Bearing Sealing Technologies: Open, Shielded, and Sealed
Rolling element bearings are precision components that depend on clean lubrication to achieve optimal performance and service life. Yet, over 40% of premature bearing failures are attributed to contamination or lubrication degradation—problems directly addressed by the bearing's sealing system. Choosing between open, shielded, or sealed bearings isn't merely a matter of price; it is a critical engineering decision that affects rotational speed, friction torque, operating temperature, and maintenance intervals. This article examines these three sealing configurations, their technical characteristics, and application-specific selection criteria, enabling engineers and procurement professionals to specify the right bearing for demanding environments.
The Role of Sealing in Bearing Reliability
Bearings operate with a thin elastohydrodynamic (EHD) oil film separating rolling elements and raceways. When contaminants such as dust, moisture, or metal particles enter the bearing, they compromise this film, causing abrasive wear, surface fatigue, and eventual spalling. Conversely, loss of lubricant via leakage or evaporation increases friction and accelerates wear. Sealing systems serve two primary functions: excluding contaminants and retaining lubrication. However, these benefits come with trade-offs—seals add friction, reduce speed capability, and increase operating temperature.
Effective sealing also supports bearing maintenance strategy. Non-contact seals and shields allow for relubrication, while fully sealed bearings are typically "lubricated for life," eliminating the need for periodic lubrication and reducing maintenance costs. The choice of sealing therefore influences not only bearing performance but also the entire lifecycle management of rotating equipment.
Open Bearings: Design and Applications
Open bearings have no integral sealing or shielding. The rolling elements and cage are fully exposed, requiring the housing and shaft design to manage contamination and lubrication through external means.
Key Characteristics:
Typical Applications: Gearboxes, high-speed spindles, turbine shafts, and industrial transmissions where oil lubrication and filtration are already present. Open bearings are also used when bearing operating temperatures exceed the capability of integral seals (e.g., >150°C).
Shielded Bearings (ZZ and Z Type)
Shielded bearings incorporate a metallic shield—typically made of cold-rolled steel or stainless steel—on one side (Z) or both sides (ZZ). The shield is pressed into the outer ring but leaves a small annular gap (usually 0.2–0.3 mm) relative to the inner ring.
Technical Attributes:
Shielded bearings are typically pre-greased at the factory (e.g., with lithium-based grease, NLGI grade 2) and are considered "greased for life" in many applications, though an inner grease fill can be adjusted. They suit environments where contamination is moderate and where low friction is critical.
Applications: Electric motors, alternators, small gearboxes, and household appliances—where high speed is required, but fine contaminants are not present.
Sealed Bearings (RS and 2RS Type)
Sealed bearings use a contact rubber seal, commonly nitrile (NBR) or fluoroelastomer (FKM), bonded to a steel or polymer insert. The seal lip presses against the inner ring land with a controlled interference, creating an effective barrier.
Design Variations:
Performance Data:
| Parameter | Shielded (ZZ) | Sealed Contact (2RS) |
|---|---|---|
| Friction torque | Very low (≈5–10% of bearing torque) | Higher (≈25–40% of bearing torque) |
| Maximum speed reduction | 0% (same as open) | 20–30% lower than open |
| Operating temperature | Up to 150°C (steel shield) | –40°C to +120°C (NBR); –30°C to +200°C (FKM) |
| Contaminant exclusion | >0.3 mm particles | >0.01 mm particles (dust-tight) |
| Relubrication | Not practical | Not required (lubricated for life) |
| Initial grease life | 20,000–40,000 hours (typical) | 15,000–30,000 hours (temp dependent) |
Sealed bearings have lower allowable speeds due to seal lip friction, which also increases heat generation. However, they excel in dirty environments—agricultural machinery, automotive wheel hubs, electric motors, and conveyor systems—where the cost of lubrication maintenance outweighs the slight performance penalty.
Comparative Analysis and Selection Criteria
Selecting the optimal sealing technology requires balancing contamination resistance, speed, friction, and maintenance strategy. The table below summarizes key selection parameters for a 6204-2Z vs. 6204-2RS bearing (bore 20 mm, OD 47 mm, width 14 mm):
| Parameter | 6204 Open | 6204-ZZ (Shielded) | 6204-2RS (Sealed) |
|---|---|---|---|
| Basic dynamic load rating, C | 12.7 kN | 12.7 kN | 12.7 kN |
| Reference speed (oil) | 24,000 RPM | 24,000 RPM | 18,000 RPM |
| Limiting speed (grease) | 20,000 RPM | 20,000 RPM | 12,000 RPM |
| Friction factor (μ) | 1.0 (baseline) | 1.1 | 1.8–2.5 |
| Relubrication capability | Yes | Limited (via groove) | No (lubricated for life) |
| Relative cost | Low | Moderate | Moderate-High |
Application Guidelines:
Furthermore, bearing maintenance is greatly simplified by sealed bearings since greasing intervals are eliminated. For existing machinery, replacing open bearings with sealed versions may require housing modifications to prevent pressure buildup, especially if the housing contains a grease relief valve.
Conclusion
Bearing sealing technology is a direct driver of reliability, operational temperature, and maintenance requirements. Open bearings offer maximum speed and external lubrication flexibility; shielded bearings serve as a low-friction compromise in mild environments; and sealed bearings provide the highest contamination protection with reduced speed capacity and lifetime lubrication. Engineers should evaluate operating conditions, access for relubrication, and speed requirements before specifying a seal type. For applications ranging from high-speed spindles to rugged industrial gearboxes, understanding these trade-offs ensures optimal bearing performance and minimized downtime.
At Haihe Bearings (yandianbearing.com), we supply a comprehensive range of open, shielded, and sealed bearings with matched sealing materials and grease options, helping you engineer the perfect fit for your machinery.