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M7 > 7.6 Fits & Clearances > Flashcards

7.6 Fits & Clearances Flashcards

1
Q

How are drill sizes given in the metric system?

A
  • assigned a number or aletter
  • largest drill is ‘z’ (10.5mm)
  • number system starts from 1 (5.79mm) down to 80 (0.34mm)
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2
Q

What is the imperial drill size range?

A

1/16-1 inch

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3
Q

What are the three classes of fit?

A
  • clearance fit
  • interference fit
  • transition fit
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4
Q

What is the clearance fit?

A
  • some play after assembly
  • assess,bled by light hand pressure (running) or without any force (free running)
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5
Q

What is an interference fit?

A
  • inner part is larger than the part it fits into
  • no movement is possible after assembly
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6
Q

What are the three classes of interference fit?

A
  • driving fit, when the shaft is slightly larger than the hole so has to be fitted by driving e.g tapping w a hammer
  • force fit , parts are forced together. Interference is larger than in driving fit
  • shrink fit, parts are joined to be held in place permanently. Outer member can be neared or the shaft cooled. Interference is greater than force fit
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7
Q

What is the transition fit?

A
  • in between interference and clearance fit
  • when mating parts are almost the same size
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8
Q

What does the term “fitting” mean?

A

Putting parts together either by them touching or joining each other

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9
Q

What is the joining process defined as?

A

That the parts slide or turn in to each other

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10
Q

What are some examples of fitted parts?

A
  • shafts fitted to a bearing
  • piston running Ina cylinder
  • propeller splined to its shaft
  • bolt fitting in to a nut
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11
Q

How is the basic size of a dimension given?

A

By tolerances expressed as a plus/ minus range

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12
Q

What is an allowance?

A

The difference in dimension necessary to give a particular class of fit

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13
Q

What is the correct allowance?

A

The difference between the high limit if the shaft and the low limit of the hole

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14
Q

What is the largest and smallest size I’d the dimension 2.5mm/ 0.00 mm?

A

Largest size is 2.55mm
Smallest size is 2.50mm

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15
Q

What is the bursitis standards system developed by the British standards institute hole classification system?

A
  • 21 hole classification from A to Z
  • 21 shaft classifications from a - z
  • 16 grades of accuracy given by a number
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16
Q

What is the international organisation for standardisation (ISO) system?

A

28 classifications for holes and shafts
20 grades of accuracy

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17
Q

What is the American National Standards Institute (ANSI) system?

A
  • Fir metric fits and clearances follows the ISO standard
    For imperial it uses symbols
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18
Q

What symbols does the ANSI use for imperial based units?

A

For running and sliding fits: RCx
For force and shrink fits: FNx

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19
Q

What was the first standard system in the UK?

A

Newall system

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20
Q

What can the newall system values be interchanged with?

A

BS 1916-1

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21
Q

What sizes does the newall system cover?

A

Clearance, transition and interference fits for sizes up to 12 inches

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22
Q

How are holes classified in the Newall system?

A
  • two grades of holes A or B
  • with bilateral tolerances
  • six grades of shaft tolerances
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23
Q

Newall system: What are the six grades of shaft tolerances?

A

Interference:
F: Force
D: Driving

Transition:
P: push (slight manual effort required)

Clearance: X,Y, Z (Running, Class Z provides finest fit)

24
Q

What is the new standard intro used by the British Institute?

A

British Standard Specification 4500

25
Q

How are the various fits obtained in the BS 4500?

A

By using shafts of different sizes

26
Q

Can manufacturing tools like reamers be used to meet BS 500 specs?

A

Yes

27
Q

What is deviation?

A

Difference between a size and the corresponding basic or normal size?

28
Q

What is the international tolerance grade?

A
  • Numbers that have the same relative level of accuracy for a particular IT number
  • vary depending upon the nominal or basic size
29
Q

What is the fundamental deviation known as in holes and shafts?

A

Holes: H
Shafts: h

30
Q

Where are the tolerances of parts specified?

A

On the design drawing

31
Q

What fit does an engine shaft and its bearing have? Why

A
  • Clearance fit
  • Allows for space for lubricant
32
Q

What are heat/ shrink fits used to achieve?

A

An interference fit

33
Q

What fit is produces by using a press fit process?

A

Transition fit

34
Q

Where is the heat/ shrink fit used when assembling an engine?

A
  • Turbine shaft shrunk with dry ice or liquid nitrogen
  • Turbine wheels heated with a heating element
35
Q

What part of assembling an engine requires press (transition fit)?

A
  • turbine shaft bearing pressed on to the turbine shaft assembly
    • when torquing tie bolt mints in to the turbine wheel bolt holes
  • spacers between the turbine wheels are light press type fit
36
Q

What part of the engine assembly requires a clearance fit?

A
  • turbine blades are inserted in to the turbine wheels by hand and just slide in to the slot (fir tree)
37
Q

When is an item termed as ‘bowed’?

A

When the centre,one of the shade is not straight

38
Q

What happens when a shaft or tube is rotating at high speeds in a bow condition?

A
  • risk of vibration
  • can lead to mechanical failures, loss of fasteners and fatigue
39
Q

What does twist result from?

A

Applying a twisting force in a round or square sectioned shaft

40
Q

What happens if a force above the elastic limit is applied to a shaft?

A

It remains twisted

41
Q

How can wear be detected in an object?

A

By comparing it’s dimensions to a new component

42
Q

What are the four key dimensions?

A
  • Dimension new: when the part is new
  • Permissible worn dimension: size to which a part can wear before it’s unserviceable
  • Clearance new: desired clearance in limit form
  • Permissible worn clearance: max allowable clearance when reassembling the component
43
Q

What are the four key dimensions?

A
  • Dimension new: when the part is new
  • Permissible worn dimension: size to which a part can wear before it’s unserviceable
  • Clearance new: desired clearance in limit form
  • Permissible worn clearance: max allowable clearance when reassembling the component
44
Q

How do you check for bow?

A
  • place a straight edge along the entire length of the shaft parallel to its axis
  • insert feeler gauges at the point of max clearance
  • calculate amount of bow
45
Q

What is the maximum bow usually allowed?

A

1 in 699

46
Q

Where can wear limits be found?

A

In the repair or overhaul manual

47
Q

When can bow not be checked with a straight edge and a set of feeler gauges?

A

When there are objects protruding

48
Q

What are some marks that a shaft has been subjected to a high torsional load?

A

Visible marks engraved or etched at each end of the shaft

49
Q

How can shafts be checked for twist?

A
  • mount in v blocks and check the locating marks in the horizontal position
  • using strain gauges that emit electric current indicating the torsion load applied on a calibrated instrument
50
Q

Where are “witness” marks found?

A

Where the drive and driven flanges meet the shaft

51
Q

What creates ovality?

A

As a result of the surface wearing through friction or linear movement

52
Q

How can holes be checked for ovality?

A

Using go/ no- go gauges
Internal micrometers
Callipers

53
Q

How can shafts be checked for wear?

A

Using a micrometer or ovality gauge

54
Q

What has to be done before testing for bow?

A

Test the shaft for ovality

55
Q

What are dial test indicators used for?

A
  • measure amount of movement between precision machines components
  • can determine roundness and trueness in a shaft or plane if rotation of a brake disc or bearing

M7 (6 decks)

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