When Should You Specify C3 Instead of CN Bearing Clearance?

The marked clearance is not the operating clearance

The number stamped on the box is measured before the bearing is installed. Every interference fit and every degree of ring-to-ring temperature difference is spent out of that number before the bearing carries load^13,15,16.

That is why two identical bearings in different housings need different clearance groups. C3 exists to leave margin so a small positive operating clearance survives the combined fit and thermal losses; if CN plus those losses reaches zero, the bearing becomes heavily preloaded, friction rises, temperature climbs and lubrication breaks down^13,16.

A separate source states that reduced clearance increases friction and drastically reduces bearing life². For a steel shaft, steel housing, room-temperature service at light-to-moderate load, the catalogue CN number survives installation essentially intact, so CN is the correct specification and C3 buys nothing^7,13.

Picture a 6205 in that position: nothing in the mounting or the duty removes clearance, so the standard group is the right call. The rule is to choose the smallest group that keeps a positive operating clearance under the worst realistic combination of fit and temperature¹³.

Before you write a clearance group on the requisition, ask what your mounting and your running temperature will take out of it.

Rule out CN when the fit is m6 or tighter

An interference fit expands the inner ring or compresses the outer ring, and most of that dimensional change comes straight out of internal clearance^15,16.

The two sources put the fraction in the same band but not the same number: one reports 60–80% of the average interference disappears from clearance¹³, the other applies a factor of 70–90% to the effective interference¹⁵ (supplier-reported figure). Either way, a heavy fit can consume the whole CN allowance before the bearing turns.

One source treats a heavy fit at m6 or tighter as sufficient justification on its own to move from CN to C3¹³. The direction of the loss depends on which ring is fitted: an inner-ring interference expands the inner ring, an outer-ring interference compresses the outer ring, and both reduce clearance^15,16.

A worked instance from one source: a 6205 with a mean diameter of 38.5 mm loses roughly 8–11 μm to the fit, before any thermal term is added¹³. That is a large share of a typical CN band, and it is why the fit check comes before the temperature check.

If your drawing calls for an m6 shaft fit, the clearance group is already the line of the requisition to attack first.

Step up when the rings run 10–15 °C apart

When the inner ring runs hotter than the outer ring it grows more than the outer ring, squeezing the rolling elements and eating clearance the fit has already reduced^7,16. The thermal loss adds to the fit loss rather than replacing it.

One source sets the trigger at an inner-ring-to-outer-ring differential above roughly 10–15 °C, or continuous hot running, and says to move up one clearance group¹³. The same source gives the mechanism: motor rotor heat conducts down the shaft and pump housings transfer process heat, so the inner ring usually runs hotter¹³.

For the 6205 above, a 15 °C differential costs roughly 7 μm on top of the 8–11 μm from the fit — the two losses together consume 15–18 μm of the 5–20 μm available¹³ (supplier-reported figure). That is the arithmetic that makes CN fail on a typical motor shaft.

The temperature term scales linearly with size, so a large motor bearing loses two or three times as much clearance to heat as a small one¹³. Where heat is conducted through the journal, one source calls for C4 rather than C3⁵.

Estimate your ring-to-ring differential before you pick the group.

Check the free side before adding clearance

On the free side of a shaft system the design deliberately uses a clearance fit between outer ring and housing, or in some cases between shaft and inner ring, to compensate for expansion or shrinkage caused by operating temperature change¹².

That growth is absorbed by the fit, not by internal clearance, so the clearance group does not have to carry it. A non-separable bearing on the free side is the case where this matters most, because the housing fit is doing the thermal work¹².

The opposite mistake is cooling only the housing of a hot bearing: the outer race cools while the inner race stays hot, radial internal clearance is removed, friction rises and life drops drastically, with smaller bearings at greatest risk because their radial internal clearances are small².

So the ring-rotation question is not only about which ring turns — it is about which ring is free to move and which one you are cooling. For a position with a free-side clearance fit, adding a clearance group to cover thermal growth double-counts the same expansion.

Confirm the fixed-side and free-side arrangement on your drawing before you step up a group.

CN, C3 or C4: which conditions fit?

The group is chosen by the worst realistic combination of fit and temperature, not by the normal case. A group that is too small reaches zero operating clearance and preloads the bearing; a group that is too large costs rigidity and noise^13,16.

Read the table against your own mounting and duty, then confirm the numerical range for your part number with the supplier (supplier-reported).

Where C3 is the wrong answer

A larger initial clearance is not automatically better. A C3 bearing has slightly less rolling-element contact support, so it may generate a bit more noise and offer slightly lower rigidity than a CN bearing¹⁶.

