A Guide to RU Series Crossed Roller Bearings: Open vs Sealed

As a design engineer, you have likely selected the RU series for its high rigidity and integrated mounting structure, which simplifies installation and reduces component count. However, the next decision is critical: choosing between the standard open model and the double-sealed RU-UU variant. This choice directly impacts rotational performance, maintenance intervals, and the long-term reliability of your equipment. An incorrect selection can lead to premature failure from contamination or excessive friction torque in sensitive systems.

At BY Bearings, with over 15 years of experience in precision manufacturing, we guide customers through this selection process daily. This article provides a clear framework for deciding between open and sealed Precision Crossed Roller Bearings, ensuring you specify a bearing that meets both performance and environmental demands. All our bearings are manufactured under a strict ISO 9001 certified quality system, providing a foundation of reliability for your design.

Key Differences: Open RU vs. Sealed RU-UU Construction

The fundamental distinction between the two models lies in their protection against the operating environment. Understanding this structural difference is the first step in making an informed choice.

RU Series (Open Type): This is the standard configuration. The bearing has no integral seals, leaving the internal rollers and raceways exposed. It relies entirely on the machine's housing or external sealing systems to prevent the ingress of dust, moisture, and other contaminants. It also requires an external lubrication system to be designed into the assembly for periodic re-greasing.
RU-UU Series (Double-Sealed Type): This model features synthetic rubber seals fitted on both sides of the bearing. These seals are designed to retain the pre-filled lubricating grease and provide a barrier against external contaminants. The "UU" suffix denotes seals on both sides. This makes the bearing a self-contained, lubricated-for-life unit in many applications, simplifying both design and maintenance.

While a sealed bearing offers obvious convenience, the seals themselves introduce trade-offs in friction, speed, and heat generation. The following sections will break down these factors to help you align the bearing type with your specific application needs.

Decision Framework: Selecting Between Open and Sealed Models

Choosing between an open and sealed bearing is a balancing act between protection, performance, and maintenance effort. Use the following table to weigh the critical factors against your application's specific requirements. This framework is based on common questions we address for customers in industries from medical imaging to industrial automation.

Decision Factor	RU (Open)	RU-UU (Double-Sealed)	Selection Guidance
Contamination Exposure	Highly vulnerable. Requires robust external sealing.	Good protection against dust, debris, and light fluid splash.	For dirty environments (e.g., CNC machining, woodworking, welding robots), the RU-UU is the default choice. For cleanrooms or sealed enclosures (e.g., semiconductor equipment, optical instruments), an open RU is often sufficient and preferred for lower torque.
Rotational Friction Torque	Lower friction. Only inherent roller and lubricant drag.	Higher friction due to seal lip contact with the inner ring.	For low-torque applications like direct-drive motors or sensitive instrumentation, the open RU is superior. The added drag from seals in an RU-UU can impact positioning accuracy and increase power consumption.
Maximum Rotational Speed	Higher speed capability. No seal-related heat generation.	Lower speed limit. Seal friction generates heat, limiting the maximum permissible rpm.	If your application pushes the speed limits of the bearing size, the open RU model provides a higher thermal safety margin. Always check the catalog's limiting speed for both variants.
Maintenance & Lubrication	Requires a planned re-lubrication schedule and procedure. Access for maintenance is necessary.	Generally considered lubricated-for-life, eliminating the need for re-greasing in most applications.	For difficult-to-access locations or to reduce total cost of ownership, the RU-UU is a clear winner. If a specific, high-performance grease is required for the application, the open RU allows for custom lubrication.
Design & Installation Complexity	Requires the design of an effective external sealing system, increasing component count and assembly complexity.	Simplifies housing design. The bearing is a self-contained unit, reducing engineering time and potential assembly errors.	To accelerate design-to-production cycles and simplify assembly, the RU-UU offers a significant advantage. It reduces the number of parts to source and assemble.

Application Scenarios: Where Each Bearing Type Excels

Theory is useful, but real-world examples clarify the decision. Here are common applications for both open and sealed RU series bearings, reflecting the trade-offs discussed above.

