Shaft and Housing Fits: A Practical Guide

A bearing that is 20 µm too loose in its housing creeps, wears the housing bore oval and eventually spins the outer ring. The same bearing 20 µm too tight on its shaft consumes its radial clearance and runs hot from the first hour. Both failures come from the same decision, and it is made long before the bearing is fitted: the fit.

Fits are where ISO 286 metrology meets bearing engineering. This guide translates the ISO 286-1 and ISO 286-2 tolerance grades into the combinations that actually work for ball and roller bearings, shows the interference magnitudes in micrometres, and works through the clearance loss, the thin-wall and aluminium derating and the thermal case.

Why the inner ring is usually an interference fit

A bearing ring must not creep on its seating. Creep - slow relative rotation between the ring and its seat - is driven by the load direction relative to the ring. Whichever ring rotates relative to the load vector must be held firmly enough that friction cannot be overcome, and that means an interference fit for the rotating-load ring.

For the common case of a shaft that turns and a load that stays in one direction, the inner ring sees a rotating load and the outer ring sees a stationary load. The rule that follows is simple: inner ring interference, outer ring clearance or light interference. When the outer ring rotates instead - a wheel hub or a conveyor roller - the roles reverse and the housing becomes the interference fit.

The classic shaft and housing combinations

Decades of practice have settled on a small set of ISO 286 combinations that work for most radial bearings. They are worth knowing by name because they cover the overwhelming majority of positions.

Rotating inner ring, stationary load

The shaft carries a transition or interference fit and the housing a clearance fit. This is the default for motors, pumps and most gearbox shafts.

Stationary inner ring or rotating outer ring

The shaft takes a clearance fit - often h6 or g6 - and the housing takes an interference fit such as K7 or M7. Wheel hubs, idler rollers and conveyor drums live in this column.

Which ring should locate axially

Only one ring should be axially clamped; the other must be free to slide so that thermal growth does not load the bearing. A clearance fit on the non-locating ring is what allows that movement, and it is why the fit choice is tied to the axial arrangement as much as to the load.

Conventional ISO 286-1 combinations for solid steel shafts and rigid steel or cast iron housings. For thin-wall, hollow or aluminium parts, back the interference off as described below.

What the interference actually adds up to

A fit grade is abstract until you convert it to micrometres. The table gives the diametral interference for a 30 mm to 50 mm shaft against a bearing bore that itself is toleranced to 0 to -10 µm.

Approximate diametral interference against a bearing bore toleranced to 0 to -10 µm at this size. Always confirm the exact ISO 286-2 limits for your nominal diameter before releasing a drawing.

The magnitudes are small and decisive. A k6 fit that looks like a normal press on a 40 mm shaft adds 20 µm of interference, and that figure will reappear below as a clearance loss. This is why the fit and the clearance cannot be specified independently.

From interference to clearance loss

The 70 to 80 % rule and a worked example

When the inner ring is pressed onto the shaft, the ring is compressed, the ring circumference is effectively stretched, and the raceway diameter grows. The result is a reduction in radial internal clearance. In practice roughly 70 to 80 % of the diametral interference appears as clearance loss.

Take a 40 mm bore bearing with 20 µm of interference and assume 75 % conversion. The clearance loss is 0.75 × 20 = 15 µm. If that bearing started life with 20 µm of CN clearance, it now has about 5 µm before any temperature effect. Add a hot running condition and the clearance can cross into preload. The internal clearance guide works the temperature half of the calculation in detail.

Thin-wall, hollow shaft and aluminium housing derating

The conversion factor above assumes a rigid steel shaft and a stiff housing. When either is flexible or expands differently, the same nominal fit behaves more tightly.

• Thin-wall housings. A housing ring that is thin relative to its diameter distorts under the outer ring, so a K7 that was designed for a solid housing can crush or oval the bearing. Drop back one grade, usually to J7 or H7.
• Hollow shafts. A hollow shaft compresses under the inner ring instead of resisting it, increasing the effective interference. Use a lighter grade such as j6 or k5 and check the wall section.
• Aluminium housings. Aluminium expands roughly twice as fast as bearing steel, so a hot aluminium housing grips the outer ring more tightly than a cold measurement suggests. The operating fit is tighter than the shop-floor fit.
• Split housings. A split or clamped housing cannot hold a full interference, so it relies on the clamp and usually needs a tighter nominal grade to compensate.

