Hole and Shaft Tolerances

This topic in our Technical Drawing lessons section will cover tolerance fit systems. It will be about hole and shaft tolerances.

Fit Systems

Before covering fits and fit systems in tolerancing, it helps to learn some key tolerance terms. These are terms like basic size, maximum limit of size, minimum limit of size, lower deviation, upper deviation.

Basic Size (Nominal Size)

Tolerances placed on a dimension in production drawings show the lower and upper deviations. The actual dimension that should be kept in mind when choosing these deviations is the basic size. That is, in toleranced dimensions, the part’s underlying dimension is called the basic size. If a part is specified with limit dimensions, for example, 50 is the basic size here. 

Tolerance = T                      Basic Size = BS

Minimum Limit of Size = LLS       Maximum Limit of Size = ULS

Lower Deviation = EI        Upper Deviation = ES

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Zero line: In limit dimensions, the basic size is accepted as the zero line. For shafts and holes, when the maximum diameter dimension equals the basic size, the upper deviation is zero, and when the minimum diameter dimension equals the basic size, the lower deviation is zero.

Upper and lower deviation: After the part is machined, the difference between the final measured dimension and the basic size is called the deviation. In other words, the deviation is the difference between the limit dimensions and the basic size.
The difference between the basic size and the maximum limit of size is called the upper deviation, and the difference between the basic size and the minimum limit of size is called the lower deviation.

Maximum and minimum limit of size: The larger of the two limit dimensions is called the maximum limit of size, and the smaller one the minimum limit of size. The part absolutely must be machined between these two dimensions.

Tolerance: The difference between the maximum limit of size and the minimum limit of size is called the tolerance. This is actually the machining tolerance. Parts are machined according to this tolerance.

Minimum Clearance = LC              Maximum Clearance = UC
Minimum Interference = LI               Maximum Interference = UI

Fits

Interference and clearance: there’s a relationship (a fit) between the holes and shafts, or the inner and outer parts, that work together. These fits result in either an interference or a clearance. The interference that occurs when the hole diameter is smaller than the shaft diameter, and the clearance that occurs when the hole diameter is larger than the shaft diameter, both arise from the difference between the hole and shaft dimensions.

Since certain suitable tolerances are accepted for both the hole and shaft dimensions when making a fit, two separate values — one maximum and one minimum — are found for interferences and clearances.

Maximum interference is the difference between the shaft’s maximum dimension and the hole’s minimum dimension.
Minimum interference is the difference between the shaft’s minimum dimension and the hole’s maximum dimension.
Maximum clearance equals the difference between the hole’s maximum dimension and the shaft’s minimum dimension.
Minimum clearance, on the other hand, is the difference between the hole’s minimum dimension and the shaft’s maximum dimension.

The differences that occur between maximum interference and minimum interference, or between maximum clearance and minimum clearance, are called the Fit Tolerance. The fit tolerance equals the sum of the hole’s and shaft’s tolerances (the machining tolerances).

The values given for interferences and clearances produce a fit. In these fits, depending on the state of the workpieces, if the shaft moves inside the hole these are called clearance fits (running fits), and if it doesn’t move, these types of fits are called interference fits (fixed fits).

 Hole-Basis System:

In the hole-basis system, a single dimension is accepted for the holes of the various fits belonging to a given diameter. The desired interference or clearance is obtained by giving values larger or smaller than the hole dimension to the shaft dimension.

In the hole-basis system, the hole’s minimum dimension is always equal to the basic size.

Shaft-Basis System

In the shaft-basis system, the shaft dimension is kept constant for all fits belonging to the same diameter. To obtain a desired fit, interferences or clearances are found by giving the hole dimension values smaller or larger than the shaft dimension. In the shaft-basis system, the shaft’s maximum dimension is always equal to the basic size.

In both systems, the basic size marks the location of the deviations. In the hole-basis system, there’s no difference between the hole’s minimum dimension and the basic size. That is, the hole’s lower deviation is zero. In the shaft-basis system, the shaft’s upper deviation is zero. Here too, there’s no difference between the shaft’s maximum dimension and the basic size. For these reasons, when it comes to hole and shaft tolerances, the line that passes through the basic size and forms the basis for the deviations is called the zero line.

The Meaning of the Letters in Tolerances

Both the tolerances shown on drawings and the symbols written on gauges, and the position of a tolerance zone relative to the zero line, are also indicated with letters in the ISO tolerance system.

Hole-basis system (above)

The tolerance zone — the region between the maximum limit of size and the minimum limit of size — is shown with uppercase letters from A to Z, ZA, ZB, ZC for holes, and with lowercase letters from a to z, za, zb, zc for shafts.

In the hole-basis system, the upper deviation equals the hole’s tolerance and the lower deviation is zero; in the shaft-basis system, the lower deviation equals the shaft’s tolerance and the upper deviation is zero.

Shaft-basis system (above)

Based on this property, the letter H for holes shows that the lower deviation equals zero, while the letter h for shafts shows that the upper deviation equals zero. Since tolerance zones based on the zero line this way are marked with the letters H and h, the hole-basis system is always denoted with the letter H, and the shaft-basis system with the letter h. Shafts from a to zc are fitted to H holes in the hole-basis system, and holes from A to ZC are fitted to the h shaft in the shaft-basis system.

Meanwhile, the uppercase letters I, L, O, Q, V, W, Y and lowercase letters i, l, o, q, v, w, y aren’t used, to avoid any confusion.

The Numbers Next to the Letters

In the ISO tolerance system, to specify the tolerance to be given to any dimension, a grade number like F7, G9, H8, f8, g10, h7 is written next to the letter showing that tolerance zone. For example, h7 shows that the shaft’s tolerance is for an h fit in grade 7.

The figure above shows a comparison of three different tolerances, each with a few different grades, and their position relative to the zero line.

Showing Tolerances in Technical Drawing

In production drawings, tolerances for holes and internal dimensions are written with uppercase letters to the upper right of the dimension, and shaft tolerances are written with lowercase letters to the lower right (figure above). A tolerance table is also drawn at a suitable location on the drawing.

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In this article, we tried to cover hole and shaft tolerances and fit systems. We covered topics like the fits between holes and shafts, the meaning of the letters used in tolerances, and how tolerances are shown in technical drawing. See you in future articles.

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