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Thursday, 26 July 2018

DIFFERENT TYPES OF FITS

Fits:
          The relationship between two parts, which are to be assembled, with respect to difference of their sizes, before assembly is called 'Fit'.
          It is the general term used to signify the range of tightness or looseness that may result from the application of a specific combination of allowances and tolerances in  mating parts.

There are four types of fits between parts, they are
          1. Clearance Fit
          2. Interferance Fit
          3. Transition Fit
          4. Line Fit

1. Clearance Fit:
          When an internal member fits in an external member (as a shaft in hole), in clearance fit there is always an air space or clearance between the shaft and hole. Such type of fit give loose joint. A clearance fit always have a positive allowance.
Clearance fit also sub-classified as
a) Slide fit
b) Running fit
c) Slace running fits
d) Loose running fits

2. Interference Fit:
          When the internal member is larger than the external member then there is always an actual interferance between those materials. This negetive difference between diameters of shaft and hole  is called interference. An interferance fit always have a negetive allowance.
          From the below example, the smallest size of the shaft is 2.2313" and the largest hole is 2.2305" so that least interference among the materials is 0.0008". Under maximum material condition the interference is 0.0018".  
Interferance fit can be sub-classified as
a) Shrink fit (or) Heavy force fit
b) Medium force fit
c) Tight fit (or) Press fit.

3. Transition Fit:
          It may result in either Clearence fit or Interference fit condition depending on the actual value of the individual tolerances of mating parts. This type of fit is used for applications where accurate location is important but either a small amount of clearence or interference is acceptable. 
Transition fit can be sub-classified as 
a) Push fit
b) Force fit
c) Wringing fit

4. Line Fit:
          The limits of size are so specified that a clearance or surface contact may result when mating parts are assembled.
ex: Shrink fit   



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