Shaft and Housing Tolerance Classes for Rolling Bearings: How Fit Choice Sets Internal Clearance

First decide which ring rotates relative to the load

Fit selection starts with one question: does the load direction rotate relative to the ring, or does the ring stay under the load? A ring that rotates relative to the load tends to creep around its seat unless the fit holds it, so the seat needs an interference or transition fit.

A ring whose load direction is stationary stays put, so a clearance fit is acceptable and often wanted because it lets the ring displace axially. The two seats are tabulated separately.

On the shaft, a rotating inner ring load calls for interference classes such as h5, js5, js6, k5, k6, m5, m6, n6, p6 or r6 depending on load and diameter, while a stationary inner ring load uses g6 where the ring must move smoothly or h6 where it need not².

On the housing, a stationary outer ring load uses clearance classes H7, H8, G7 or F7, while a rotating outer ring load uses M7, N7 or P7⁵. Both tables are functions of load type, bearing type and size, so the class cannot be picked from the load direction alone²⁵.

Getting this wrong has two costs. A loose seat lets the race rotate and fret against the shaft or housing, and a seat that does not support the thin race properly shortens service life and can force the shaft or housing to be reconditioned before a replacement bearing goes in¹¹.

Reading the shaft class off the load-and-diameter table

For a rotating inner ring, the shaft class is tabulated against load magnitude and shaft diameter, and the fit is defined relative to the bearing bore Normal tolerance — so the same letter class means different absolute interference at different diameters²¹².

The table below is the light and normal load case for cylindrical-bore bearings in classes 0, 6X and 6; heavy or impact load moves the class to n6, p6 or r6 and requires a bearing with larger internal clearance than standard². Two relaxations matter.

Where high accuracy is wanted, js5, k5 and m5 replace js6, k6 and m6². For single-row tapered roller and angular contact ball bearings, k5 and m5 may be replaced by k6 and m6 because internal clearance reduction due to fit need not be considered².

Reading the housing class off the load-and-displacement table

The housing class turns on whether the outer ring must be axially displaceable, on the load type and on how hot the shaft and inner ring run. A single default class is therefore wrong for a rotating outer ring or a hot shaft⁵.

Where the outer ring load is stationary and the ring must displace easily, H7 is the ordinary choice; G7 may be applied for a large bearing or a large temperature difference between outer ring and housing⁵.

Where the shaft and inner ring run hot, the housing moves to G7, with F7 for a large bearing or large temperature difference⁵. A rotating outer ring load is not displaceable and takes M7, N7 or P7 depending on load and housing section⁵.

Check whether the interference eats the internal clearance

An interference fit expands the inner ring, and that expansion reduces the manufactured, or bench, internal clearance of the bearing¹⁹. A tight housing fit does the same from the outside by compressing the outer ring, so the two reductions add³.

If the reduction is not accounted for, there may not be enough bench clearance left to leave any running clearance, and the bearing can move into negative clearance, excess friction and early failure³¹⁹. The reduction is not simply the nominal interference.

The effective interference is calculated as two-thirds of the nominal interference minus a flattening dimension, for the inner ring and for the outer ring, and the resulting clearance change depends on the effective interference, the mating shaft diameter and the housing bore wall thickness⁹.

That is why the fit class and the clearance class have to be chosen together rather than in sequence.

Where the fit is heavy, or the operating temperature is consistently high, or speeds exceed the standard range, a larger clearance class such as C3 is the appropriate choice; C4 is reserved for exceptionally high temperatures, significant thermal differential between shaft and housing, thin-walled housings or severe misalignment²⁰.

One supplier states C3 is the default recommendation for many common industrial motors and gearboxes, which is that supplier's position rather than a rule for every application²⁰.

Temperature differential and the free-side arrangement

Clearance also moves with temperature.

Where the inner ring or shaft is expected to run hotter and expand more than the outer ring or housing, the play in the bearing falls in operation, so a higher radial play is wanted from the start; if the outer ring expands more than the inner ring, play increases instead⁴.

