Why Temperature Matters in Bearing Selection
Standard spherical roller bearings are typically rated for continuous operation at temperatures up to 120°C with conventional grease lubrication. Above this threshold, every aspect of bearing performance changes: the steel expands, the grease degrades faster, the internal clearance closes, and the risk of premature failure increases exponentially.
For applications like steel mill continuous casting lines, paper machine dryer sections, cement kilns, and industrial fan hot-gas recirculation — where operating temperatures routinely reach 150–200°C — bearing selection must account for these thermal effects from the start.
Dimensional Stabilization: The Foundation
Standard bearing steel (GCr15 / 100Cr6) undergoes microstructural changes at elevated temperatures that cause dimensional growth — the rings expand slightly and permanently. A bearing that is precision-ground at room temperature can lose its ABEC tolerance class after hours of operation at 180°C.
The solution is dimensional stabilization — a special heat treatment process applied to the bearing rings during manufacturing:
| Stabilization Class | Maximum Operating Temperature | Typical Application |
|---|---|---|
| S0 (standard) | 120°C | General industrial |
| S1 | 150°C | Moderate heat, fans, kilns |
| S2 | 200°C | Steel mills, paper dryers |
| S3 | 250°C | Extreme heat, special applications |
For steel mill and paper machine applications, specify S2-stabilized bearings as a minimum.
Internal Clearance: C3, C4, or C5?
The single most common mistake in high-temperature bearing selection is specifying standard (CN) internal clearance. At 200°C, the inner ring expands more than the outer ring because it is hotter and has less surface area for heat dissipation. This differential expansion closes the internal clearance.
A bearing with CN clearance at room temperature may have zero or negative clearance at 200°C — leading to rapid overheating and seizure.
Clearance guide for high-temperature applications:
- C3: Suitable up to approximately 120°C with normal temperature differential between inner and outer rings
- C4: Required for 150–200°C applications, steel mill run-out tables, paper machine dryers, kiln fans
- C5: For applications above 200°C, or where the inner ring runs significantly hotter than the outer ring (e.g., directly heated shafts)
Always specify C4 as the minimum for continuous operation above 150°C.
Cage Material Selection
The cage must maintain its mechanical properties at the operating temperature:
- Pressed steel cage (CC type): Acceptable up to 150°C. Above this temperature, the steel loses strength and the cage may deform under roller forces.
- Machined brass cage (CA, MB, MA types): Required for operation above 150°C. Brass retains strength better than steel at elevated temperatures and provides superior guidance accuracy.
- Machined steel cage (special): Available for applications where brass is incompatible with the lubricant or environment.
For steel mill and high-temperature fan applications, specify machined brass cage (CA) as the standard.
Lubrication for High Temperatures
Standard lithium grease has a dropping point around 180–200°C and an operating temperature limit of approximately 120°C for continuous service. For high-temperature applications:
- Polyurea grease: Operating range -30°C to 160°C. Good oxidation stability. Standard for many sealed bearings.
- PTFE-thickened (PFPE) grease: Operating range -40°C to 200°C+. Excellent chemical resistance. For extreme high-temperature applications.
- Oil circulation systems: For continuous operation above 180°C, oil circulation with external cooling is generally preferred over grease.
Relubrication intervals should be calculated using the bearing manufacturer’s formula, with a temperature correction factor. As a rule of thumb, grease life halves for every 15°C increase in operating temperature above 70°C.
Practical Selection Checklist
When specifying spherical roller bearings for a high-temperature application:
- Confirm the actual sustained operating temperature — not the peak, not the ambient, but the temperature the bearing rings reach at steady state
- Specify S2 dimensional stabilization for applications above 150°C
- Specify C4 or C5 internal clearance depending on the temperature differential
- Specify machined brass cage (CA type)
- Select lubricant with operating range exceeding the bearing temperature by at least 20°C
- Calculate relubrication intervals using the temperature-corrected formula
- Consider sealed SB series bearings for applications where field relubrication is difficult
For complex high-temperature applications, send your operating conditions to our engineering team for application-specific recommendations.