Axial location: shoulders, fillets and radii

A fit only locates the ring radially. Axially, the ring must seat against a shoulder, and the shoulder geometry has to match the bearing.

r min, da min and Da max

The bearing drawing gives a minimum chamfer radius r for the ring corners, a minimum shaft shoulder diameter da, and a maximum housing shoulder diameter Da. The shaft fillet radius must be smaller than the bearing chamfer, or the ring cannot seat flat and the bearing runs misaligned from the first revolution. The shaft and housing geometry guide covers the shoulder and fillet relationships, and the mounting and dismounting guide covers how the fit is applied without damaging the seat.

Seat tolerance and surface finish

Two seats machined to the same diameter can behave differently because of finish and form. A rough seat contact only on its high spots, so the effective interference is lower than the measured value and the ring can creep; a distorted seat transfers its ovality straight into the raceway.

• Surface roughness. Aim for Ra 1.6 µm or better on both shaft and housing seats. A finer finish is not always better - an over-polished seat can be too smooth to hold.
• Cylindricity and roundness. Form error adds directly to the running accuracy of the bearing, so a fine bore that is oval is worse than a slightly rougher bore that is round.
• Squareness of the shoulder. A shoulder that is not perpendicular to the seat tilts the ring and loads one side of the raceway.

Thermal differential in hot-running equipment

When the shaft runs hotter than the housing, the shaft grows more than the housing and the inner ring fit tightens; the clearance it consumes is the thermal loss described above. When the housing runs hotter - a bearing in a hot housing with a cooled shaft - the opposite happens and the outer ring can loosen. Both cases are handled with the same approach: estimate the temperature differential at steady state, convert it to a diametral change using the coefficient of thermal expansion for each material, and add it to the fit loss before choosing the clearance. Equipment that runs above roughly 80 °C almost always needs this step to be done on paper.

How to select a fit

Work through the sequence and keep the numbers on the drawing.

1. Establish the load direction and which ring rotates. This decides which ring takes the interference fit.
2. Estimate the load magnitude and any shock. Heavier and shock-loaded positions need a tighter grade than the light-load default.
3. Pick the base combination from the table above, for example k6 on the shaft and H7 on the housing.
4. Derate for thin walls, hollow shafts, aluminium or split housings. Move one grade lighter where the seat is flexible or expands faster.
5. Convert the chosen fit to micrometres using the ISO 286-2 tables for the exact nominal diameter, rather than trusting a rule of thumb.
6. Deduct the clearance loss at 70 to 80 % of the interference, then add the thermal differential. The worked fit calculation takes a full example from grade to operating clearance.
7. Confirm the resulting operating clearance is still positive for the duty at the working temperature before the fit is released.
8. Specify the seats with diameter, grade, roughness and form on the drawing, and note the mounting method. Heat-assisted mounting is described in the induction heater guide.

Common mistakes

These are the errors that turn a correct bearing into a warranty claim.

• Re-using a worn seating. A seat that has already crept is worn oversize and will not hold the new ring at the intended interference. Measure it before re-fitting.
• Putting a k6 on a thin-walled housing. The housing distorts and the outer ring loses its roundness, which shows up as noise and heat rather than a fit failure.
• Ignoring an aluminium housing's higher expansion. The hot operating fit is tighter than the cold fit, which is how a housing that measured correct still crushes a bearing.
• Choosing the fit and the clearance separately. The fit consumes clearance, so a tight fit with a CN bearing is really a preloaded bearing with a different part number.
• Forgetting the non-locating ring. Clamping both rings axially and then running the shaft hot loads the bearing axially until it fails.
• Machining the seat to size but not to form. Diameter alone does not hold a ring; roundness and roughness do.

Our deep groove ball bearings, the cylindrical roller range and the self-aligning roller range all publish the bore and outside diameter deviation you need for the calculation, and any SKF or FAG listing can be checked against the ISO 286-2 limits. The bearing's own bore and outside diameter deviations are set by ISO 492, and the radial internal clearance the fit consumes is defined by ISO 5753-1, so the three standards have to be read together. The terms used here are defined in the glossary, and the selector filters by fit-critical dimensions across the catalogue. Recall too that the bore and outside diameter deviation are themselves controlled by the tolerance class, so a change of class shifts the interference - the tolerance class guide explains that link.