Precision spindles and low-noise applications sometimes need CN or even C2 instead¹⁶. That is the trade a spindle position cannot make, however hot or heavily fitted it is: the extra clearance that protects a motor shaft from preload is the same extra clearance that costs a spindle its stiffness and its quiet running.

The same source notes that larger clearance better accommodates thermal expansion and fit compression, but too much can raise running noise and vibration, while smaller clearance gives higher positioning accuracy⁷. So the decision is not "more clearance is safer" — it is "the smallest group that still keeps a positive operating clearance"¹³.

If your position is a precision spindle or a low-noise application, the burden of proof runs the other way: you need the fit and thermal numbers to justify leaving CN. Write down the rigidity and noise requirement next to the clearance group before you release the requisition.

Above DN 1,500,000, change the bearing

At high speed the heat problem is solved by the bearing design rather than by a larger clearance group. Ceramic hybrid bearings reduce centrifugal forces, lower operating temperatures and extend lubricant life, and one supplier states that for motorized spindles above DN 1,500,000 the improved reliability typically justifies the higher initial cost⁸.

A second supplier states that for spindles running above DN 1,500,000, ceramic hybrids are strongly recommended, and that the lower coefficient of thermal expansion of the ceramic balls maintains preload stability across operating temperature ranges¹⁰ (supplier-reported). Both are supplier claims, not independent measurements.

The practical point for the clearance decision is that the answer above that threshold is a different bearing, not simply a bigger clearance group: if the speed is high enough to drive the heat, the design measure that addresses it is the ball material and the lubrication, not the clearance class.

Note that the sources do not state a speed threshold above which C3 itself is required, and they do not state how inner-ring versus outer-ring rotation changes the required group. If your position runs above DN 1,500,000, raise the hybrid option with the supplier before you fix the clearance group (supplier-reported).

Confirm the temperature band you will quote

The clearance decision turns on temperatures you have to supply from the application, and the sources give bands rather than a single figure. A requisition written without a measured or estimated band is a guess, and the guess is discovered only when the bearing fails early.

The bands below are the ones the sources give; the wider alarm and heat-stabilisation ranges are context for why a measured number is required, not clearance-selection triggers (supplier-reported).

• ✓Measure or estimate the actual bearing temperature, not the housing skin temperature — one source notes measured skin temperatures run 15–25 °F cooler than the bearing itself.
• ✓Check whether the position sits in the 180–200 °F caution band, where a C3 clearance bearing is typically considered and monitoring should continue.
• ✓Confirm whether heat reaches the bearing through the journal — that condition calls for C4 rather than C3.
• ✓Record the ring-to-ring differential you are assuming, since the step-up trigger is stated as roughly 10–15 °C.
• ✓Note the alarm band of 250–300 °F, where the bearing is likely already in some stage of failure, and the 300–400 °F heat-stabilisation range where integrity and geometry can be severely compromised.

Ask the supplier before you freeze the part number

The engineering decision can be made from fit and temperature, but the requisition cannot be released until the supplier confirms that the chosen clearance group exists for that part number and what it costs. The sources do not carry those figures, so they must be obtained rather than assumed.

C3 is widely used in electric motors, pumps, small machinery and power tools, but that is a usage pattern, not a rule that your position needs it¹⁶.

One supplier states that choosing between C3 and CN mainly comes down to operating temperature rise and how tight the fit is, and that for ambient-temperature, low-load conditions CN is usually enough⁷.

• ✓Ask for the numerical CN, C3 and C4 clearance ranges in µm for the exact part number and size.
• ✓Ask for the price premium of C3 over CN for that part number.
• ✓Ask for the minimum order quantity and the lead time for the C3 variant.
• ✓Ask which clearance class the applicable equipment standard requires for this equipment type.
• ✓Ask the supplier to confirm your fit and temperature inputs against their own selection guidance.

Where the sources disagree

The sources give different numbers for the same quantity and different verdicts on the same trade-off. A reader who takes one figure as fact will over- or under-specify clearance.

Treat the ranges as a band and confirm the applicable figure with the bearing supplier (supplier-reported).

What the sources do not establish

• No numerical clearance ranges in µm for CN, C3 or C4 for a specific bearing type and size
• No formula or worked example converting a temperature differential into a clearance loss for a specific bearing
• No speed threshold or rule for how inner-ring versus outer-ring rotation affects the required clearance group
• No price premium, MOQ or lead time for C3 versus CN for any part number
• No manufacturer selection chart with speed, temperature and fit axes
• No standard cited that mandates a clearance class for a specific equipment type
• No quantified life, noise or vibration penalty for too little or too much clearance