Most spherical roller bearing failures that show up within the first few weeks of service trace back to one of two things: the wrong radial clearance class, or a mounting fit that was never actually verified. Both are avoidable, and both come down to two numbers a fitter can measure on the shop floor with a feeler gauge or a dial indicator.
Cylindrical Bore vs. Tapered Bore: Choosing the Mounting Method
Cylindrical bore bearing is pressed or shrink-fitted directly onto a straight shaft. It's simpler to size and dismount, and it's the default choice for shafts under roughly 80 mm where a mechanical press or induction heater can do the job.
Tapered bore bearing (designation suffix K, taper 1:12 as standard, 1:30 for some large sizes) mounts on a tapered shaft seat, or more commonly on an adapter sleeve for plain cylindrical shafts. As the bearing is driven up the taper, the inner ring expands and grips the shaft — no shaft shoulder or keyway is required, which is why tapered-bore SRBs are standard on pillow block and take-up bearing housings.
The trade-off: a tapered-bore mount can't be checked by measuring an interference fit directly. Instead, you measure how much the internal clearance has been reduced, or how far the bearing has traveled up the taper. Both methods are covered below.
Radial Internal Clearance: What C2 Through C5 Actually Mean
Radial internal clearance (RIC) is the total distance one ring can move relative to the other before the bearing is mounted, measured in micrometers (µm) per ISO 5753. Spherical roller bearings ship in Normal (CN) clearance by default across virtually all manufacturers.
Larger clearance classes — C3, C4, C5 — exist because mounting itself removes clearance (interference fit expands the inner ring) and running heat removes more of it (the inner ring runs hotter than the outer ring and expands faster).
Selected values for cylindrical bore, per ISO 5753-1991:
Bore range (mm) | C2 (µm) | Normal (µm) | C3 (µm) | C4 (µm) |
|---|---|---|---|---|
50–65 | 20–40 | 40–65 | 65–90 | 90–120 |
80–100 | 35–60 | 60–100 | 100–135 | 135–180 |
100–120 | 40–75 | 75–120 | 120–160 | 160–210 |
140–160 | 60–110 | 110–170 | 170–220 | 220–280 |
180–200 | 70–130 | 130–200 | 200–260 | 260–340 |
Tapered bore clearance ranges run higher than cylindrical bore at the same nominal size — for a 100–120 mm bore, Normal clearance on a tapered-bore bearing is 100–135 µm versus 75–120 µm for cylindrical bore — because the interference from sleeve mounting is inherently greater and needs a larger starting clearance to land in the right operating range.
When to step up from Normal to C3: any application with an interference shaft fit tighter than standard, or an operating temperature consistently above roughly 70°C. The clearance loss from a tight fit and thermal expansion can otherwise eat into Normal clearance enough to preload the bearing — and a preloaded spherical roller bearing runs hot, which accelerates lubricant breakdown and shortens fatigue life.
C4 and C5 exist mainly for vibrating screens and high-speed, high-temperature duty where even more clearance loss is expected in service.
Mounting a Tapered-Bore Bearing: The Two Practical Methods
Method 1 — Feeler gauge (mechanical drive-up), for bore ≤ 120 mm
Measure the unmounted clearance at the top of the bearing (12 o'clock, standing bearing) using a feeler gauge between the outer ring and the uppermost roller.
Drive the bearing up the taper — using a lock nut and tab washer for bores up to about 80 mm, or a hydraulic nut for larger sizes — while re-checking clearance at the bottom (6 o'clock) periodically.
Stop once the clearance reduction falls inside the recommended range for that bore size (see worked example below).
For bearings above 120 mm bore, ring deformation under the bearing's own weight distorts a single reading. The standard correction measures clearance at three points instead of one: "a" at 9 o'clock, "b" at 3 o'clock, and "c" at 12 or 6 o'clock. The true clearance is then 0.5 × (a + b + c). A three-point average cancels out the sag that a single reading would wrongly count as clearance.
