7.6 Fits & Clearances Flashcards
How are drill sizes given in the metric system?
- assigned a number or a letter
- largest drill is ‘z’ (10.5mm)
- number system starts from 1 (5.79mm) down to 80 (0.34mm)
What is the imperial drill size range?
1/16-1 inch
What are the three classes of fit?
- clearance fit
- interference fit
- transition fit
What is the clearance fit?
- some play after assembly
- assess,bled by light hand pressure (running) or without any force (free running)
What is an interference fit?
- inner part is larger than the part it fits into
- no movement is possible after assembly
What are the three classes of interference fit?
- 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
What is the transition fit?
- in between interference and clearance fit
- when mating parts are almost the same size
What does the term “fitting” mean?
Putting parts together either by them touching or joining each other
What is the joining process defined as?
That the parts slide or turn in to each other
What are some examples of fitted parts?
- shafts fitted to a bearing
- piston running Ina cylinder
- propeller splined to its shaft
- bolt fitting in to a nut
How is the basic size of a dimension given?
By tolerances expressed as a plus/ minus range
What is an allowance?
The difference in dimension necessary to give a particular class of fit
What is the correct allowance?
The difference between the high limit if the shaft and the low limit of the hole
What is the largest and smallest size of the dimension 2.5 mm +/- 0.05 mm?
Largest size is 2.55mm
Smallest size is 2.45mm
What is the british standards system developed by the British standards institute hole classification system?
- 21 hole classification from A to Z
- 21 shaft classifications from a - z
- 16 grades of accuracy given by a number
What is the international organisation for standardisation (ISO) system?
28 classifications for holes and shafts
20 grades of accuracy
What is the American National Standards Institute (ANSI) system?
- Fir metric fits and clearances follows the ISO standard
For imperial it uses symbols
What symbols does the ANSI use for imperial based units?
For running and sliding fits: RCx
For force and shrink fits: FNx
What was the first standard system in the UK?
Newall system
What can the newall system values be interchanged with?
BS 1916-1
What sizes does the newall system cover?
Clearance, transition and interference fits for sizes up to 12 inches
How are holes and shafts classified in the Newall system?
- two grades of holes A or B
- with bilateral tolerances
- six grades of shaft tolerances
Newall system: What are the six grades of shaft tolerances?
Interference:
F: Force
D: Driving
Transition:
P: push (slight manual effort required)
Clearance: X,Y, Z (Running, Class Z provides finest fit)
What is the new standard intro used by the British Institute?
British Standard Specification 4500
How are the various fits obtained in the BS 4500?
By using shafts of different sizes
Can manufacturing tools like reamers be used to meet BS 500 specs?
Yes
What is deviation?
Difference between a size and the corresponding basic or normal size?
What is the international tolerance grade?
- Numbers that have the same relative level of accuracy for a particular IT number
- vary depending upon the nominal or basic size
What is the fundamental deviation known as in holes and shafts?
Holes: H
Shafts: h
Where are the tolerances of parts specified?
On the design drawing
What fit does an engine shaft and its bearing have? Why
- Clearance fit
- Allows for space for lubricant
What are heat/ shrink fits used to achieve?
An interference fit
What fit is produces by using a press fit process?
Transition fit
Where is the heat/ shrink fit used when assembling an engine?
- Turbine shaft shrunk with dry ice or liquid nitrogen
- Turbine wheels heated with a heating element
What part of assembling an engine requires press (transition fit)?
- turbine shaft bearing pressed on to the turbine shaft assembly
- when torquing tie bolt mounts in to the turbine wheel bolt holes
- spacers between the turbine wheels are light press type fit
What part of the engine assembly requires a clearance fit?
- turbine blades are inserted in to the turbine wheels by hand and just slide in to the slot (fir tree)
When is an item termed as ‘bowed’?
When the centreline is not straight
What happens when a shaft or tube is rotating at high speeds in a bow condition?
- risk of vibration
- can lead to mechanical failures, loss of fasteners and fatigue
What does twist result from?
Applying a twisting force in a round or square sectioned shaft
What happens if a force above the elastic limit is applied to a shaft?
It remains twisted
How can wear be detected in an object?
By comparing it’s dimensions to a new component
What are the four key dimensions?
- 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
What are the four key dimensions?
- 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
How do you check for bow?
- 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
What is the maximum bow usually allowed?
1 in 600
Where can wear limits be found?
In the repair or overhaul manual
When can bow not be checked with a straight edge and a set of feeler gauges?
When there are objects protruding
What are some marks that a shaft has been subjected to a high torsional load?
Visible marks engraved or etched at each end of the shaft
How can shafts be checked for twist?
- 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
Where are “witness” marks found?
Where the drive and driven flanges meet the shaft
What creates ovality?
As a result of the surface wearing through friction or linear movement
How can holes be checked for ovality?
Using go/ no- go gauges
Internal micrometers
Callipers
How can shafts be checked for wear?
Using a micrometer or ovality gauge
What has to be done before testing for bow?
Test the shaft for ovality
What are dial test indicators used for?
- 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
What is the most common system?
Hole based system
What is the hole basis?
A system of fits relating to basic or nominal hole size
