7.6: Fits and Clearances Flashcards
What does the number system in drill sizes range from?
From 1 to 80
What are the three types of fit?
Clearance fit
Transition fit
Interference fit
What is an inteference fit?
When there is no possible movement after the assembly because the inner part is larger than the part it fits into
What are the three classes of interference fit?
Driving fit
Force fit
Shrink fit
What is the driving fit?
When the shaft is slightly larger than the hole and the allowance is such that the parts can be assembled by driving, like hammer taps
What is a force fit?
When the parts are forced together, usually by presses and the interference is larger than that for a driving fit
What is a shrink fit?
When the parts are joined permanently, with the outer member heated or the shaft cooled. The interference is greater than the force fit
What is a transition fit?
When the mating parts are almost the same size
What are allowances?
The difference in dimension that is necessary to give a particular class of fit between two parts
What are tolerances?
The total amount of variation in the size of the part, which is the smallest size subtracted from the largest size
What are the British Standard System divided into?
21 Hole classifications with a capital letter
21 shaft classifications with a small letter
16 grades of accuracy with a number
What are the ISO standards divided into?
28 hole and shaft classifications
20 grades of accuracy
What is the Newall system?
Provides a range of clearance, transition and interference fits for sizes up to 12 in
What is hole basis?
A system of fits relating to basic or nomial hole size with fundamental deviation being H
What is shaft basis?
A system of fits relating to basic or nominal shaft size with fundamental deviation being h
What are shrink/heat fits in terms of class of fit?
Interference fits
What are press fits in terms of class of fits?
Transition fits
What does Bow mean?
When the centreline of a shaft is not straight. Only a small amount of bow is permitted, usually 1 in 600
What are the four dimensions covered in wear tables?
Dimension new
Permissible worn dimensions
Clearance new
Permissible worn clearance
What is dimension new?
The size of the shaft when it is new and shows relevant tolerances
What is permissible worn dimensions?
The size of which a part can wear to before it is rejected
What is clearance new?
The desired clearance in limit form
What is permissible worn clearance?
Relates to the maximum allowable clearance when reassembling the component
How can bow be checked?
With a straight edge and a feeler gauge along a shaft
Where are witness marks on some shafts?
Where the drive and driven flanges meet the shaft, provided by the manufacturer
What is ovality?
Applies to holes and shafts, and occurs as a result of the surface wearing through friction or linear movement
How can holes be checked for ovality?
GO/NO-GO gauge
Internal micrometers
Callipers
How can shafts be checked for ovality?
Using a micrometer or ovality gauge
What order should you test a shaft for bow and ovality
Should test a shaft for ovality before bow
