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A system of fits is a systematically arranged series of fits with different assignments of internal and external dimensions, it takes into account cylindrical and flat fits.
The setting up of internationally standardized systems of fits made it possible to exchange ready-to-assemble component parts produced in different countries and to insert them without expensive subsequent mechanical treatment.
In countries with the metric system of units the worldwide effective ISA System of Fits has been adopted in addition to national systems:
ISA - International Federation of the National Standardizing Associations (founded in 1926)
ISO - International Organization for Standardization
3.1. Fundamental Problem
In the production engineering it is not possible to produce analogeous parts without any deviations from each other.
The actual size of the completed workpiece will always deviate from the size given in the drawing by some tenths, hundredths and thousandths of a millimetre.
To achieve a certain fit, though, the occurring dimensional variations have to be kept within certain limits.
These limits are given by the use of the elements.
Example:
The spindle of a vice may have greater dimensional variations than the spindle of an external micrometer.The permissible dimensional variations are given in the engineering drawing in order to make possible an exact manufacturing of the elements.
3.2. Dimensions and their Representation by the Example of a Shaft
Nominal size
Dimension (200 mm) given in the drawing without further specification in the millimetre unit.
Actual size
Actually obtained size of the completed element (200.1 mm) which equals the nominal size or shows slight deviations and can be read from the measuring instrument.
Figure 15 Nominal size
Figure 16 Actual size
Since a specified size described as “nominal size” can never exactly be found two dimensions are necessary between which the actual size may vary. These dimensions are called limit dimensions where the greater of the two is called maximum limit, and the smaller one is called minimum limit
Dimensions which are important for the function must, consequently, have additional specifications on the nominal size to determine the dimensional limits!
These additional specifications are given by the “dimensional variations”.
Dimensional variations
Permissible deviations from the nominal size, given in the drawing behind the nominal size as a number with the symbol + or - in the millimetre unit (+0.2 mm and -0.1 mm).
Figure 17 Nominal size with
dimensional variations
The combination of nominal size and upper dimensional variations results in the maximum limit 200.2 mm) and the combination of nominal size and lower dimensional variation results in the minimum limit (199.9 mm).
Tolerance (dimensional tolerance)
Range of the permissible dimensional variation from the nominal size, corresponds to the permissible gap of dimensions between maximum limit and minimum limit (0.3 mm).
The graphic representation of the tolerance limits between maximum and minimum limit is called tolerance zone.
All elements of a manufactured series the actual size of which lies within the required tolerance zone can be used for assembling.
Figure 18 Series corresponding
to the manufacturing requirements with the permissible tolerance
For what reason has the internationally valid ISA System of Fits
been
adopted?
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3.3. Representation of Fits
A fit can be recognized in an engineering drawing by the dimensional specification marked by fit sizes.
Fit size:
Nominal size with specifications of dimensional variations or nominal size with ISA symbol. Fit sizes are used to specify lengths and diameters.If the shaft is efficiently connected with a bore (gear hub), a fit results.
For each fit the dimensions of the diameters of shaft and bore have to be coordinated to obtain, according to the use, interference fits, clearance fits or the intermediate stage called transition fits.
Figure 19 Fit size
specification with dimensional variations
For this reason two tolerance specifications are necessary for each fit - for the external dimension and the internal dimension which refer to the same nominal size of the two mating parts.
In engineering drawings the representation of the dimensional specifications is very extensive, for assembled basic units the dimensional specification by numbers is very space-consuming. For this reason symbols of the Latin alphabet are used for tolerance specifications.
Capital letters (A to Z) are used for internal dimensions (bores), and small letters (a to z) are used for external dimensions (shafts).
In addition to the specification of the tolerance zone by a letter, a number is added containing a statement on the size of the tolerance zone. This number (1 to 16) is called “quality number”.
|
Quality 1 to 7 |
: no or small dimensional variation permissible - (for gauges) - small tolerate zone |
|
Quality 5 to 13 |
: medium dimensional variation permissible - (for fits) - medium-sized tolerate zone |
|
Quality over 13 |
: greater dimensional variations permissible - large tolerance zone |
Example:
Figure
Dependent upon nominal size there are dimensional specifications for fixed tolerance zones which can be read from an ISA table, if these dimensions are necessary.
Parts of an ISA Table
Dimensional Tolerances for Shafts
|
Range of the nominal |
Dimensional variations in micrometre | |||
|
size in mm |
d 11 |
e 8 |
h 11 |
k 6 |
|
over 6 |
- 40 |
- 25 |
0 |
+ 10 |
|
up to 10 |
- 130 |
- 47 |
- 90 |
+ 1 |
|
over 10 |
- 50 |
- 32 |
0 |
+ 12 |
|
up to 14 |
- 160 |
- 59 |
- 110 |
+ 1 |
Dimensional Tolerances for Bores
|
Range of the nominal |
Dimensional variations in micrometre | |||
|
size in mm |
D 11 |
E 8 |
H 11 |
K 6 |
|
over 6 |
+ 130 |
+ 47 |
+ 90 |
+ 2 |
|
up to 10 |
+ 40 |
+ 25 |
0 |
- 7 |
|
over 10 |
+ 160 |
+ 59 |
+ 110 |
+ 2 |
|
up to 14 |
+ 50 |
+ 32 |
0 |
- 9 |
According to given fit sizes the maximum and minimum limits can be determined from the table.
Example:
Fit size of a shaft:
|
Æ 10 e 8 |
tabular value of the dimensional variations:
-25 mm = -0.025 mm
-47 mm = -0.047 mm
|
maximum limit: |
10 - 0.025 = 9.975 mm |
|
minimum limit: |
10 - 0.047 = 9.953 mm |
fit size of a bore:
|
Æ 10 E 8 |
tabular value of the dimensional variations:
+ 47 mm = + 0.047 mm
+ 25 mm = + 0.025 mm
|
maximum limit: |
10 + 0.047 = 10.047 mm |
|
minimum limit: |
10 + 0.025 = 10.025 mm |
If the two parts are joined to form a fit, the specification reads as follows:
|
|
Figure 20 Fit size
specification with ISA symbols
3.4. Practical Use of the ISA System of Fits
The application of the standardized systems of fits has special advantages:
1. The fabrication of series-manufactured fits is possible also in case of separated single-part production.2. The spare part production for defective component parts with ISA fits is easy and can quickly be caried out.
3. Fits are partly produced and reworked with tools the dimensions of which correspond to the standardized system of fits.
4. The testing of fit sizes is performed with standardized gauges containing maximum and minimum limits in accordance with the standardized system of fits. This makes possible a time-saving testing of the quality of the fit without determination of the actual size.
Example:
A pulley shall get a centred bore with the fit size Æ 10 H 8.After drilling and countersinking the finishing is performed with a reamer Æ 10 H 8, and subsequently the fit size is tested with a plug limit gauge Æ 10 H 8.
Conclusion:
Coordinated dimensions of tool and testing instrument render possible an interaction of manufacturing and control process.
Figure 21 Tools and testing
means according to ISA standard
Hints on Testing:
To obtain exact test results it is necessary to take care to see to it that testing means and workpieces have the same reference temperature.For testing means a reference temperature of 20º C is given by the producer.
Testing means are precision instruments, they have to be protected against damages caused by blows, impacts and corrosion!
Which specifications have to be recognizable in engineering
drawings for fit
sizes?
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How is the specification of an ISA fit size
marked?
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What practical advantage does the ISA System of Fits have for
testing?
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Which hint has to be paid attention to during the
testing?
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