformation. The broken pin has surpassed the failure load.

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21
Q

What is a benefit of the size and location of the drive pin?

A

This may or may not be a design feature considered by the engineers, but due to the set up of the rods there is no room for the drive pin to move if it breaks, this means the system may still work, and you may still get some drive from the pin. This is not to say that the breakages doesn’t matter.

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22
Q

Which loads do thrust bearings take?

A

Thrust bearings only take an axial load

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23
Q

What are the thrust bearings inside the MAGEC rod made of and why?

A

The thrust bearings inside the MAGEC rod are made of stainless steel in order to resist corrosion.

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24
Q

What are the thrust bearings, and are they easy to buy?

A

Yes, the F3-8M thrust bearings are fairly standard and can be bought off the shelf.

25
Q

What is the static load capacity of the F3-8M thrust bearing?

A

93kgf

26
Q

How would you calculate the axial load capacity of the thrust bearings?

A

(static load capacity) x 9.81

93 x 9.81

27
Q

What are the radial loads in the thrust bearings?

A

The thrust bearings cannot support radial loads, only axial.

28
Q

What do the thrust bearings have in each washer?

A

They have a groove or raceway in each washer.

29
Q

What are the washers and balls made for in the thrust bearings and why?

A

The washers and the balls are made from chrome steel due to its high load capacity.

30
Q

What is the drawback of using chrome steel for the thrust bearing washers and balls?

A

Chrome steel is not corrosion resistant.

31
Q

How are the washers and balls in the thrust bearings treated to ensure protection against corrosion?

A

As they are not corrosion resistant, they are instead coated in a layer of preservative oil to protect against corrosion, or they can use more specified lubricants too.

32
Q

What was the quality of the thrust bearings explanted at Newcastle Uni like?

A

Some of the explanted thrust bearings were found to be of standard quality, however some were of very poor quality.

33
Q

What type of loads are taken by radial bearing?

A

Radial bearings take axial and radial loads.

34
Q

Which load do radial bearings primarily take?

A

Radial bearings primarily take radial loads.

35
Q

What are the radial bearings in the MAGEC rods made of?

A

The radial bearings in the MAGEC rods are made of stainless steel.

36
Q

What is special about the radial bearings in the MAGEC rods?

A

The radial bearings in the MAGEC rods are shielded on one side.

37
Q

What are the dimensions of the radial bearings inside the MAGEC rods?

A

The outer diameter is 5/16’’
The inner diameter is 3/16’’
The width is 1/8’’

38
Q

Why are the radial bearings measurements given in imperial units?

A

They are an American company.

39
Q

What is the bearing code of the radial bearings used in the MAGEC rods?

A

SR156-Z, this is made up of the dimensions.

40
Q

How can you find the load capacity of the radial bearings?

A

You can look in bearing catalogues to find the load capacity of the radial bearing.

41
Q

What is the dynamic load rating of the radial bearing in the MAGEC rods?

A

37kgf

42
Q

What is the static load rating of the radial bearing in the MAGEC rods?

A

15kgf

43
Q

What is the most important load rating for the radial bearing in the MAGEC rod and why?

A

The most important load rating in the radial bearing is the dynamic load as it is the loads they can take under motion that’s important.

44
Q

What does the L10 of a bearing show?

A

The L10 is the life (millions of revolutions) after which 10% of bearings will fail, i.e. lifetime for 90% probability of survival.

45
Q

What is the equation for the L10 of the bearing, and what do they stand for?

A

The equation for L10;
L10=(C/P)^3
C = basic dynamic loading (kN)
P = actual radial load on bearing (kN)

46
Q

Why is the L10 equation not so useful for the radial bearings used in the MAGEC rods?

A

The L10 equation isn’t so useful for estimating the life-time of the bearings in the MAGEC rod, this is because there is little rotation taking place in the bearings in the MAGEC rod.

47
Q

What is the radial bearing in the MAGEC rod being used for as oppose to rotation?

A

The radial bearing inside the MAGEC rod is being used for location more than rotation.

48
Q

How can the static radial load on the radial bearing be calculated?

A

Using moment calculation.

49
Q

What is the most common type of seal?

A

The o-ring seal.

50
Q

What type of seal is used in the MAGEC rod?

A

An o-ring seal

51
Q

Why are o-ring seals so popular?

A

They are effective and inexpensive.

52
Q

How do the o-ring seals work?

A

They need to deform in order to work.

53
Q

What are the different types of o-ring seal?

A

They can be static-contained in a non-moving gland, or they can be dynamic-contained in a moving gland e.g a piston.

54
Q

How are o-ring seals defined?

A

O-ring seals are defined by the inner diameter, cross section, hardness and material. This is similar to how bearings are defined.

55
Q

What type of o-ring seal is in the MAGEC rod?

A

The majority of the time it is a static o-ring seal, it is only when it extends that it becomes a dynamic seal.

56
Q

What is the purpose of the o-ring seal in the MAGEC rod?

A

The purpose of the o-ring seal in the MAGEC rod is to prevent fluid ingress and prevent debris egress.

57
Q

How are the o-ring seals found in most explanted MAGEC rods?

A

They are usually found broken, with localised material lost and sharp edges, the sharp edges may be from wear and tear or because of cutting.

58
Q

What equation is needed to find the buckling load of the extending bar in the MAGEC rod?

A 
F=n(pi)^2El/L^2
F=load
n=constant (0.25)
E=Youngs Modulus (114GPa for the MAGEC rod)
I= second moment of area
L=length
59
Q

What is the best component to alter first inside the MAGEC rod?

A

The drive pin may be the best option to alter first as calculations would suggest that its strength is the lowest.

Biomedical Engineering (13 decks)

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