Method 2 — Hydraulic nut drive-up method, for sealed bearings or large bore
Sealed spherical roller bearings can't be checked with a feeler gauge because the seal blocks access to the rollers. For these, and for any large tapered-bore bearing, use a hydraulic nut fitted with a dial indicator: hydraulic pressure drives the bearing up the taper while the dial indicator reads axial travel directly, which correlates to clearance reduction through the taper angle.
Worked example — 190 mm bore, 1:12 taper, Normal clearance:
Parameter | Value |
|---|---|
Unmounted radial clearance | 0.14–0.20 mm |
Target clearance reduction | 0.09–0.13 mm |
Axial drive-up on solid shaft | 1.4–2.0 mm |
Axial drive-up on adapter sleeve | 1.5–2.2 mm |
Minimum residual clearance after mounting | 0.07 mm |
Note the drive-up distance is roughly 16 times the clearance reduction for a 1:12 taper on a solid shaft, and about 18 times for sleeve mounting — the taper angle magnifies a small radial change into a distance you can actually measure with a dial indicator. For a 1:30 taper, those multipliers rise to roughly 39× and 42× respectively, since a shallower taper needs more axial travel to produce the same radial expansion.
Axial Load Limit on Adapter and Withdrawal Sleeves
When a spherical roller bearing is mounted on a smooth shaft using an adapter or withdrawal sleeve with no fixed shoulder, the axial load the assembly can carry is limited by friction between the shaft and sleeve, not by the bearing itself:
Fap = 3 × B × d
where Fap is the maximum permissible axial load in newtons, B is bearing width in mm, and d is bore diameter in mm. For the 22320 bearing (bore 100 mm, width 73 mm), that caps sleeve-mounted axial load at roughly 21,900 N — well below the bearing's own static rating, which is why sleeve-mounted arrangements are specified for radial-load-dominant applications, not thrust-heavy ones.
Pillow Block and Take-Up Housing Applications
Tapered-bore SRBs on adapter sleeves are the standard bearing in SNW/SAF-style pillow block housings and in take-up bearing units on conveyor systems, precisely because the sleeve eliminates the need for a shaft shoulder or keyway — a plain, uniform shaft can be cut, and the housing position adjusted along its length. The clearance and drive-up principles above apply identically inside these housings; the only difference is that the housing bore, not a bearing seat machined into a larger shaft, provides the outer ring location.
Common Mounting Mistakes
Forcing the bearing onto the shaft without verifying it's seated line-to-line first — causes ring deformation that shows up as premature vibration.
Uneven lock nut tightening — pulls the bearing up the taper at an angle, leading to misalignment that the bearing's self-aligning capability can mask for a while before it doesn't.
Skipping the clearance check entirely and mounting "by feel" — the single most common cause of early-life bearing failure in tapered-bore installations.
Using a feeler gauge on a sealed bearing — physically impossible to get an accurate reading; use the drive-up method instead.
FAQ
Is radial clearance the same thing as bearing preload?
No — clearance is the gap that exists before any external force is applied; preload is a deliberately induced negative clearance used in some precision bearing pairs. A correctly mounted spherical roller bearing retains some positive residual clearance after mounting (see the 0.07 mm minimum in the worked example); it is not preloaded.
How often should mounted clearance be re-checked after installation?
It isn't a running maintenance check — clearance reduction is verified once, during mounting. After that, thermal growth during operation is expected and accounted for by the clearance class chosen up front.
Does a tighter shaft fit always mean I should jump to C3 or C4?
Only if the resulting clearance loss would push the bearing toward zero clearance under running conditions. For moderate interference fits at normal operating temperatures, Normal clearance with a correctly measured drive-up is often sufficient — jumping straight to C4 without checking the numbers can leave a bearing with too much play, which shows up as noise and roller skidding rather than overheating.
Clearance values and drive-up ratios shown conform to ISO 5753-1 and standard 1:12/1:30 taper geometry. Figures are representative for the stated bore and taper combination; confirm exact values against your supplier's mounting instructions for the specific bearing before installation.
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