The core purpose of any limit system is to offer guidance on factors that are inherently complex and difficult to analyze. In situations involving mating parts with relative motion, variables such as load conditions, rotational speed, lubrication methods, and environmental factors present challenges that are not easily quantified. A detailed analysis for every scenario would rarely be practical or cost-effective.
Therefore, the designer must rely heavily on experience or seek guidance from others. The main objectives of any general system of standard fits and limits are to assist the user in
✔ Choosing appropriate functional clearances and interferences for a specific application or type of fit.
✔ Defining tolerances that strike a practical and cost-effective balance between fit precision and manufacturing costs.
ISO (International Organization for Standardization) System of Limits and Fits.
The ISO system applies to holes and shafts ranging from the smallest sizes up to 3150 mm. For sizes beyond this range, a wide variety of fits are available, with tolerance grades spanning from very fine to broad tolerances. The standard provides the option to use either a hole-based or shaft-based system, depending on the application requirements.
For any given basic size, there are 28 different hole sizes. These are created by progressively increasing the size (oversized holes) and progressively decreasing the size (undersized holes) relative to the basic size. The variation from the basic size for each hole is determined by the fundamental deviation. These size differences define the required fit between mating parts.
The "28 holes" are designated by the capital letters: A, B, C, CD, D, E, EF, F, FG, G, H, J, JS, K, M, N, P, R, S, T, U, V, X, Y, Z, ZA, ZB, and ZC. Each of these 28 holes is associated with a selection of 18 tolerance grades, labeled from IT0 1, IT0, and IT1 to IT16. The chosen tolerance grade determines the level of manufacturing accuracy required for the part.
Similarly, for shafts, there are 28 designated sizes for a given basic size, represented by lowercase letters from 'a' to 'zc'. Each of these shafts is also associated with 18 tolerance grades, which are designated in the same manner as for the holes.
The general arrangement of holes and shafts is shown in the figure below.
For shafts designated from 'a' to 'g', the upper deviation lies below the zero line, indicating that these shafts are undersized relative to the basic size. For shafts from 'j' to 'zc', the upper deviation is above the zero line, meaning these shafts are oversized relative to the basic size. This arrangement ensures that the corresponding fits between holes and shafts are accurately defined according to the required tolerances.
Similarly, the lower deviations for holes designated from 'A' to 'G' are above the zero line, indicating that these holes are oversized relative to the basic size. For holes from 'J' to 'ZC', the lower deviations are below the zero line, meaning these holes are undersized relative to the basic size. This variation in deviations helps define the specific fits between the holes and shafts based on the tolerance grades.
The shaft designated as 'h', where the upper deviation is zero, is referred to as the Basic Shaft. Similarly, the hole designated as 'H', where the lower deviation is zero, is known as the Basic Hole. These serve as reference points, with all other holes and shafts being specified in relation to these basic sizes.
The general trend for the shafts is that for shafts 'a' to 'g', both the upper and lower limits fall below the zero line. As a result, these shafts are undersized relative to the basic size and typically result in clearance fits when paired with corresponding holes. In these fits, there is always a gap or clearance between the shaft and the hole, ensuring easy assembly and movement.
For the 'h' shaft, the upper limit coincides with the basic size, meaning this shaft is manufactured to the exact nominal size. When assembled with the 'H' hole, which has a lower deviation of zero, this shaft will provide close running fits. These fits offer minimal clearance, ensuring a precise and smooth connection between the shaft and the hole, suitable for applications where accuracy and tight tolerances are essential.
For 'j' shafts, the tolerance zone is spread both above and below the zero line, resulting in transition fits when assembled with corresponding holes. This means the fit can either provide a slight clearance or a slight interference, depending on the actual size of the shaft and hole, allowing for a range of fitting conditions.
For 'k' to 'zc' shafts, the entire tolerance zone lies above the zero line (the basic size), meaning these shafts are oversized relative to the basic size. As a result, these shafts will always provide interference fits when paired with holes. In these fits, the shaft is larger than the hole, creating a tight, press-fit connection that ensures a firm and secure assembly, often used for applications requiring high strength and rigidity.
In conclusion, the limit system helps designers navigate complex factors like load, speed, and lubrication by providing standardized guidance. The ISO System of Limits and Fits defines 28 hole and 28 shaft sizes with 18 tolerance grades, balancing precision and cost. These tolerances create clearance, transition, or interference fits, ensuring the right fit for each application. Designers use this system to select the best fit for efficient and reliable assembly.