Mechanical drawings for complex assemblies often become congested with dimensions, tolerances, leaders, notes, datums and a variety of other markers. This can make it difficult to keep correct tolerances straight across large complex assemblies. To address this the International Standards Organization (ISO) created a standard notation system for common assembly types—like shafts and holes—denoted easily by a single letter and number. This system helps prevent errors and ensures the accurate interpretation of design intent across engineering projects.
ISO sets up the classification of fits into three main categories: clearance, transition, and interference fits.
A clearance fit is one where two parts can slide and rotate with relation to one another when assembled. In interference fits the parts cannot move in relation to one another when assembled. This fit takes a high level or force or heat to assemble/disassemble. Finally, a transition fit is one where there is a very small amount of interference, just light enough that someone could move the parts relative to one another with a light amount of force, typically this can be done by hand.
Classifying a fit only by these terms may have been acceptable hundreds of years ago, but as the complexity of machines have improved, so has the ability to measure features. Because of this, further sub-categories needed to be established.
Clearance Fits:
- Loose Running: Non-accurate clearance suitable for moving parts that may experience corrosion or contamination.
- Free Running: Designed for high-speed parts that do not require high accuracy.
- Close Running: Small clearances ideal for light accuracy.
- Sliding: Offers high accuracy with easy assembly.
- Location: Tight assembly may require lubrication, very accurate.
Transition Fits:
- Similar Fit: Zero clearance to light assembly force required.
- Fixed Fit: Interference fit with light assembly force.
Interference Fits:
- Press Fit: Interference fit that requires force to assemble, parts may be at room temperature.
- Driving Fit: Interference fit requiring force; parts must be heated or cooled.
- Forced Fit: Interference where one part is heated or cooled before assembly, typically resulting in permanent bonding.
By establishing these sub-groups engineers can now select a fit based on their design intent. Each of these conditions have an alpha numeric code that conveys the tolerance the feature should be manufactured to. The code consists of an uppercase letter and number for the hole fit (e.g. H7), and a lowercase letter and number for shaft fit (e.g. h6). These codes are easily referenced in the Machinery’s Handbook and allow the machinist to look-up the tolerance for the specified fit.
The proper sizing of shaft and hole fits can have a large impact on the function, serviceability, and reliability of a mechanical system. Familiarity with these standards allows engineers to make design decisions quickly, ensuring that parts fit correctly the first time, thereby reducing costly redesigns and downtime.
Comments