Engineering Standardization, Tolerances, and ISO System Fits

Standardization

Standardization is defined as a set of technical requirements that specify, unify, and simplify aspects of industrial processes. This standardization affects:

• a) The form, composition (wt% specifies the components), dimensions, and physical and chemical properties of materials.
• b) Terminology and symbols (standard symbols must be used for universal interpretation).
• c) Calculation methods, materials testing, measurement, and usage.

Certification is the action carried out by a recognized, independent organization, testifying that a company, product, process, service, or person meets the requirements defined in standards.

A standard is a document containing technical agreements made between manufacturers, users, and technical groups over a certain period. It depends on a technical committee for approval. The standard serves as a model for judging a product.

UNE: This may be an adaptation of another standard or document deemed relevant. The scope of UNE is national (state-level).

ISO: These are internationally recognized standards, meaning they must be met and recognized worldwide.

Tolerances and Fits

Tolerance

Tolerance is a dimensional quantity indicating the acceptable range of dimensions for manufacturing a piece. Formulas include: CM = C + ds, cm = c + di, T = CM - cm = ds - di.

Fit (Adjustment)

Fit (or Adjustment) refers to how one piece fits into another with a pre-defined relationship based on their dimensions before assembly. There are 3 main types:

• Clearance Fit: Ensures pieces can slide relative to each other. Achieved when the minimum diameter of the hole is larger than the maximum diameter of the shaft.
• Interference Fit: Prevents parts from moving relative to each other once assembled. Achieved when the minimum diameter of the shaft is larger than the maximum diameter of the hole.
• Transition Fit: It cannot be known in advance whether the pieces will slide or be fixed once assembled; either a clearance or interference may result depending on the actual sizes of the mating parts.

Fit Calculations

• Maximum Clearance (JM): The difference between the maximum hole dimension and the minimum shaft dimension. Formula: JM = ds - di'
• Minimum Clearance (Jm): The difference between the minimum hole dimension and the maximum shaft dimension (must be positive for a clearance fit). Formula: Jm = di - ds'
• Maximum Interference (SM): The difference between the maximum shaft dimension and the minimum hole dimension. Formula: SM = ds' - di
• Minimum Interference (Sm): The difference between the minimum shaft dimension and the maximum hole dimension (must be positive for an interference fit). Formula: Sm = di' - ds
• Tolerance of Fit (Ta): Equal to the sum of the tolerance of the shaft (Te) and the hole (Tf). Formula: Ta = Te + Tf

ISO System of Dimensional Tolerance

To determine ISO tolerances, two distinct concepts are used:

• Quality (Grade): A degree of tolerance precision informing about the piece, corresponding to the magnitude (value) of the tolerance (e.g., IT7).
• Position: Indicates where the tolerance zone lies relative to the basic size (zero line). It is indicated with a letter (e.g., H for hole, h for shaft).

ISO System of Fits

To simplify fit selection, ISO established two primary systems:

• Hole-Basis System: Uses a fundamental deviation of 'H' for the hole, meaning its lower deviation is zero (minimum hole size equals basic size). The desired fit is achieved by varying the shaft tolerance position (letter):
  • Clearance fits: shafts a-h
  • Transition fits: shafts j-n
  • Interference fits: shafts p-z
• Shaft-Basis System: Uses a fundamental deviation of 'h' for the shaft, meaning its upper deviation is zero (maximum shaft size equals basic size). The desired fit is achieved by varying the hole tolerance position (letter):
  • Clearance fits: holes A-H
  • Transition fits: holes J-N
  • Interference fits: holes P-Z