Bearing fit tolerance is the controlled interference or clearance between a bearing’s inner or outer ring and its mating shaft or housing bore. Get it wrong too loose and the bearing spins in its seat, fretting the housing and generating debris that destroys the rolling elements. Too tight and the bearing preloads internally, runs hot, and fails early. This guide covers ISO fit classes, when to use interference versus clearance fits, how to call out tolerances on your machining drawing, and the most common fit specification mistakes that show up in design review.<\/p>\n\n\n\n
Bearing selection gets the attention. Bearing fit tolerance gets ignored until something fails.<\/p>\n\n\n\n
The bearing manufacturer’s load ratings assume the bearing is correctly installed which means the inner ring is supported uniformly by the shaft and the outer ring is supported uniformly by the housing. Both assumptions require correct fit tolerance. An interference fit that’s 0.01 mm too loose on a shaft rotating under load allows the ring to micro-slip. That micro-slip generates fretting corrosion, produces iron oxide debris, and initiates fatigue at the raceway all while the bearing’s load rating remains unaffected on paper.<\/p>\n\n\n\n
The fit tolerance that prevents this is not complicated to specify. What’s complicated is that it interacts with shaft material, housing material, operating temperature, load direction, rotational speed, and installation method in ways that generic guidelines don’t fully capture. This guide gives you the systematic approach.<\/p>\n\n\n\n
Au Pr\u00e9cision Yicen<\/a>, bearing bores and shaft journals are regularly held to IT5 and IT6 tolerances \u2014 the range where most bearing fit specifications live. What follows is the technical framework for specifying them correctly.<\/p>\n\n\n\n Bearing fits are specified using the ISO 286 tolerance system, which defines the position and size of a tolerance zone using a letter (position relative to nominal) and a number (tolerance grade, or IT grade).<\/p>\n\n\n\n For shafts, lowercase letters are used: k, m, n for interference fits; h for a sliding fit; g, f, e for clearance fits. For housing bores, uppercase letters: K, M, N for interference fits; H for neutral; G, F, E for clearance fits.<\/p>\n\n\n\n The IT grade number defines the total tolerance range. IT5 allows less variation than IT6, which allows less than IT7. For bearing seats:<\/p>\n\n\n\n Combining these: a shaft specification of k6 means an interference fit (k) with IT6 tolerance grade. A housing specification of H7 means a neutral-to-clearance fit (H) with IT7 tolerance grade. The actual interference or clearance produced depends on the nominal bore size and the specific tolerance values from the ISO 286 tables.<\/p>\n\n\n\n This is the governing question. The rule is straightforward: the ring that rotates relative to the load direction must have an interference fit. The ring that is stationary relative to the load may use a clearance fit or light interference.<\/p>\n\n\n\n In a typical electric motor shaft application, the shaft rotates and the housing is stationary. The load on the bearing (from belt tension, gear force, or gravity) is fixed in space \u2014 it always points in the same direction regardless of shaft rotation. So the inner ring rotates relative to the load, and the outer ring is stationary relative to the load.<\/p>\n\n\n\n Result: inner ring needs interference fit (k6 or m6 on the shaft). Outer ring can use a looser fit (H6 or H7 in the housing).<\/p>\n\n\n\n If the load rotates with the shaft \u2014 as in a wheel hub where the hub rotates and the shaft is stationary \u2014 the logic inverts: outer ring gets the interference fit in the rotating hub bore, inner ring uses a lighter fit on the stationary shaft.<\/p>\n\n\n\n Getting this wrong is one of the most common bearing fit specification errors. An inner ring with a clearance fit on a rotating load application will spin in place within months.<\/p>\n\n\n\n Normal radial loads, shaft rotating:<\/strong><\/p>\n\n\n\n Heavy or shock radial loads, shaft rotating:<\/strong><\/p>\n\n\n\n Axial loads only (thrust bearings):<\/strong><\/p>\n\n\n\n Rotating outer ring (wheel hub applications):<\/strong><\/p>\n\n\n\n Precision machine tool spindles:<\/strong><\/p>\n\n\n\n Room-temperature interference fit calculations are a starting point, not an endpoint. Operating temperature changes the interference.<\/p>\n\n\n\n Steel shafts and steel bearings have similar thermal expansion coefficients (approximately 12 \u00b5m\/m\u00b0C). A steel bearing on a steel shaft in a steel housing doesn’t change fit dramatically with temperature.<\/p>\n\n\n\n Aluminum housings are the problem. Aluminum expands at roughly 23 \u00b5m\/m\u00b0C \u2014 nearly double steel’s rate. An interference fit specified at 20\u00b0C for an aluminum housing can become a clearance fit at 80\u00b0C operating temperature.<\/p>\n\n\n\n The calculation: for a 62 mm housing bore in aluminum with a steel outer ring, a 60\u00b0C temperature rise produces approximately: 62 mm \u00d7 (23 \u2212 12) \u00b5m\/m\u00b0C \u00d7 60\u00b0C = 40.9 \u00b5m of additional clearance. A fit that was 15 \u00b5m interference at room temperature becomes 26 \u00b5m clearance at operating temperature. The outer ring is now spinning.<\/p>\n\n\n\n For aluminum housings, tighten the initial interference by the thermal expansion differential at maximum operating temperature. Or specify a steel sleeve insert in the housing bore, which eliminates the aluminum expansion problem at the bearing seat.<\/p>\n\n\n\n The tolerance call-out on your drawing determines what your machining supplier produces. Vague call-outs produce correct-looking parts that fail fit checks at assembly.<\/p>\n\n\n\n Shaft journal:<\/strong> Specify the nominal shaft diameter, the tolerance designation (e.g., \u03c625 k6), and the explicit upper and lower deviation values in parentheses for clarity. For \u03c625 k6, the deviations are +15\/+2 \u00b5m, meaning the finished shaft must measure between 25.002 and 25.015 mm. The feature should carry a cylindricity tolerance (not just roundness) of half the IT grade value or better.<\/p>\n\n\n\n Housing bore:<\/strong> Specify the nominal bore, tolerance designation (e.g., \u03c652 H7), and explicit deviations. For \u03c652 H7, the deviations are 0\/+30 \u00b5m, meaning the bore must measure between 52.000 and 52.030 mm. Specify cylindricity and perpendicularity of the bore axis to the shaft centerline.<\/p>\n\n\n\n Surface finish:<\/strong> Bearing seats require Ra 0.8 \u00b5m or better. Coarser surfaces reduce the effective interference \u2014 surface peaks crush during installation, and the effective bearing contact is less than the nominal interference. For precision applications, Ra 0.4 \u00b5m is standard on ground shaft journals.<\/p>\n\n\n\nUnderstanding Bearing Fit: What the Numbers Mean<\/strong><\/h2>\n\n\n\n
ISO Tolerance System Basics<\/strong><\/h3>\n\n\n\n
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When to Use Interference Fits vs. Clearance Fits<\/strong><\/h2>\n\n\n\n
Which Bearing Ring Rotates Relative to the Load?<\/strong><\/h3>\n\n\n\n
Standard Fit Recommendations by Application<\/strong><\/h2>\n\n\n\n
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How Thermal Expansion Changes Your Fit Calculation<\/strong><\/h2>\n\n\n\n
Calling Out Bearing Tolerances on a Machining Drawing<\/strong><\/h2>\n\n\n\n