ISO 286

ISO 286. It is an international standard from the International Organization for Standardization that defines the system of limits and fits for cylindrical parts and other features. The standard provides a code for tolerances and deviations to ensure the interchangeability and precise function of mechanical components in manufacturing and assembly. Its principles are fundamental to mechanical engineering design and are widely adopted in industries such as automotive manufacturing, aerospace engineering, and precision engineering.

Overview and Scope

The standard establishes a comprehensive system based on the concept of the basic size, which is the common nominal dimension for both a shaft and a hole. The scope of ISO 286 encompasses the definitions for tolerances, deviations, and the resulting fits between mating parts. It is applicable to cylindrical features and also to parallel surfaces where similar fit conditions apply, as seen in dovetail slides or prismatic guides. The system is intended to simplify design communication between engineers, draftsmen, and machinists across global supply chains, promoting economic efficiency in production. Its development was influenced by earlier national standards like DIN standards from Germany and practices from the Soviet Union.

System of Limits and Fits

The core of the standard is the classification of fits into three main categories: clearance fit, transition fit, and interference fit. A clearance fit always ensures a gap between the shaft and hole, crucial for plain bearings or piston assemblies in an internal combustion engine. A transition fit may result in either a clearance or a slight interference, often used for dowel pin locations in assembly jigs. An interference fit guarantees the shaft diameter is larger than the hole diameter, creating a force that requires press fitting or shrink fitting, commonly applied in mounting gears onto transmission shafts. The system uses a series of tolerance grades and fundamental deviation letters to mathematically derive the limits for any given basic size.

Fundamental Deviations and Tolerance Grades

Fundamental deviation is a key parameter, representing the deviation closest to the basic size for either a shaft or a hole. For holes, fundamental deviations are designated by capital letters such as A, H, or Zc, where H represents a hole with the lower deviation at zero. For shafts, lowercase letters like a, h, or zc are used, with h representing a shaft with the upper deviation at zero. The tolerance grade is a number (e.g., IT01, IT7, IT16) that defines the magnitude of the tolerance zone, irrespective of the fundamental deviation. These grades are part of the ISO system of limits and fits, with finer grades like IT6 used for precision ground spindles and coarser grades like IT12 for stamped components.

Designation and Application

A fit is designated by combining the hole tolerance symbol with the shaft tolerance symbol, such as H7/g6 or P7/h6. The H7/g6 designation indicates a clearance fit suitable for sliding fits in hydraulic cylinders, while P7/h6 indicates an interference fit for bearing housings. Engineers select fits based on functional requirements, lubrication needs, and thermal expansion considerations, as seen in turbine blade roots or camshaft journals. The standard includes extensive tabular data for common basic size ranges, which are referenced in engineering drawings and CAD software like SolidWorks and Autodesk Inventor.

Related Standards and Influence

ISO 286 is part of a family of geometrical product specifications standards. It is directly related to ISO 1, which defines the standard reference temperature for dimensional metrology, and ISO 2768, which provides general geometric tolerances. Its predecessor was ISO/R 286. The standard harmonizes with regional systems, including ANSI B4.1 in the United States, JIS B 0401 in Japan, and GOST 25347 in the Commonwealth of Independent States. Its principles underpin quality assurance systems like ISO 9001 and are taught globally in engineering curricula at institutions like the Massachusetts Institute of Technology and the University of Cambridge. The widespread adoption of ISO 286 facilitates international trade in capital goods and complex machinery.