LIMITS and FITS

In present - day mass production it would be uneconomical to attempt to produce components

whose sizes are exact. On the other hand, the requirement that batches of mating parts should be
interchangeable makes it necessary to control the variations in size which may result from
differing qualities of workmanship, worn tools, etc.
The error that will be tolerated must be indicated on the detail drawing from which the machine
will work. As an example, consider the two mating parts shown. The theoretical basic size of
40mm should be dimensioned using limits as shown on the right.

40 mm 39.98
39.95
40.05
40.02

For each part the actual finished size must lie between its upper and lower limits. The difference
between a pair of size limits is called the tolerance which should be as large as possible to ease
production and so keep down costs.

100

80
UNITS
OF 60
COST
40

20

0.1 0.2 0.3 0.4 0.5

TOLERANCE (mm)
Bilateral and unilateral tolerances

Alternative ways of tolerancing dimensions are:

1. Bilateral tolerance allow variations above and below the basic size and are both positive
and negative.

2. Unilateral tolerances allow variations in only one direction from the basic size and are
either wholly positive or wholly negative.
Fits between mating parts - To ensure that a design will function correctly, its component parts
must fit together in a predictable manner.

It is customary to show the shaft and hole in contact at the bottom so that all the clearance appears
at the top.
Basic size is the required theoretical size and is the same for both members of a fit.

Zero line represents the basic size and is the line to which all deviations are referred.

Deviation is the amount by which a size limit differs from the basic size. Taken to be positive
when measured upwards from a horizontal zero line and negative when measured downwards
from a horizontal zero line.

Tolerance is the amount by which a dimension is allowed to vary in order to ease production.
It is the tolerance that determines cost.

Clearance is the difference between the size of the hole and the size of the shaft when the hole
is larger than the shaft. For there to be a clearance, the shaft must be free in the hole.

Interference is the difference between the size of the hole and the size of the shaft when the
shaft is larger than the hole. If there is interference the shaft will be fixed in the hole.

Allowance is the difference between the low limit for the hole and the high limit for the shaft.
Will be positive when there is clearance, and negative when there is interference.

Tolerance zone is a graphical representation of the tolerance and is positioned with reference
to the zero line. For a hole, the tolerance zone is shown by a cross-hatched area, and for a shaft
by a blackened area.
Types of fits
For general engineering work these are three types of fits between mating parts.
1. Clearance fit in which the shaft is always smaller than the hole into which it fits,
allowance always positive. Used where one of the mating parts has to rotate or slide in the
other.

2. Transition fit in which the shaft may be either bigger or smaller than the hole into which
it fits, allowance may be positive or negative. Assembly of the mating parts may require
slight pressure.

3. Interference fit in which the shaft is always bigger than the hole into which it fits,
allowance always negative. Assembly of the mating parts requires pressure and/ or heat.

Size limits
When determining the size limits of two mating parts either the hole or the shaft is used as the
basis. In the hole- basis system, the diameter of the hole is considered to remain constant and
the different fits are obtained by varying the diameters of the shaft.
Alternatively, in the shaft- basis system, the diameters of the shaft is considered to remain
constant and the different fits are obtained by varying the diameter of the hole.

The hole -basis system is preferred because, whereas shafting can be machined to any
required sizes, holes are usually produced by tools having fixed sizes. However, there are cases
where it is better to adopt the shaft-basis system. Such a case, occurs where a single shaft has to
carry a number of different components such as bearings, pulleys, collars.

Selected ISO fits: hole basis

The ISO system of fits provides a great many hole and shaft tolerances to cater for a very wide
range of conditions, however, experience shows that the majority of fit conditions required for
normal engineering products can be provided by a quite limited selection of tolerances. The
following selected hole and shaft tolerances are commonly applied.

Hole tolerance: H11, H9, H8, H7

Shaft tolerance: c11, d10, e9, f7, g6, h6, k6, n6, p6, s6.

1. Clearance fits
• Easy running: H11 - c11, H9 – d10, H9 – e9. For bearings where an appreciable clearance
is permissible, for example, several bearings in line, as on a camshaft.
• Close running: H8-f7, H7-g6, The amount of clearance is small and should not be used for
continuously running bearings. Used, for example, in precision sliding- spigot fits for journal
bearings.
• Slide: H7-h6. This is the closest available clearance fit. The upper shaft limit of the fit is
zero, however, there will be a slight clearance present. This fit is widely used for non-
running assemblies such as location fits and splined shafts.

2. Transition fits

• Push: H7-k6. This is a true transition fit with virtually no clearance, recommended for
location fits where a slight interference is required, for eliminating vibrations, dowels and the
inner ring of ball bearings.
• Drive: H7-n6. The grade of fit is suitable for tight assembly fits and gives, in effect, an
interference fit when the fitting surfaces are long and errors of straightness make the fit
noticeably tighter.
The table shows the range of fits divided into the three main classes - clearance, transition,
interference- for nominal sizes from below 3mm to 250mm.

3. Interference fits
• Light press fit: H7-p6. This is a slight interference fit. The amount of interference, although
small, is sufficient to give ferrous parts a press fit which can be assembled and dismantled
without overstraining the parts.
• Press fit: H7-s6. This is a definite interference fit and is used for fixing components such as
bearing bushes.
To obtain size limits from symbol for fit The symbol designating a particular fit is made up of
numbers and letters. The numbers indicate the tolerance grades and the letters the derivations
- capital letters for holes and small letters for shafts.
Take as an example an 85mm diameter hole designated H8. This means that the hole is given an
H deviation and a grade 8 tolerance. From the table on the previous page, the lower deviation is
zero and the upper deviation is +54 units (0.054mm), which means that the grade 8 tolerance is 54
units.
Similarly, the mating shaft could designated f7. From the tables, for an 85mm diameters shaft,
the upper deviation is - 36units (-0.036mm), and the lower deviation is -71units (-0.071mm),
which means that the grade 7 tolerance is 35units. The resulting tolerance diagram shown in
Fig.1, shows that mating the 85H8 hole with the 85f7 shaft, gives close running clearance fit,
which is indicated by the 85H8-f7, (see Fig.2). Fig.3 is a diagrammatic representation of the
corresponding size limits which can be obtained from the tolerance diagram as follows:

HOLE:

Higher limit = 85 + 0.054 = 85.054mm

Lower limit = 85 + 0.000 = 85.000mm

SHAFT
Higher limit = 85 - 0.036 = 84.964mm
Lower limit = 85 - 0.071 = 84.929mm

Now consider the light press interference fit, H7-p6, for a diameter of 20mm. From the table, for
the hole, the lower deviation is zero and the upper deviation is +21 units which is zero and the
upper deviation is +21 units which means that the grade 7 tolerance is 21 units. Also from the
table, for a 20mm diameter shaft, the upper deviation is +35units and the lower deviation is +22
units, which means that the grade 6 tolerance is 13 units. The tolerance diagram for this fit is
shown in Fig.4. The mating pair would be dimensioned as shown in Fig.5. Fig. 6 is a
diagrammatic representation of the size limits which are obtained by making use of the tolerance
diagram:
HOLE:
Higher limit = 20 + 0.021 = 20.021mm
Lower limit = 20 + 0 = 20.000mm

SHAFT:
Higher limit = 20 + 0.035 = 20.035mm
Lower limit = 20 + 0.022 = 20.022mm

Who decides the type of fit required?

When a new project is begun, the designer draws a layout, to illustrate this thoughts and ideas.
During this layout stage, each individual part is considered for the strength, material and
functional requirements, this includes the fit of mating parts. A good designer will indicate, by
means of the appropriate symbol, the type of fit he requires.