“Shielded” and “sealed” are often used casually, but they describe different closure behaviour. In a miniature bearing, even a small change in drag or heat can matter because the motor has little thermal and torque margin. At the same time, an open bearing that performs well in a clean test fixture may be poorly protected in field operation.
Open
No integral closure. Lowest obstruction around the rolling system, but depends on a clean assembly and external protection.
Shielded
A non-contact metal shield helps retain grease and limit larger contaminants with little or no rubbing contact.
Sealed
A rubber closure can be non-contact or contact type. Contact improves sealing but usually increases torque.
Four closure types to distinguish
Open bearing
An open bearing has no integral shield or seal. It can suit a protected, clean assembly or a design with external lubrication and sealing, but leaves the rolling elements more exposed during handling and operation.
Metal shield
A metal shield is fixed to the outer ring and normally does not contact the inner ring. MinebeaMitsumi describes its ZZ steel shields as non-contact closures. The small clearance helps limit contamination and grease leakage while avoiding seal rubbing torque.
Non-contact rubber seal
A non-contact rubber seal reduces the gap around the inner ring without intentional rubbing contact. It can provide more protection than a simple shield while retaining lower torque than a contact seal, depending on the exact design.
Contact rubber seal
A contact seal touches the inner-ring land. MinebeaMitsumi notes that its contact DD type has stronger sealing and higher rotating torque than non-contact enclosures. NTN likewise explains that contact sealing improves contamination control while lowering limiting speed because friction increases torque.
The main engineering trade-offs
| Factor | Open / shielded direction | Contact-sealed direction |
|---|---|---|
| Rotating torque | Generally lower because there is no rubbing lip. | Generally higher because the seal contacts the inner ring. |
| Speed and heat | Often preferred where low drag and high speed dominate. | Seal friction can add heat and reduce the permissible speed. |
| Contamination | Protection depends on shield clearance and external assembly design. | Better barrier against fine dust, moisture and lubricant loss. |
| Handling | Open types require especially clean storage and assembly. | Integral protection reduces direct exposure, but still requires clean handling. |
| Lubrication | Shielded designs can retain grease; open designs may use an external strategy. | Seals help retain grease, but lubricant type and fill remain important to torque and heat. |
These are directions, not absolute guarantees. Closure geometry, material, lubricant, size, speed, temperature and manufacturing details all change the actual result. Compare supplier data for the exact part.
Match protection to the real flight environment
An indoor development bench, a cinematic platform over dry terrain and an FPV aircraft flown through grass or dusty ground do not present the same contamination risk. Describe the operating environment in concrete terms:
- Dust, sand, grass particles or carbon-fibre debris;
- Humidity, rain exposure, salt air or agricultural chemicals;
- Motor airflow that can carry contamination toward the bearing;
- Assembly-room cleanliness and the time a bearing remains exposed before installation;
- Whether the motor is treated as serviceable or replaced as a complete unit.
SMB Bearings notes that shielded miniature bearings can be supplied with low-noise, low-torque grease and warns that adding oil to a grease-filled shielded bearing can change grease viscosity. Maintenance advice must therefore match the supplied bearing and motor design rather than rely on a universal “add lubricant” rule.
Validate torque, heat and protection together
Do not evaluate a closure only by hand feel. Use a controlled motor-level test and record the complete bearing specification. A useful comparison can include no-load current, starting behaviour, temperature rise, vibration or sound, endurance at defined speed, and inspection after controlled contamination exposure.
- Keep shaft, housing, rotor and assembly method constant.
- Use the same lubricant specification and fill basis where the comparison allows.
- Test nominal and peak speed, not only a brief spin.
- Record ambient temperature and motor winding condition.
- Inspect for grease migration, contamination ingress and seal damage after testing.
- Select the closure that meets the complete system requirement—not simply the lowest torque or strongest seal.
Questions to send with the enquiry
What are the nominal RPM, peak RPM and time at each condition?
Which dust, moisture, chemicals or handling risks must the closure resist?
Is there a starting-torque, current or temperature-rise limit?
What grease or oil, fill amount and service expectation apply?
Vohann can coordinate a review around the application, dimensions, closure preference, lubricant requirements, samples and production demand. See the broader drone and FPV bearing application guide or submit the available data through the project enquiry form.