Hole Clearance Fit Class Calculator

A clearance fit is specified by two tolerance bands rather than by two sizes. In the hole basis system the hole's lower limit sits at the nominal size and the shaft band is displaced downward, so clearance is guaranteed no matter where each part falls within its own band. Minimum clearance equals the smallest hole minus the largest shaft; maximum clearance equals the largest hole minus the smallest shaft. ANSI B4.1 tabulates nine running and sliding classes, RC1 through RC9, with tolerance values that grow roughly as the cube root of nominal diameter, because achievable machining accuracy scales that way rather than linearly. The classes map onto function. RC1 and RC2 are location fits where parts must assemble without perceptible play. RC3 and RC4 suit accurately located plain bearings running at moderate speed. RC5 and RC6 cover general purpose journal bearings, RC7 gives free running clearance where temperature swings or shaft deflection are expected, and RC8 and RC9 are loose fits for agricultural and rough machinery. A 25 mm shaft at RC4 needs only a few hundredths of a millimetre of clearance, so at that scale surface finish, roundness, and cylindricity matter as much as the diameter reading. The important caveats are that these bands are stepped by nominal size range rather than computed continuously, that ISO 286 uses an entirely different notation of IT grades and letter deviations such as H7/g6, and that thermal expansion and lubricant film thickness can consume a small running clearance completely at operating temperature.

Fits were originally a matter of hand fitting. A machinist scraped and lapped a shaft to its own bearing, and the resulting pair was not interchangeable with any other pair on the same machine. Systematic limit gauging emerged from armouries and from firms specializing in precision production, notably the Newall Engineering Company in England, whose limit system published around the turn of the twentieth century tabulated allowances for holes and shafts. Early British Standards Institution work on limit gauges in the first decade of the century pushed the concept into general engineering, and wartime production, where components had to be assembled from many independent suppliers, made it unavoidable. American practice consolidated into ANSI B4.1, Preferred Limits and Fits for Cylindrical Parts, which defined the familiar letter groups: RC for running and sliding clearance, LC, LT, and LN for the locational clearance, transition, and interference fits, and FN for force and shrink fits. Internationally the ISO system took a different structure, defining tolerance grades from IT01 through IT18 and fundamental deviations denoted by letters, with the hole basis H used as the default. That system was issued as ISO Recommendation R286 in the early 1960s and later became ISO 286, now published in parts covering the tolerance code, the tables, and gauging. Metric drawings today almost always cite ISO fits, while ANSI RC classes remain common on older inch drawings and in North American maintenance work.