Module 7.6 Flashcards
What are the 3 classes of fit?
-Clearance
-Interference
-Transition
How are parts with clearance fits assembled?
By light hand pressure or without any force (running or free running) there is play between components
Why is no movement possible after assembly of parts with an interference fit?
Inner part is larger than what it fits into
What are the 3 classes of interference fit?
-driving
-force
-shrink
When and how are shrink fits used?
When parts are joined to be held in place permanently
Outer member heated to expand or inner member cooled to shrink then allowed to contract/expand to fit tight
What is a transition fit?
A fit in between clearance and interference. Mating points are almost the same size
What is an allowance?
Difference in dimensions that is necessary to give particular fit
What is the correct allowance calculated from?
Difference in high limit of shaft and low limit of hole
What is a tolerance?
Total amount of variation in size of a part
What are the two types of tolerances?
Uni-lateral
Bi-lateral
How are holes and shafts classified in the BSI?
Hole uses 21capital letters
Shaft uses 21 lowercase letters
How is the accuracy of each letter classified?
Numbered 1-16
How does ISO 286?differ from BS? (ANSI same as ISO)
28 hole classes and shafts
20 accuracy degrees
Is the new all system still used?
No
What is the BS4500 system based on?
On standard sized holes
In an engine shaft the inner part must be smaller than the outer, why is this?
The clearance allows for lubrication
During engine/component assembly what are the 3 processes for a correct fit?
-heat/shrink fits, interference fit
-press fits, transition fit
-clearance fit
Where is a press fit used in the aircraft engine?
Turbine shaft bearing pressed into a turbine shaft
What is an example of a clearance fit on an engine?
Turbine blades into the turbine wheels
What is essential to check for on a shaft or tube?
Ends are square with eachother
What word describes if the centreline of a shaft is not straight?
Bowed
What is increased if a shaft or tube is bowed?
Risk of vibration
When does a shaft remain twisted?
When force is applied above the elastic limit
With a square sectioned shaft how can twist be checked?
Dial test indicator mounted on a surface gauge
What are the 4 wear dimensions?
-dimension new
-permissible worn dimension
-clearance new
-permissible worn clearance
How do you check for bowing?
Place a straight edge to the whole length and then insert feeler gauge at max clearance point (1mm over 600mm)
What are the methods of checking for twists?
-strain gauges in the shaft
-witness marks between drive and driven
What are the 3 methods for checking hole ovality?
-go/no go gauges
-internal micrometers
-callipers
What are the two methods for checking shaft ovality?
-micrometer
-ovality gauge
What are the DTIs commonly used for?
Checking bowing + ovality