Typical Applications for RU (Open) Bearings

Open bearings are the preferred choice when the environment is controlled or when performance parameters like low torque and high speed are non-negotiable.

Semiconductor & Medical Equipment: In wafer handling systems or CT scanners, operations occur in a clean, controlled environment. Here, the primary concerns are ultra-low vibration and minimal friction torque. An open bearing provides the smoothest rotation. This aligns with our experience supplying high-precision bearings to customers in Germany for sensitive semiconductor equipment.
High-Speed Indexing Tables: For rotary tables that require rapid, repetitive positioning, the lower heat generation of an open bearing allows for higher sustained speeds without thermal expansion affecting accuracy.
Custom Lubrication Systems: Applications requiring specialty lubricants, such as vacuum-rated or food-grade greases, necessitate an open bearing that can be packed with the specified lubricant during assembly.

Typical Applications for RU-UU (Sealed) Bearings

Sealed bearings are the workhorses for general industrial machinery where reliability in less-than-ideal conditions is paramount.

Robotic Joints: The arms of Bearings for Industrial Robotics are exposed to dust, welding spatter, and cutting fluids. A sealed RU-UU bearing protects the raceways, ensuring long life and consistent positioning accuracy without requiring frequent maintenance in hard-to-reach joints.
CNC Machine Tool Rotary Axes: Whether in a 4th-axis rotary table or a turn-mill center, these bearings are constantly exposed to coolant and metal chips. The integrated seals of the RU-UU are essential for preventing contamination that would otherwise quickly destroy the bearing.
General Factory Automation: In conveyor systems, packaging machines, and automated assembly lines, a sealed, pre-lubricated bearing drastically reduces maintenance demands and simplifies machine design, lowering the total cost of ownership.

Common Selection Mistakes to Avoid

Over my 15 years in precision bearing manufacturing, I have seen several recurring mistakes in specifying RU series bearings. Avoiding these simple errors can save significant time and cost down the line.

Ignoring "Minor" Contamination: Engineers sometimes specify an open bearing for an application they consider "mostly clean," such as a machine in an enclosed cabinet. However, airborne dust, condensation, and microscopic debris from other moving parts can still enter an unsealed bearing over time, causing premature wear. Rule of Thumb: If the environment is not a certified cleanroom, start by considering the sealed RU-UU as your baseline.
Overlooking Seal Drag in Torque-Sensitive Systems: Conversely, for direct-drive turntables or optical scanning heads, the consistent, low-running torque is a primary design requirement. The added (and slightly variable) friction from the UU seals can introduce positioning errors or motor instability. Action: If your torque budget is measured in N-cm, the open RU model is almost always the correct choice, paired with a non-contact labyrinth seal if protection is needed.
Assuming Sealed Means Invincible: While RU-UU seals are effective against dust and splash, they are not designed for high-pressure fluid jets or full submersion. Aggressive chemical washdowns can degrade the seal material. Verification: Always confirm the chemical compatibility of the NBR seals with any fluids present in your operating environment.
Failing to Plan for Re-lubrication Access: When choosing an open RU bearing, the design must include accessible grease fittings and a clear path for purging old grease. Burying an open bearing deep inside a complex assembly without a re-lubrication plan is a recipe for future failure.

Our manufacturing process includes 100% pre-shipment inspection of key parameters, including runout, which we control within 1.5 μm for our highest precision P2 grade bearings. This ensures that whether you choose an open or sealed model, the core geometric accuracy of your BY Bearings product is guaranteed. For details on our inspection processes, you can review our Quality Control procedures.

The RU series, with its integrated design, is an excellent solution for achieving high-rigidity rotation in a compact space. By carefully evaluating your application's environment, torque requirements, and maintenance strategy, you can confidently select between the open RU and sealed RU-UU models to achieve optimal system performance and reliability.

If you require custom mounting holes, special preload settings, or have questions about the best bearing for your specific design, our engineering team is ready to assist. Send us your technical specifications to get a quote and a tailored recommendation.

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