Fit types and tolerances – An overview

Tolerances and fits are critical in the manufacturing industry. In the technical sense, tolerances define a permissible deviation from a defined property, such as a nominal dimension. They ensure that products and components meet the required quality standards.

Fits and tolerances

Manufacturing and design engineering differentiate tolerances into dimensional tolerances, form tolerance and positional tolerances. Dimensional tolerances define the permissible deviation of a component from an expected dimension (nominal dimension) by a maximum permissible upper limit and a maximum permissible lower limit. Each component is manufactured with a certain tolerance. If two components are to be combined with each other, the tolerance fields of both components interact. The connection between two or more design elements is called fit. For example, a round fit is the connection of the shaft and bore, which are matched by their dimensioning, dimensions and tolerances. The fit refers to the mechanical contact point at which the elements interact.

Types of fits

The following fit types exist:

• Clearance fit
• Interference fit
• Transition fit

Clearance fit

Clearance fits are fits iaw. DIN-EN-ISO 286, wherein the minimum dimension of the bore is greater than or equal to (in the limit case) the maximum dimension of the shaft (c). This always creates play when assembling the bore and shaft. A clearance fit may be necessary in some cases to account for thermal expansion, mounting or operating conditions. In bearing applications, for example, a clearance fit always results in the rolling elements or sliding surfaces having freedom of movement in the bearings. Examples are:

• H8/d9 - lots of clearance, presence of a gap
• H7/g6 - low clearance, narrow gap

Interference fit

Interference fits, also called press fits, are a fitting method used in mechanical engineering designs. A component is intentionally manufactured with oversize so that it fits tightly into the base dimension of the mating component.

This press fit provides a fixed, permanent connection between a shaft and a bore. Joining is only possible with great force and, if necessary, additional heating. An example is the fit H7/p6, which is joined under pressure.

Transition fit

Transitional fits are an intermediate variant of clearance fit and oversize fit. This means that a clearance fit or a press fit results depending on where the actual measurements are located in the tolerance field. Transition fits can no longer be joined manually but can e. g. be joined under slight pressure (hammer). An example is H7/n6.

Fit systems

Fit systems have been introduced to reduce the number of tolerances and to make the use of tolerances more practical in manufacturing.

Basic bore

Because it is easier to produce the outer diameter than the inner diameter, the principle of the bsic bore is often applied due to its simplicity and cost efficiency. The bore is always manufactured with the same tool and tolerated iaw. the ISO tolerance system. DIN EN ISO 286-1 and DIN EN ISO 286-2 provide international tolerance standards for dimensions and fits to ensure that components are manufactured accurately and meet quality standards. The diameter of the bore is tolerated iaw. the ISO tolerance system, wherein the corresponding shaft is assigned to any tolerance field position. Basic bores are marked with capital letters, such as H7.

Basic shaft

For the basic shaft system, the tolerance refers to the shaft. The tolerance is determined iaw. the ISO tolerance system within an h-field. The basic shaft is also defined in DIN EN ISO 286-1. The associated tolerance is shifted to the bore. Basic shafts are specified with lower case letters, e.g. h7.

Basic shafts are less common, but are for example used on transmissions with long shafts or when a corresponding shaft is specified and is also the guide element.

More information on dimensional tolerances can be found in the blog Fundamentals of dimensional tolerances and the selection of fits.

Various tolerances

Tolerances are permissible deviations from characteristics of a technical component or a functional group; inside said tolerances, the functional reliability of the component or functional group is ensured. Geometric tolerances, which relate to dimensions, shapes, positions, waviness, and roughness are particularly important for the design-engineering process.

But why are tolerances necessary? Components are shown to scale on engineering drawings. Theoretically, the nominal dimensions can be read there. However, tolerances must be included because in reality there will always be deviations from the nominal dimensions when components are manufactured (100% manufacturing accuracy is not possible). These are generally determined on a function-specific basis, i.e. the future use, environmental conditions and the connection to other components (tolerance chains) are already taken into account in the design. When specifying tolerances, either the tolerance fields can be specified or the permissible deviations (dimensions) can be specified directly. For form and position tolerances, the tolerated parameters are determined by the corresponding symbols iaw. the standard.

Applicable standards for tolerances (as of 04/2024) are e.g.:

• DIN ISO 2768-1 and DIN EN ISO 22081: Regulations on general tolerances
• DIN EN ISO 1101: Regulations on form and position tolerances
• DIN EN ISO 5459: Regulations on references and reference systems
• DIN EN ISO 8015: Determining and specifying tolerances

General tolerances

General tolerances apply to all dimensions for which a tolerance is not explicitly specified. DIN ISO 2768-1 regulates the general tolerances for lengths and angle dimensions, DIN EN ISO 22081 for shape and position tolerances. An example callout on an engineering drawing can be, for example: ISO 2768-mf.

For example, there are the following accuracy classes for length and angle dimensions:

• f (fine), used e.g. in precision engineering
• m (medium), typical machine shop classification
• c (coarse), used e.g. for castings
• v (very coarse), used e.g. for coarse woodworking

Manufacturing tolerances

In manufacturing, tolerances allow components to be interchangeable, provided they were produced within the same tolerances. This is also goes hand-in-hand with manufacturer independence. Manufacturing tolerances form the basis for mass production.   Depending on the application, it may be useful for the design to either reference the upper or the lower limit dimension. As a result, when rework is required, adjustments can be made accordingly upwards or downwards without running the risk of exceeding the manufacturing tolerance. For example, it makes sense to reference the lower limit dimension on bores, and for example to reference the upper limit dimension on shafts.

Dimensional tolerances

Dimensional tolerances are dimensional specifications, e.g. by the design engineer, that must be observed to ensure that the design works, e.g. 110 mm (-5 mm, +10 mm). The tolerances indicate the maximum permissible deviations (up/down) from the nominal value. This can be percentage specifications or maximum deviations.

The upper or lower dimensional tolerance is calculated from the difference of the permitted largest dimension (upper limit dimension, maximum dimension) and the smallest dimension (lower limit dimension, minimum dimension). The tolerance field is within these limits. The more precision is specified by the tolerance, the more expensive the manufacturing process becomes. Tolerances should therefore generally not be selected too narrow.

Ball bearing tolerances

The specification of the tolerance class can be used as a simple gauge for the rolling accuracy of a rolling bearing (e.g. radial bearing, axial bearing). As a Japanese manufacturer, MISUMI supplies its products in tolerance classes iaw. the Japanese standard JIS B0401. In the DIN or ISO standard range, the JIS B 1514 standard covers the ISO 492, ISO 199 and DIN 620 standards for the corresponding bearing types. The bearing accuracy can be selected, for example, in the tolerance classes 2 (P2), 4 (P4), 5 (P5), 6 (P6) and 0 (P0) iaw. JIS B 1514 (specs shown in parentheses are iaw. DIN 620). Bearing class 2 (P2) refers to the highest-quality bearings, and to the more cost-effective bearings with the larger tolerances rising up to class 0 (P0).