Metrology, standardization and
certification
Theme 3: Types of fits in Hole and Shaft systems
Lecture plan:
1. Types of fits
2. Hole and shaft systems
3. ISO code system for tolerances on linear sizes
Types of fits
Fit: the nature of connections details, determined by the difference in their sizes
before assembly
Depending on the relative motion of mating parts or their resistance to relative
displacement fits is divided into three types:
1. Clearance fit
2. Interference fit
3. Transition fit
Clearance (S)- the difference between the sizes of the hole and shaft, if the hole size
is larger than the size of the shaft.
Interference (N) - the difference between the shaft and the hole sizes before
assembly, if the shaft size is larger than the hole size.
Clearance fit Interference fit
Clearance fit
Clearance fit: the fit, in which the gap is always formed in the connection, i.e.
minimum limit of size of hole larger than the maximum limit of size of shaft or equal
to it
Schematic image with clearance fit
TD – diameter tolerance of the hole, Td – diameter tolerance of the shaft,
Smin - the smallest clearance; Smax – the largest clearance
Interference fit
Interference fit: the fit, in which always formed interference in the connection, i.e.,
the maximum limit of size of the hole smaller than the minimum limit of size of the
shaft or equal to it
Schematic representation of the interference fit,
TD – diameter tolerance of the hole, Td – diameter tolerance of the shaft,
Nmin – the smallest interference, Nmax – the largest interference
Transition fits
Transition fit: the fit in which it is possible to obtain a clearance or interference in the
joint.
There are the following types of transition fits:
1. With the most probable interference in the joint.
2. With the most probable clearance in the connection.
3. With equiprobable clearance and interference in the connection.
Transition fit with the most probable interference in the joint.
Transition fits
Transition fit with the most probable clearance in the joint
Transition fit with equiprobable clearance and interference in the connection
Function of transition fit is to ensure the accuracy of the centering of the elements of the
parts. (EXAMPLE: Connection of pulley or toothed wheel with a gear shaft)
Hole and shaft-basis systems of fits
b)
Fits with the same clearance
This "freedom" of choice is uneconomical. If any tolerance zones will be appointed in
the design, of such zones may be countless. But this means that it is practically
impossible to produce centrally for market dimensional processing tool for making
holes (drills, countersinks, reamers) - the tool directly forming a size.
A tool for making holes
а) b)
A tool for making holes,
а) drill b) reamer
Hole and shaft-basis systems of fits
Basic hole - hole lower deviation is zero
Basic shaft - shaft upper deviation is zero
Tolerance zone of the hole for clearance fit
Tolerance zone of the hole for transition fit
Tolerance zone of the hole for interference fit
Fits in shaft-basic system
Tolerance zone of the shaft for interference fit
Tolerance zone of the shaft for transition fit
Tolerance zone of the shaft for clearance fit
Fits in hole-basic system
Example of the fits in hole-basic and
in shaft-basic system
Fit in Fit in
hole-basic shaft-basic
system system
1-body; 2 – shaft; 3 – inner bearing ring; 4– outer bearing ring
ISO code system for tolerances on linear sizes
IS0 286-1 (2010) Geometrical product specifications (GPS) - ISO code system for
tolerances on linear sizes - Part 1: Basis of tolerances, deviations and fits.
IS0 286-2 (2010) Geometrical product specifications (GPS) - ISO code system for
tolerances on linear sizes - Part 2: Tables of standard tolerance classes and limit
deviations for holes and shafts.
Tolerances and fits system - a set of rows of tolerances and fits, logically constructed on
the basis of production experience, experimental researches, theoretical generalizations
and executed as a standard
This part of ISO 286 establishes the ISO code system for tolerances to be used for linear
sizes of features of the following types: a) cylinder; b) two parallel opposite surfaces.
