Rolling element is the spherical or roller-shaped component positioned between the inner and outer rings of a bearing, directly transmitting load from one ring to the other through rolling contact.
This is the component that carries the highest Hertz contact stress in the entire bearing. Contact stress on AISI 52100 bearing steel can reach 1,500–3,000 MPa — three to five times the yield strength of ordinary steel — yet the material withstands it because the stress is instantaneous, compressive, and non-continuous. The shape of the rolling element determines every key characteristic of the bearing: which load types it can carry, its limiting speed, and its failure mechanism.
Types of Rolling Elements
Balls are the most common. Point contact with the raceway produces the lowest friction — ideal for high speed. Balls carry both radial and axial loads, but the small contact area limits load capacity relative to rollers of the same size. Example: bearing 6308 (d = 40, D = 90) uses 8 balls of 15.875 mm diameter.
Cylindrical rollers make line contact with the raceway — the contact area is 5–10 times larger than that of a same-diameter ball, giving 50–100% higher radial load capacity. Series NU, N, and NJ use cylindrical rollers. Example: NU2216 E (d = 80, D = 140) carries 11 rollers each 27 mm long.
Tapered rollers make line contact on a conical surface — the cone angle generates an axial force component. When mounted in opposed pairs face-to-face (DB) or back-to-back (DF), tapered roller bearings handle large combined loads. Series 302xx and 322xx are common in gearboxes and axle assemblies.
Spherical rollers have a barrel-shaped profile — allowing the bearing to self-align up to 2–3°. Series 222xx and 223xx use spherical rollers to accommodate shaft deflection and installation misalignment.
Practical Example: 22220 EK/C3
The double-row self-aligning roller bearing 22220 EK/C3 (d = 100, D = 180, B = 46 mm, C = 365 kN, C₀ = 490 kN) uses symmetric spherical rollers. The EK designation indicates a tapered bore 1:12 — the bearing mounts on an adapter sleeve for radial clearance adjustment.
At a cement plant in Ha Nam, a 22220 EK/C3 on a screw conveyor shaft carried 180 kN radial load. After three years of operation at 480 rpm, profilometer measurement of the rollers showed uniform barrel-surface wear of 0.02 mm — within the allowable tolerance. Scheduled replacement after 25,000 hours per the preventive maintenance plan.
| Rolling Element Type | Contact | Radial Load | Axial Load | Typical ndm |
|---|---|---|---|---|
| Ball | Point | Moderate | Yes | High (800K–1.2M) |
| Cylindrical roller | Line | High | No | Moderate (400K–700K) |
| Tapered roller | Angled line | High | High (paired) | Low–Moderate (200K–500K) |
| Spherical roller | Curved line | Very high | Yes | Moderate (300K–600K) |
Rolling Element Materials and Precision
Standard rolling elements are made from 100Cr6 steel (AISI 52100) — through-hardened to 60–64 HRC. For operating temperatures above 200°C, M50 or M62 steel is used. Silicon nitride (Si₃N₄) ceramic balls are used in extreme-speed applications (ndm > 2,000,000) or where electrical isolation is required — they are 60% lighter than steel and three times harder per NSK bearing materials documentation.
Ball roundness is controlled to Grade G10 per ISO 3290 (form error ≤ 0.25 µm) for standard bearings, and G5 (≤ 0.13 µm) for high-precision bearings. Diameter variation between balls within the same bearing must not exceed 1 µm — if it does, load concentrates on the largest ball, causing localized premature failure.