1. Fits and allowances
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B406

2. Content Interchangeability of parts Fits and allowances Metric fits
Calculation of fits
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3. Interchangeability of Parts
Increased demand for manufactured products -> the development of new production techniques.
Interchangeability of parts became the basis for mass-production, lowcost manufacturing
it brought about the refinement of machinery, machine tools, and measuring devices.
design for 100% percent interchangeability.
No part can be manufactured to exact dimensions.
Tool wear, machine variations, and the human factor all contribute to some degree of deviation from perfection.
It is therefore necessary to determine the deviation and permissible clearance, or interference, to produce the desired fit between parts.
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4. Interchangeability of Parts
Modern industry has adopted 3 basis approaches to manufacturing
The completely interchangeable assembly
any and all mating parts of a design are toleranced to permit them to assemble and function properly without the need for machining or fitting at assembly.
The fitted assembly
mating features of a design are fabricated either simultaneously or with respect to one another.
Individual members of mating features are not interchangeable.
The selected assembly
all parts are mass-produced, but members of mating features are individually selected to provide the required relationship with one another.
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5. Fits and allowances
In order that assembled parts may function properly and to allow for interchangeable manufacturing, it is necessary to permit only a certain amount of tolerance on each of the mating parts and a certain amount of allowance between them.
The fit between two mating parts is the relationship between them with respect to the amount of clearance or interference present when they are assembled.
An allowance is an intentional difference between the maximum material limits of mating parts. It is the minimum clearance (positive allowance) or maximum interference (negative allowance) between such parts.
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6. Fits
Types:
Clearance fit: a fit between mating parts having limits of size so prescribed that a clearance always results in assembly.
Interference fit: a fit between mating parts having limits of size so prescribed that an interference always results in assembly.
Transition fit: A fit between mating parts having limits of size so prescribed as to partially or wholly overlap, so that either a clearance or an interference may result in assembly.
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7. Terminology
Basic Size: the size to which limits or deviations are assigned.
Deviation: the algebraic difference between a size and the corresponding basic size.
Upper Deviation: the algebraic difference between the maximum limit of size and the corresponding basic size.
Lower Deviation: the algebraic difference between the minimum limit of size and the corresponding basic size.
Tolerance: the difference between the maximum and minimum size limits on a part.
Tolerance Zone: a zone representing the tolerance and its
position in relation to the basic size.
Fundamental Deviation: the deviation closest to the basic size.
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8. Terminology
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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9. Fits Running and sliding fits Drive and force fits Locational fits
Clearance fits
Transition fits
Interference fits
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10. Running and sliding fits
A special type of clearance fit.
These are intended to provide a similar running performance, with suitable lubrication allowance, throughout the range of sizes.
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11. Drive and force fits
A special type of interference fit, normally characterized by maintenance of constant bore pressures throughout the range of sizes.
The interference varies almost directly with diameter
The difference between its minimum and maximum values is small to maintain the resulting pressures within reasonable limits.
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12. Locational Fits
Locational fits are intended to determine only the location of the mating parts
Locational clearance fits:
are intended for parts that are normally stationary but that can be freely assembled or disassembled.
They run from snug fits for parts requiring accuracy of location, through the medium clearance fits for parts such as ball, race, and housing, to the looser fastener fits for which freedom of assembly is of prime importance.
Locational transition fits
compromise between clearance and interference fits when accuracy of location is important but a small amount of either clearance or interference is permissible.
Locational interference fits
used when accuracy of location is of prime importance and for parts requiring rigidity and alignment with no special requirements for bore pressure.
Such fits are not intended for parts designed to transmit frictional loads from one part to another by virtue of the tightness of fit these conditions are covered by force fits.
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13. Standard Inch Fits RC - Running and sliding fit
LC - Locational clearance fit
LT - Locational transition fit
LN - Locational interference fit
FN - Force or shrink fit
These letter symbols are used in conjunction with numbers representing the class of fit
Each of these symbols (two letters and a number) represents a complete fit, for which the minimum and maximum clearance or interference, and the limits of size for the mating parts
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14. Standard Inch Fits RC - Running and sliding fit
LC - Locational clearance fit
LT - Locational transition fit
LN - Locational interference fit
FN - Force or shrink fit
These letter symbols are used in conjunction with numbers representing the class of fit
Each of these symbols (two letters and a number) represents a complete fit, for which the minimum and maximum clearance or interference, and the limits of size for the mating parts
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15. Metric limits and fits
Establishes the designation symbols used to define specific dimensional limits on drawings
An "International Tolerance grade" (IT tolerance grade number) establishes the magnitude of the tolerance zone or the amount of part size variation allowed for internal and external dimensions alike
The smaller the grade number, the smaller the tolerance zone.
1-4: very precise grades intended primarily for gage making and similar precision work, although grade 4 can also be used for very precise production work.
5-16: represent a progressive series suitable for cutting operations, such as turning, boring, grinding, milling, and sawing.
12-16: intended for manufacturing operations such as cold heading, pressing, rolling, and other forming operations.
For work in other materials, such as plastics, it may be necessary to use coarser tolerance grades for the same process.
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16. International tolerance grade
Establishes the magnitude of the tolerance zone
Identified by prefix IT + grade number (e.g. IT6)
The smaller the grade the smaller the tolerance zone
Grade 1-4: very precise grades intented primarily for gage making and similar precision work
Grade 5-16: a progressive series suitable for cutting operations, such as turning, boring, grinding, milling and sawing
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
scan
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17. Tolerance symbol
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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18. Fundamental deviation - shafts
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19. Fundamental deviation - holes
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20. Fundamental deviation - holes
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21. Shaft basis fits system
Hole basis Fits System
Shaft basis fits system
the basic size will be the minimum size of the hole
More recently used
the basic size is the maximum shaft size.
more than two fits are required on the same shaft, the shaft basis fits system is recommended.
Clearance: H6/g5; H8/d9
Transition: H6/j5, H7/k6
Interference: H7/p6, H8/s7
Clearance: F7/h6; D10/h9
Transition: J6/h6, K7/h6
Interference: P6/h5, S7/h6
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22. Preferred metric fits
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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23. Clearance fit
scan
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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24. Transition fit
scan
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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25. Interference fit
scan
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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26. Fits on technical drawings
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27. Example
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28. Example
Ø20 H7/s7
𝐷= 18∙30 =23,24
𝑖=0,45 3 𝐷 +0,001𝐷=0, ,24 +0,001∙23,24
𝑖=1,3
𝑞= −1 =15,85
𝑇=𝑞𝑖=15,85∙1,3=20,6≈21 
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29. Example Hole: 𝐻 𝑚𝑖𝑛 =20 𝐻 𝑚𝑎𝑥 =20+𝑇=20+0,021=20,021 Shaft 𝐸 =35 𝜇
𝑆 𝑚𝑖𝑛 =20+𝐸=20+0,035=20,035
𝑆 𝑚𝑎𝑥 =20+𝐸𝑇=20+0,035+0,021=20,056
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30. Example I 𝑚𝑎𝑥 =𝑁𝐹= 𝐶𝑆 𝐹𝐻 − 𝐹 𝐴𝐻 =20,056−20=0,056
𝐼 𝑎𝑣𝑔 =𝑀𝐹= 𝑁𝐹+𝐾𝐹 2 = 0,056+0,014 2 =0,035

31. Summary Fits and allowances Metric fits Calculation of fits
Next week: bolted connections

32. Thank You for Your attention!