The main features of the system of tolerances and fits:
1. Size intervals.
2. Standard tolerance factor.
3. Tolerance grades.
4. Tolerance zones of holes and shafts.
5. Hole-basic and shaft-basic fit systems.
6. Normal temperature.
ISO code system for tolerances on linear sizes
Size intervals
In the world there are restrictions on the use of the size of the values inherent in the
concept of preferred numbers and ranks of preferred numbers, i.e standardized values to
which it is necessary to round the calculated values. This approach makes it possible to
reduce the number of sizes of parts and components, the amount of dimensional cutting
tools and other technological and measuring equipment. Preferred number are the same
all over the world and are members of the geometric progression with ratio: 1.6; 1.25;
1.12; 1.06
5 10 20 40
10 1,6; 10 1,25; 10 1,12; 10 1,06.
For example: for R5 (denominator 1.6) take values from the set:R5: ... 10; 16; 25; 40;
63; 100; 160; 250; 400; 630, etc.
In R10 (denominator 1.25) take values from the set:R10: ... 10; 12.5; 16; 20; 25; 31.5;
40; 50; 63; 80; 125; 160; 200; 250; 315; 400; 630, etc
ISO 286 standard provides 13 intervals of sizes in the range of 1 to 500 mm, within
which tolerances are constant. These intervals are called basic. Intervals increase along
with the size, accounting for an approximate geometric progression with ratio 1.6
Standard tolerance factor
Standard tolerance factor (i) - a measure characterizing parts manufacturing complexity
depending on its size
For sizes from 1 up to 500 mm:
i 0, 45 3 Dm 0, 001 Dm ,
i - standard tolerance factor, m
Dm- geometric mean extremes sizes of each
interval, mm
Dm D min D max .
0,001 Dm - measurement error, m
Value of Tolerance depends on the standard tolerance factor (i) and quantity of these
factor (a): T= a · i
For example: The value of Tolerance for quality class 7 is taken equal to 16 units of
standard tolerance factor. IT7 = 16i
Tolerance grades
In order to meet the requirements of various products for accuracy of the components,
the ISO system implements 20 grades of accuracy, which are called tolerance grades.
The bigger is grade number, the bigger is tolerance value for a given dimension.
Tolerance grades is denoted as IT01, IT0, IT1, IT2….up to IT18.
From IT1 up to IT4
From IT5…IT14 up to IT 18
Tolerance zones for holes and shafts
Fundamental deviation - one of two deviations (upper or lower) that is used to
determine the position of the tolerance zone relative to the zero line (deviation closest
to the zero line)
Hole There are 28 deviations of holes
and 28 deviations of shaft.
Basic size
The tolerance zone is formed like
a combination of fundamental
deviation and grade of tolerance.
For example:
for the shaft: h6, g6, p6,
Basic size
Shaft and for the hole: H7, F8, JS6
Fits in hole and shaft-basic systems
Fits in hole-basic system - fits, in which the various clearances and interferences is
formed by combination of various tolerance zones of shafts with one basic hole.
Fits in shaft-basic system - fits, in which the various clearances and interferences is
formed by combination of various tolerance zones of holes with one basic shaft.
Usually, the fits is designated in the form of a fraction. Tolerance zone of hole is always
indicated in the numerator, and the shaft tolerance zone - in the denominator
Example: Ø20 H7/g6 - fit in hole-basic system; Ø20 G7/h6 - fit in the shaft-basic system
Graphic image of a clearance fit in the Graphic image of a clearance fit in the
hole-basic system (Ø20 H7/g6) shaft-basic system (Ø20 G7/h6)
Normal temperature
In the world considered that the dimension values, which are listed in the
regulations pertain to the details of when their temperature of 20 ° C. If the parts
are different from the temperature of 20 ° C, it is necessary take into consideration
the amendment:
Δl ≈ l · (α1 · Δt1 - α2 · Δt2),
Δl - temperature error;
l - the measured size in mm;
Δt1 - the difference between the parts temperature and the temperature of 20 ° C.
Δt2 - the difference between the temperature of measuring means and a normal
temperature of 20 ° C.
α1, α2 - temperature coefficients of linear expansion of the materials and details of
the measuring means, C-1.
Designation of tolerances and fits in the drawing
ISO
Hole Shaft Fit
GOST
Thank you for attention
