A bearing shaft fit depends on which ring carries a rotating load, the bearing type, shaft size and load severity. SKF’s fit table gives recommended ISO tolerance classes for solid steel shafts, but a class by itself does not predict the final assembled fit. The rows below are limited to light or variable loads with a rotating inner-ring load, or an indeterminate load direction, and the size ranges shown by SKF.

Metric bearing shaft fit classes

How to read the recommendations

Start with the bearing type and the nominal shaft-seat diameter. Then identify how the load moves relative to the inner ring. SKF describes a rotating inner-ring load as the inner ring rotating while the applied load direction remains fixed. The table also groups variable, shock or uncertain load directions as indeterminate. A light/variable load in these selected rows is defined as P ≤ 0.05 C, where P is the equivalent dynamic bearing load and C is the basic dynamic load rating.

The fit class describes the shaft’s dimensional tolerance zone; it is not the bearing bore tolerance and is not a direct interference value. For example, the SKF table recommends j6 for a ball-bearing shaft in the range above 17 mm through 100 mm under the selected light-load condition, while the stainless-bearing entry in parentheses is j5. Do not apply that parenthetical note to a standard bearing or silently interchange the tolerance classes.

Worked selection example

Suppose a solid-steel shaft carries a standard radial ball bearing with a 40 mm bore. The inner ring rotates under a light, mostly steady load, and the load ratio is below the table’s 0.05 C threshold. The matching SKF row is the ball-bearing interval above 17 to 100 mm, so j6 is the listed shaft-seat recommendation. If the bearing is stainless steel, SKF shows a different class in parentheses. The designer must still check the bearing manufacturer’s bore limits and the required internal clearance before deciding whether the fit is acceptable.

If the same shaft sees heavy load, shock, vibration, a rotating outer ring or an indeterminate load direction outside the table assumptions, this selected row is not sufficient. Use the appropriate SKF row and current catalog guidance. A tighter shaft fit can reduce bearing internal clearance and may require a bearing with greater initial clearance. Temperature difference between the shaft and rings also changes the operating fit.

Common mistakes

First, do not select the shaft fit based only on the shaft rotating: the key condition is load rotation relative to the bearing ring. Second, do not read an ISO class such as k6 as a fixed micrometre interference without the size-dependent tolerance limits. Third, do not treat a recommended shaft class as a universal specification for all bearing styles. Check shaft material, wall thickness, hub stiffness, ring creep risk, axial displacement, runout and installation method.

SKF’s table is for solid steel shafts and distinguishes bearing construction and operating conditions. The source also notes that normal-to-heavy ball-bearing applications can require increased radial clearance when tighter shaft tolerances are used. Confirm the exact bearing designation and the manufacturer’s current recommendation before releasing a drawing. Source: SKF General Catalogue 6000/1, Table 1.

The SKF intervals use parenthesized boundary values in the source table. At a shared boundary, confirm the exact interval convention in the current catalog instead of assigning a diameter to two rows. A tolerance designation such as j6 also refers to a size-dependent tolerance zone under the ISO limits-and-fits system; it is not a single numeric shaft diameter allowance for every bearing size. To calculate a minimum and maximum assembled fit, pair the shaft limits with the bearing bore limits for the exact tolerance class.

Use P/C consistently: compare the equivalent dynamic bearing load P with the bearing’s basic dynamic load rating C using compatible units. This selected dataset omits SKF’s normal-to-heavy and high-accuracy fit rows, stationary-inner-ring cases, stainless exceptions beyond the notes shown, and non-solid-steel shaft cases. Those are not covered by this short lookup and should be checked in the full catalog.