Bearing Internal Clearance C1–C5: Choosing a Group When Temperature and Fits Consume It

The group on the drawing is not the clearance the bearing runs at

The most expensive mistake here is treating a bigger group as the safe choice. The suffix only sets the manufactured, or bench, clearance; what has to stay positive is the clearance left once the bearing is fitted and hot.

The groups are ordered C2 < CM/C0(CN) < C3 < C4 < C5, and C0 is the default when no clearance suffix appears in the designation²⁵.

SKF's designation system lists C3 as greater than normal, C4 greater than C3 and C5 greater than C4, so the suffix is a position in a sequence, not a tolerance in itself⁸. Two effects consume that bench clearance in service.

An interference fit on the shaft expands the inner ring, and a tight housing fit compresses the outer ring, both reducing clearance³. If the reduction is not accounted for, the bearing can run at negative clearance, which raises friction and heat and ends in early failure³¹⁷.

So the number to design around is the running clearance, not the catalogue group. Before you fix a suffix, you need the fit you will actually use and the temperature each ring will reach.

Which ring runs hotter decides the direction of the correction

Thermal growth does not always close clearance. The sign depends on which ring expands more relative to the other.

When the inner ring or shaft runs hotter than the outer ring or housing, the radial play reduces in operation, and a larger initial clearance may be needed². When the outer ring expands more than the inner ring, play increases instead².

JTEKT makes the same point from the fit side: an inner ring under load generally runs hotter than the shaft, and a bearing hotter than its shaft needs greater interference⁹.

That gives you a directional rule you can apply before any numbers: hot shaft, step toward a looser group; hot housing, the thermal effect works the other way. Neither source gives a clearance change in micrometres or a worked expansion calculation, so the magnitude has to come from the bearing maker.

How much clearance the interference fit will take

Press-fitting is the second consumer of bench clearance, and it acts before the bearing ever turns. An interference fit expands the inner ring and reduces clearance; a similarly tight housing fit on a bearing with low radial play compresses the outer ring and reduces it further.

Which can produce negative internal clearance and early failure³. American Roller Bearing describes the same loss as a reduction of the manufactured bench clearance, and warns that without enough running clearance the bearing can enter thermal runaway¹⁷.

SKF frames the tight-fit side as too little operating clearance, which raises temperature and accelerates lubricant deterioration¹⁵. Three publishers agree on the direction, so you can treat fit-induced clearance loss as real and plan for it.

None of them gives a percentage rule or a reduction formula, so ask the maker for the expected clearance loss at your fit and bore rather than estimating it yourself.

Picking a starting group from speed, load, temperature and fit

With the direction and the consumers identified, you can pick a starting point. Treat these as screening recommendations, not a standard that mandates a group.

One peer guide puts CN on most applications, C3 where speeds, operating temperatures or press-fit mounting on solid shafts raise the demand, and C4 for extreme temperature differentials⁴. A manufacturer-style selection table is finer-grained, and is worth reading as a ladder rather than a set of rules.

Which side of the range is riskier for your application

Both ends of the clearance range carry failure modes, and they are not symmetric in how fast they develop. Too little clearance, or negative clearance, causes excessive friction, rapid heating and seizure¹¹.

American Roller Bearing describes the mechanism as a loop: heat from initial operation raises bearing temperature, which drives clearance negative, which generates more heat, until the lubricant is exceeded and failure follows quickly¹⁷. Too much clearance shows up differently: inaccurate shaft guidance, increased vibration and uneven load distribution¹¹.

JTEKT ties inappropriate operating clearance to short rolling fatigue life and to heat, noise and vibration²⁰. For most applications the binding constraint is the minimum permissible running clearance, because the low-clearance failure escalates on its own.

If your calculation leaves little margin above zero, that is the number to take back to the maker.

Checks before the clearance code goes on the drawing

A suffix on a purchase order is only as good as what the supplier measures and declares against it. These checks cover what the sources support; the rest is in the boundary list below.

• ✓Confirm the supplier's declared standard for the group — one supplier states C3 is defined by ISO 5753 and ABMA as larger than Normal (CN).
• ✓For small deep groove, cylindrical or spherical roller bearings, ask how radial clearance is gauged: one race fixed, the other pushed and pulled 180°, read on a dial indicator.
• ✓For two- and four-row tapered roller bearings, ask for axial clearance or bench end play instead, since radial measurement is generally not done on these.
• ✓Ask which clearance code will appear in the designation and on the packaging, since the suffix is part of the designation system.
• ✓Confirm the gauging load and the measurement uncertainty the supplier works to, and whether clearance is verified per bearing or per batch.

Commercial route for a non-standard group

C1, C2, C4 and C5 are not shelf-default items in the way CN and C3 often are, so the commercial side belongs in the design decision, not after it.

The only commercial data in this set is a supplier statement that MOQ for C3 bearings varies by type, specification and whether customization is needed, with quote and lead time on request²³. That same supplier asks for the model, clearance code, quantity and application before quoting²³.

For any group outside CN and C3, you have no stock, premium or lead-time figure here. Raise the clearance code with the maker at the shortlist stage, before the drawing is released, so a special-order group does not surface as a schedule problem after the design is frozen.

Where the recommendations disagree on electric motors

The clearest disagreement in this set is which group suits a normal electric motor. It matters because motor bearings are a high-volume choice, and the three positions below do not resolve into one answer.

Before you request a quote

• The evidence leaves this open — put it to the supplier: no source in this set gives numeric clearance ranges in micrometres for C1, C2, CN, C3, C4 or C5 at any bore size or bearing type — ask the maker for the tabulated values against the standard it declares.
• The evidence leaves this open — put it to the supplier: no worked thermal expansion calculation or clearance-change value in micrometres is provided — ask the maker to compute the expected clearance change from your ring temperatures.
• The evidence leaves this open — put it to the supplier: no percentage rule or formula converting interference to clearance loss is given — ask the maker for the expected loss at your fit and bore.
• The evidence leaves this open — put it to the supplier: gauging load values and measurement uncertainty for clearance checking are not stated — confirm both with the supplier.
• Request certificate numbers, issuing bodies, scope and validity for the exact model and destination market.
• Request a written quote with price basis, MOQ, tooling, payment terms and production lead time.
• The evidence leaves this open — put it to the supplier: which standard a given supplier declares for its clearance groups, and whether the clearance code is marked on the bearing itself, is not stated — confirm in writing.
• The evidence leaves this open — put it to the supplier: c1 is not addressed by any source in this set.