What a bearing designation actually tells you
A bearing designation is not a part number in the ordinary sense — it is a coded specification. The base number identifies the type, series and bore. The suffixes that follow it define internal clearance, cage material, sealing and precision class, and two bearings with the same base number but different suffixes are not interchangeable in a demanding application.
When you raise a requirement, quote the complete designation stamped on the bearing you are replacing, including suffixes. A designation given without its clearance suffix leaves the supplier to assume standard clearance, which is wrong for any shaft running hot or fitted with interference.
| Designation element | What it defines |
|---|---|
| First digit(s) | Bearing type — ball, taper roller, spherical roller, needle |
| Series digits | Dimension series — governs outer diameter and width for a given bore |
| Last two digits | Bore code — multiply by 5 for bores of 20 mm and above |
| C2 / CN / C3 / C4 | Radial internal clearance, from reduced through standard to extra loose |
| 2RS / ZZ / RS | Sealing arrangement — contact seals, metal shields, single seal |
| P5 / P6 / P0 | Precision class — tighter running accuracy for higher speeds |
Choosing the bearing type
Type selection follows the direction and magnitude of the load, the speed, and whether the arrangement has to tolerate shaft misalignment.
| Type | Load characteristics | Typical use |
|---|---|---|
| Deep groove ball | Moderate radial with some axial capacity; highest speed capability | Electric motors, fans, pumps, general light-duty drives |
| Taper roller | Heavy combined radial and axial in one direction; mounted in pairs | Gearboxes, wheel hubs, heavily loaded shafts |
| Spherical roller | Very heavy radial load with self-aligning capability | Crushers, mill drives, conveyor pulleys, vibrating screens |
| Cylindrical roller | Very heavy radial, low axial; tolerates axial shaft expansion | Large electric motors, rolling mills, gearbox floating positions |
| Needle roller | High radial capacity in a small radial envelope | Compact gearboxes, linkages, tight-space assemblies |
| Pillow block / housed unit | Radial with built-in self-alignment and integral housing | Conveyors, agricultural and general plant equipment |
Internal clearance — the specification most often got wrong
Radial internal clearance is the play inside the bearing before it is mounted. Fitting a bearing onto an interference-fit shaft expands the inner ring and consumes part of that clearance; running temperature consumes more, because the inner ring typically runs hotter than the outer. If the starting clearance was too small, the bearing preloads itself in service, temperature climbs further, and it fails early.
- CN (standard) suits most general applications with normal fits and modest temperature rise
- C3 is the usual choice where there is an interference fit, a significant temperature difference across the bearing, or a hollow shaft
- C4 is used for high temperature differentials or heavy interference fits, common on mill and crusher duty
- C2 is reduced clearance, used only where running accuracy matters more than thermal margin
Load rating and expected life
Bearing catalogues quote a dynamic load rating (C) and a static load rating (C₀). The dynamic rating feeds the basic rating life calculation, which predicts the hours 90% of a population of identical bearings will exceed under a given load and speed. The static rating governs permanent deformation in a stationary or slow-oscillating bearing.
For procurement, the practical point is that rating life falls very steeply as load rises — for ball bearings it varies with the cube of the load ratio, and for roller bearings faster still. A modest increase in actual operating load over the design assumption shortens life far more than most buyers expect, which is why an undersized replacement chosen on price rarely saves money.
Lubrication and sealing
Most industrial bearing failures trace back to lubrication — the wrong grease, too much or too little of it, or contamination getting past a seal. Select the arrangement against the operating temperature, the speed and how dirty the environment is.
- Sealed-for-life bearings (2RS) suit inaccessible positions and moderate speeds and temperatures
- Shielded bearings (ZZ) run cooler and faster than contact seals but keep out less contamination
- Open bearings with a relubrication route suit high temperatures, high speeds and dirty environments where grease has to be renewed
- Match the grease base oil viscosity to the speed and temperature, not to whatever is already on the shelf
Mounting and handling
A correctly specified bearing can still fail in the first hours if it is mounted badly. Mounting force must be applied to the ring being fitted — never through the rolling elements — and induction heating should be used for interference fits rather than a hammer and drift. Keep bearings in their original packaging until the moment of fitting; the corrosion protection they ship with is doing real work.
What to include in a bearing enquiry
- The complete designation including clearance, cage and sealing suffixes
- Quantity, and whether the requirement is a breakdown or planned maintenance
- The equipment and duty, if the original designation is unknown or illegible
- Shaft and housing diameters where a bearing has to be identified from dimensions
- Any brand preference, and whether an equivalent from another manufacturer is acceptable
- Delivery city and required date