Among the myriad of codes and symbols found on technical drawings, "20h8" stands as a classic example of the ISO system of limits and fits. To the uninitiated, it looks like a cryptic password. To an engineer, it is a precise instruction dictating the boundaries of perfection.
All measured diameters must fall within 19.967 mm and 20.000 mm, inclusive. If the shaft has out-of-roundness > 0.010 mm, even if all diameters are within band, it may still cause fit issues—consider adding a circularity tolerance.
Using standard ISO 286 tables, we calculate the limits for a 20h8 shaft. 20h8 tolerance
The notation refers to a specific fit and tolerance specification under the ISO 286 system, commonly used in mechanical engineering for shafts. It describes a shaft with a nominal diameter of 20 mm , a fundamental deviation of h (indicating it is a shaft-based system where the upper limit starts at the nominal size), and an IT Grade 8 level of precision. 1. Understanding the Components of 20h8
Therefore, the shaft dimensions for are: Among the myriad of codes and symbols found
Here’s a technical feature on the , focusing on its meaning, application, and practical implications in engineering and manufacturing.
When used correctly, 20h8 reduces manufacturing costs (compared to tighter grades like h6), ensures reliable assembly, and serves as the lingua franca of metric clearance fits. When misunderstood, it leads to seized assemblies, loose rattling parts, and rejected batches. All measured diameters must fall within 19
Breaking down the notation: