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ISOFits
ISO 286:1988 System of Limits and Fits
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ISO 286 standardizes the way designers assign and designate tolerances
to holes and shafts and, in particular, mating hole / shaft
assemblies. It consists of a number of formula-defined
tables and the rules for applying them. Below I offer a
layman's explanation of the ISO system of "Limits and Fits":
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The system consists of three primary tables:
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The "IT Tolerance"
table. This table indicates the total
tolerance to be applied to either a shaft or a hole
diameter. The range of sizes covered is
divided into a number of groups spanning from 0 to
3150 mm. For each group of sizes, as many as
20 different tolerance "grades" are specified,
labeled IT01, IT0, IT1, IT2, and so on up to IT18.
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The "Shaft Fundamental Deviations" table. This table
indicates the deviation or offset from basic
(nominal) size; the way in which the tolerance (see
above) is applied to the shaft diameter. Once
again, the range of sizes is divided into groups
spanning 0 to 3150 mm. For each of these size
groupings, there are 28 deviation "zones", labeled
"a" to "zc".
Given a shaft of "Basic Size" 13 mm, a deviation
zone of "d" (-50 um), and a tolerance grade of IT8
(27 um), we end up with a shaft size/tolerance
designated as "13 d8" which will measure 12,950 to
12,923 mm in diameter, the deviation of -50 um
having been applied to the basic size and then the
tolerance of 27 um applied negatively (because this
is a shaft; it would be applied positively for a
hole). This shaft has a total tolerance of 27
um, an "upper deviation" (abbreviated as "es")
of -50 um and a "lower deviation" (ei) of -77 um.
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The "Hole Fundamental Deviations" table works in essentially
the same way except the zones are specified as "A"
through "ZC". In fact, throughout the system,
hole features are identified with upper-case
characters while shaft features are identified with
lower-case characters. A hole of basic size
"13 D8" will have a diameter of 13,050 to 13,077 mm,
an "ES" or upper deviation of 77 um and an EI of 50
um.
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A "Fit" consists of a hole and shaft combination which has
been designed taking into account goals such as the amount
of clearance or interference ("press fit") required and the
relative manufacturing techniques and costs associated with
producing the hole and shaft. Fits are designated as
Clearance, Interference, or Transition:
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For a Clearance fit, the hole, even at its smallest possible
diameter, is always larger than the shaft even when
at its maximum diameter.
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An Interference fit is characterized by a hole which is always smaller
than it's mating shaft.
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A Transition fit will, depending on the actual diameters of the hole
and shaft, be either a clearance or an interference
fit. At one range of extremes (hole near
maximum size and shaft near minimum), a clearance
fit exists. At the other extreme (hole near
minimum, shaft near maximum), an interference fit
occurs.
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A "Fit" may be (but does not need to be) "Hole basis" or
"Shaft basis". This would indicate that one side of
the hole (or shaft) is dimensioned at its nominal or basic
size. In other words, the fundamental deviation is
zero. The "h" and "H" deviation zones are the basis
zones.
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Given all of the above, a typical hole/shaft "fit" might be designated as "22
H11/g9". This fit would be a "Clearance, Hole-basis"
fit and look like the following:
| Feature |
Specification |
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| Hole Designation: |
22 H11 |
| Total tolerance: |
130 um |
| ES (upper deviation): |
130 um |
| EI (lower deviation): |
0 um |
| Limits of size (LMC / MMC): |
22,130 / 22,000 mm |
| |
|
| Shaft Designation: |
22 g9 |
| Total Tolerance: |
52 um |
| es (upper deviation): |
-7 um |
| ei (lower deviation): |
-59 um |
| Limits of size (LMC / MMC): |
21,993 / 21,941 mm |
Do some math and we see that this fit has a maximum
clearance ("Smax") of 189 um and a minimum clearance
(Smin) of 7 um. |
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