The same logic drives the clearance class: C3 where operating temperature is consistently high, fits are heavy or speeds are above standard, and C4 where the thermal differential between shaft and housing is significant²⁰. A free-side bearing is the other way to absorb that movement.

It compensates for expansion or shrinkage caused by operating temperature change, and where a non-separable bearing is used on the free side, a clearance fit is provided between the outer ring and the housing, or in some cases between the shaft and the inner ring²¹.

That clearance fit is a deliberate thermal provision, not a looser version of the locating-side fit.

Bound the interference from both ends

The fit has to be tight enough to stop the ring creeping and loose enough not to overstress it.

Proper fits prevent race rotation relative to the shaft or housing and the fretting and galling that follow, and they support the relatively thin bearing races; the allowed shaft and housing tolerances are only slightly larger than those of the mating bearing components¹¹.

Press-fitting the inner ring adds to the hoop stress in the ring. For one specific case — a 120 mm bore angular contact main-shaft thrust bearing in M50 steel for a turbojet engine — the source recommends that this press-fit component of hoop stress should not exceed about 140 MPa¹⁷.

That figure comes from a single high-speed aerospace application and is not a general industry limit; treat it as a reminder that maximum interference is bounded by ring stress, and ask the bearing supplier for the limit that applies to your bearing.

Pick a mounting method that can achieve the fit

The interference on the drawing has to be achievable without damaging the raceway. Cylindrical-bore bearings are generally mounted by press fit, or by heating the inner ring to expand its bore before assembly — a shrink fit²⁶.

For larger bearings or higher interference, heat mounting reduces the risk of raceway damage during pressing, and where the outer ring needs an interference fit the bearing may first be cooled with dry ice, with rust-prevention treatment applied beforehand²⁶. Tooling matters as much as temperature.

Bearing presses, induction heaters or hydraulic presses should be used according to the bearing type and fit, and force should never be applied directly to the rolling elements — only to the ring being mounted¹³.

What to confirm on the drawing before ordering

Fits are written as letter-plus-digit combinations such as m6 and H7, where a lowercase letter is a shaft outside diameter and a capital letter is a housing bore, and every one of them is defined relative to the Normal tolerance of the bearing bore or outside diameter¹².

Because those Normal tolerances grow with bearing size, the same class produces a different absolute fit on a larger bearing, so the drawing is incomplete without the bearing tolerance class it sits against¹². The sources give no numerical seat-geometry or surface values, so those must be written as verification items rather than copied figures.

• ✓State the bearing tolerance class the fit is defined against, using the ISO, DIN, ANSI/ABEC or JIS designation the supplier works to
• ✓Confirm the shaft class against the load case and the actual shaft diameter band, not a default class
• ✓Confirm the housing class against the outer-ring displacement need and the shaft temperature
• ✓State the internal clearance class (CN, C3 or C4) and the fit it was chosen with
• ✓Require the shop to verify shaft OD and housing bore size, surface finish and form before assembly
• ✓Ask the supplier to confirm the expected clearance reduction for the specified fit
• ✓Name the mounting method and tooling so the specified interference is achievable without raceway damage

Where the sources disagree or stay silent

The tables above come from supplier fit-selection guides, and they do not all point the same way. The rows below keep each position separate; the evidence does not resolve which applies to a given machine, so the buyer verifies against the actual application.

What the sources do not establish

• No source gives numerical Δdmp/ΔDmp bore and OD deviation values for a specific bearing, so the actual interference cannot be computed from this article.
• No source gives numerical radial internal clearance ranges for CN, C3 or C4 before mounting.
• No source gives a complete ring-ratio factor formula for clearance reduction; only the 2/3 factor and the G* flattening dimension are stated.
• No source gives numerical thermal expansion calculations or the temperature differential at which C3 or C4 becomes necessary.
• No source gives a minimum interference formula to prevent creep or fretting, or a general maximum interference limit for cracking or hoop stress.
• No source gives numerical surface roughness (Ra), cylindricity, roundness or shoulder squareness requirements.
• No source gives achievable or required interference values, or mounting force values, for press fit, induction heating, oil injection or hydraulic nut methods.
• No source gives numerical shaft or housing seat geometry tolerances relative to the bearing tolerances.