21300 Series Spherical Roller Bearings are used for carrying the heaviest radial loads in the catalog, including shock, while self-aligning under shaft deflection and housing misalignment. Two rows of barrel rollers run in a common spherical outer raceway, so the roller set swivels with the shaft while keeping full-length line contact — capacity and alignment freedom in one bearing, which no other type provides together. This page sets out the industries and machine positions where the type is specified, what has to be stated on the order, mounting practice, and the duties where a different bearing type is the correct answer. Boundary dimensions are to ISO 15 / GB/T 288 / DIN 635-2. This series is catalogued from 20 mm to 100 mm bore; the full table of boundary dimensions is further down the page.
Typical Applications for 21300 Series Spherical Roller Bearings
The machine positions and sectors where this type is specified:
- Vibrating screens: the reference duty for the type: continuous shock load with the shaft deflecting under an out-of-balance mass, and a special vibratory execution to suit it.
- Crushers and mills: eccentric and main shafts in aggregate and mining plant where load and contamination are both extreme.
- Paper machines: dryer and press roll positions, where roll deflection across a wide machine is measured in millimetres.
- Continuous casting plant: segment rolls carrying heavy load at low speed and high temperature.
- Wind turbine main shafts: the rotor position, where the shaft deflects under blade loads that reverse every revolution.
How to Specify 21300 Series Spherical Roller Bearings for Your Application
- Bore: Start from the bore the shaft or housing gives you — this series is catalogued from 20 mm to 100 mm — then check the outer diameter and width against the space available. The boundary dimensions are fixed by the standard, so they are the same from any manufacturer; the load ratings and limiting speeds are not, and belong to the specific bearing being supplied.
- Bore form and sleeve: A cylindrical bore mounts directly on a machined shaft seat. A tapered (K) bore mounts on an adapter sleeve on a plain shaft, or on a withdrawal sleeve where the bearing has to come off again in service — a decision that changes the mounting procedure, the tooling and the residual clearance, so it belongs on the order.
- Internal clearance: C3 is the usual starting point in this family rather than the exception, because the interference fit and the drive-up on a tapered bore both consume clearance before the bearing ever runs. C4 is specified where the shaft runs much hotter than the housing.
- Minimum load: A spherical roller bearing needs a minimum load to keep the rollers rolling. A lightly loaded bearing at speed lets the unloaded rollers skid as they re-enter the load zone, and the smearing that follows is a lubrication failure, not a fatigue one.
- Designation and suffixes: State the complete designation including every suffix. The base number gives the type and the boundary dimensions; the suffixes carry the sealing, the cage, the clearance group, the tolerance class and any special execution — and two bearings whose base numbers match but whose suffixes differ are not interchangeable.
- Internal clearance group: State the radial internal clearance: C2 (reduced), CN (normal, and unmarked), C3, C4 or C5. Clearance is consumed after the bearing is fitted — by the interference fit on the shaft, and by the inner ring running hotter than the outer — so a bearing with a tight fit or a hot inner ring is ordered C3 and a bearing mounted lightly at even temperature is left CN. This field is left off orders more often than any other, and the bearing then arrives in the normal group by default.
- Seal or shield: State open, shielded (ZZ, 2Z) or sealed (2RS, 2RZ). A shield is non-contacting: it excludes coarse contamination and costs nothing in speed. A contact seal excludes fine dust and water and retains grease, but it rubs — it adds drag, raises the running temperature and lowers the permissible speed. Sealed bearings are also greased for life and cannot be relubricated.
- Cage material: State the cage. Pressed steel is the default and suits most duties; machined brass takes shock, high temperature and poor lubrication better and is used in large and heavily loaded bearings; polyamide is quiet, light and low-friction but has a temperature limit of about 120 °C and is attacked by aged oil. The cage is not a detail — it is the component that most often sets the real temperature limit of the bearing.
- Shaft and housing fit: Specify the seat tolerances, not just the bearing. The rule is that the ring which rotates relative to the load direction takes the interference fit: for a rotating inner ring under normal radial load the shaft seat is typically k5 or k6 with the housing bore H7, and where the outer ring rotates the interference moves to the housing (N7 or P7) with a clearance fit (g6) on the shaft. Heavier loads and hollow shafts move the fits tighter. A correctly chosen bearing in the wrong fit fails anyway — too loose and the ring creeps and frets its seat, too tight and the interference consumes the internal clearance and the bearing runs preloaded.
- Standard: Boundary dimensions are to ISO 15 / GB/T 288 / DIN 635-2. Quoting the standard on the order guarantees the mounting dimensions are interchangeable between suppliers — it does not standardize load ratings, limiting speeds or internal design, which remain manufacturer-specific.
Mounting & Assembly Practice
- On a tapered bore, measure the reduction in radial internal clearance as you drive the bearing up the taper — with a feeler gauge between the unloaded roller and the outer raceway, before and during mounting. The drive-up distance is a check on that measurement, not a substitute for it.
- Withdrawal sleeves and hydraulic nuts exist because a bearing this size cannot be driven up accurately by hammer and drift; use them where the size makes hand mounting a guess.
- Keep the outer ring free to align in the housing while tightening the sleeve, then check that the bearing turns freely by hand before closing the housing.
- Match the housing to the duty: a split plummer block is chosen so that a bearing this size can be inspected and replaced without dismantling the shaft line.
Common Selection Mistakes with 21300 Series Spherical Roller Bearings
- Driving a tapered-bore bearing up the sleeve "until it feels right". Over-drive removes all internal clearance, the bearing runs preloaded from the first minute, and it fails within days.
- Specifying the type for a light, fast position because it is the strongest available. Below the minimum load the rollers skid, and the failure mode is smearing rather than fatigue.
- Treating the alignment capability as licence to skip alignment of a plummer block line; the bearing accepts static misalignment, but a badly aligned shaft still puts unnecessary load into the seals and the structure.
- Sizing this bearing from the boundary dimensions alone. Load ratings and limiting speeds are manufacturer-specific for this series and are deliberately not published here — take them from the catalog of the bearing actually being supplied and calculate the life against them.
Where 21300 Series Spherical Roller Bearings Are Not the Right Choice
- Not for high speed. Two rows of barrel rollers with a large sliding component at the roller ends set a limiting speed well below a ball bearing of the same bore.
- Not for light loads at speed — below the minimum load the rollers skid and smear rather than roll.
- Not a thrust bearing. Axial capacity is real and useful, but it is secondary to a radial load; a purely axial duty needs a thrust spherical roller bearing, which is a different type.
- Rarely justified for light general duty: the price and the envelope of the type only make sense where load, shock or deflection actually demand it.
- The alignment capability is for a static angle between the shaft and the housing seat; it is not a licence to leave a shaft line unaligned, and it does not absorb misalignment that changes every revolution.
