ISOtoleranssijärjestelmä

ISOtoleranssijärjestelmä (ISO tolerance system) is a standardized method for specifying permissible variations in the dimensions of manufactured parts. This system is crucial for ensuring interchangeability and proper fit between components produced by different manufacturers or at different times. The International Organization for Standardization (ISO) developed and maintains this system, which is widely adopted globally.

The core of the ISO toleranssijärjestelmä lies in defining acceptable limits for a dimension. These limits are expressed as a nominal size, followed by an upper deviation and a lower deviation. For example, a dimension might be specified as 25 mm +0.05 / -0.02. This means the actual dimension must be between 25.05 mm and 24.98 mm.

The system categorizes deviations into two main types: fundamental deviations and tolerance grades. Fundamental deviations define the position of the tolerance zone relative to the nominal size. Tolerance grades, denoted by numbers (e.g., IT5, IT6, IT7), specify the size of the tolerance zone itself, with lower numbers indicating tighter tolerances.

Hole and shaft systems are common applications of the ISO toleranssijärjestelmä. In the hole system, the basic hole is taken as the nominal size, and the shaft's tolerance is varied to achieve the desired fit. Conversely, in the shaft system, the basic shaft is the nominal size, and the hole's tolerance is adjusted. This systematic approach facilitates mass production and ensures that parts will function correctly when assembled.