The system of limits and Fits
The system of limits and fits defines how precisely mating parts must be manufactured and how they will assemble in real life. It controls clearance, interference, and functional behavior between components such as shafts and holes. A design can be dimensionally correct and still fail if the fit is not defined properly.
In engineering drawings, limits and fits are specified using standardized systems defined in ISO 286 (and ASME equivalents). Instead of giving only a nominal dimension, the designer defines an allowable tolerance range and a fit type. Common fit classes include clearance fits, transition fits, and interference fits, each chosen based on functional requirements.
The ISO system uses a combination of tolerance grades (IT01–IT16) and fundamental deviations identified by letters. For example, an H7 hole combined with a g6 shaft results in a controlled clearance fit, while H7/p6 creates an interference fit. This notation allows designers and manufacturers worldwide to communicate fit intent without ambiguity.
Limits and fits are function-driven. Bearings, sliding guides, press-fitted gears, and locating pins all require different fit strategies. Too much clearance leads to vibration, noise, and premature wear. Too much interference can cause assembly damage, distortion, or failure. Choosing the wrong fit is a common design error with serious functional and cost implications.
From a manufacturing perspective, tighter tolerances increase cost exponentially. Applying unnecessarily tight fits across a drawing leads to higher scrap rates, slower production, and complex inspection. Effective engineering drawings define tight fits only where function demands it and allow looser tolerances everywhere else.
Correct use of the system of limits and fits ensures predictable assembly, functional reliability, and cost-efficient production. The following video explains the ISO limits and fits system step by step, with practical examples that show how to select the right fit and apply it correctly in engineering drawings.
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