Cage (also called a retainer or separator) is the ring-shaped component positioned between the inner and outer rings of a bearing, responsible for spacing the rolling elements evenly around the circumference and preventing direct contact between them.
The cage does not carry the primary load — load travels through the rolling elements and raceways. But when a cage fails, the bearing fails immediately: balls or rollers lose their positioning, collide with each other, generate impact loads, and destroy the raceways within minutes. According to FAG/Schaeffler, cage failures account for approximately 2–3% of all bearing failure causes — a small percentage, but with severe consequences because they typically trigger complete assembly failure.
Cage Materials and Design
The most common material is stamped steel — low cost, adequate strength for standard applications. Stamped steel cages are identified by two halves joined by rivets or a zigzag crimp. Series 6205 through 6310 typically use stamped steel cages, designated by the suffix J in the bearing code (e.g., 6308-J).
Polyamide PA66 reinforced with 25% glass fiber is the choice for high-speed and moderate-temperature applications (up to 120°C continuous). Plastic cages are 60–70% lighter than steel, reducing centrifugal forces at high speed and allowing a higher ndm factor. The bearing code designation is TN9 (e.g., 6308-2RS/TN9).
Machined brass is used for special-duty bearings — high temperature, chemical environments, or where thermal conductivity is needed. Large angular contact ball bearings in the 7200/7300 series commonly use brass cages (designation M, e.g., 7210 BM). Above 200°C, a PEEK cage is the only practical option.
Practical Example: 6205-2RS1/C3
The 6205-2RS1/C3 bearing (d = 25, D = 52, B = 15 mm, C = 14.8 kN, C₀ = 7.8 kN) uses a standard stamped steel cage. The 2RS1 designation indicates two-sided contact seals — the cage is fully enclosed, isolated from the external environment.
At a textile factory in Nam Dinh, a 6205-2RS1/C3 on a shuttle shaft ran at 6,000 rpm. After 8 months, an abnormal high-frequency noise appeared. Disassembly revealed uneven wear on one pocket of the steel cage — caused by 0.05 mm shaft misalignment. After realigning the shaft and replacing the bearing, the noise disappeared and the replacement ran for over two more years without incident.
| Cage Type | Code | Max Temp | Typical ndm | Application |
|---|---|---|---|---|
| Stamped steel | J | 150°C | 450,000 | General purpose |
| PA66-GF plastic | TN9 | 120°C | 700,000 | High speed, lightweight |
| Machined brass | M | 200°C | 600,000 | Large bore, high temperature |
| PEEK | — | 250°C | 800,000 | Extreme environments |
Cage Failure: Identification and Root Causes
Cage failures typically result from one of three causes: speed exceeding the ndm limit (cage torn apart by centrifugal force), insufficient lubrication (friction between cage and rolling elements rises sharply), or incorrect installation (prying the cage during press-fitting with excessive force applied at the wrong location). See how each component interacts in bearing construction explained, and choose deep groove ball bearings with the right cage material for your speed range.
Early warning signs: irregular high-frequency noise (distinct from the uniform hum of normal bearing wear), and sudden vibration spikes. When disassembling a bearing and finding balls no longer evenly spaced, or a cracked or fractured cage — this is mechanical failure, not material fatigue. Root cause investigation is essential to prevent recurrence.