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Help CenterONA ORU124(G)/ORU124X Thin-Walled Crossed Roller Bearing P4/P2 High Rigidity With Mounting Holes
Product Spotlights
Minimal performance impact from mounting – Installation procedure does not affect rotational accuracy or torque, ensuring consistent performance.
High rigidity– Crossed roller design with 90° roller arrangement handles radial, axial and overturning loads simultaneously.
Stable rotation torque
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ONA RU Series – One‑Piece Inner and Outer Ring Crossed Roller Bearings with Mounting Holes
1. Product Overview
The ONA RU Series is a premium crossed roller bearing featuring an integrated (one‑piece) inner and outer ring design. Unlike conventional crossed roller bearings that require separate presser flanges or housings to secure the rings, the RU series is equipped with mounting holes on both the inner and outer rings. This allows the bearing to be bolted directly to adjacent machine structures, eliminating the need for additional retaining components and significantly simplifying mechanical design.
Because the mounting procedure has minimal influence on the bearing’s internal geometry, the RU series delivers stable rotational accuracy and torque performance even after installation. Furthermore, the integrated ring construction enables the bearing to be used for both inner ring rotation and outer ring rotation applications, providing exceptional design flexibility.
As part of Luoyang ONA Bearing Co., Ltd.’s high‑precision product portfolio, the RU series is manufactured to the highest quality standards and serves as a direct replacement for imported bearings such as THK RU series, IKO CRBS/CRB series, and NSK NRXT series.
2. Structural Design
2.1. One‑Piece Inner and Outer Rings
The defining characteristic of the RU series is that both the inner ring and the outer ring are single, non‑split components. This is fundamentally different from the RB series (split outer ring) and RE series (split inner ring). The one‑piece construction provides:
Higher overall rigidity – No split lines to compromise stiffness
Simplified handling – No risk of rings separating during transport or assembly
Easier sealing integration – Seals can be installed without interference from split joints
2.2. Mounting Holes for Direct Bolt‑On Installation
Both rings are provided with precision‑drilled mounting holes (through holes or threaded holes, depending on the model). These holes allow the bearing to be securely fastened directly to the machine frame or rotary table using standard bolts. Consequently:
No presser flange required – The bearing does not need external clamping flanges
No special housing needed – The bearing can be mounted on flat, machined surfaces
Installation is fast and repeatable – Bolt torque directly determines the preload on the mounting interface
2.3. Crossed Roller Arrangement
Inside the RU bearing, cylindrical rollers are arranged perpendicularly to each other on the rolling surfaces of 90° V‑shaped grooves. The rollers are separated by isolation blocks (spacer retainers) that maintain proper spacing and orientation. This arrangement ensures that:
Rollers do not tilt or skew
Rollers do not rub against each other
Friction torque is low and uniform
No locking or jamming occurs, even under heavy loads
2.4. Thin‑Wall / Ultra‑Compact Cross‑Section
The RU series follows the same thin‑wall philosophy as other ONA crossed roller bearings. The cross‑sectional dimensions (radial height and width) are minimized to the practical limit, approaching the smallest possible size for a given bore diameter. Despite the ultra‑thin profile, the RU series retains exceptional rigidity – a direct result of the crossed roller geometry and the one‑piece ring construction.
3. Key Performance Characteristics
3.1. High Load Capacity – Radial, Axial and Overturning Moments
Because the rollers are arranged at 90° on V‑shaped raceways, a single RU bearing can simultaneously withstand:
Radial loads – Forces perpendicular to the shaft axis
Bidirectional axial loads – Thrust forces from both directions
Overturning moment (tilting moment) loads – Critical for rotary tables, robotic joints, and any application where the load is offset from the bearing center
This combined load capability eliminates the need for multiple bearings arranged in a complex configuration, saving space and reducing assembly time.
3.2. High Rigidity – 3.4 Times Conventional Bearings
The crossed roller design, combined with the one‑piece rings and preload capability, delivers rigidity 3.4 times higher than traditional bearing arrangements (such as two pairs of angular contact ball bearings or a combination of radial and thrust bearings). This high rigidity ensures:
Minimal elastic deformation under heavy or varying loads
Excellent machining stability in machine tool applications
Precise positioning even under dynamic load conditions
3.3. Precision Grades P5, P4 and P2
ONA RU series bearings are manufactured to the highest precision standards:
P5 – Standard precision for most industrial applications
P4 – High precision for machine tool rotary tables and measuring equipment
P2 – Ultra‑high precision for aerospace, medical, and research applications
The bearings are 100% inspected for dimensional accuracy, running accuracy (radial and axial runout), and torque consistency.
3.4. Stable Rotational Accuracy and Low Torque
Isolation blocks between the perpendicular rollers prevent roller‑to‑roller contact, eliminating the friction and irregular motion that can occur with conventional steel‑plate retainers. As a result:
Starting torque is low and predictable
Running torque remains uniform across the entire speed range
No sudden torque spikes or locking phenomena
This stable rotational behavior is essential for applications requiring smooth, jitter‑free rotation, such as precision indexing and robotic arm control.
3.5. Minimal Performance Impact from Mounting
Because the RU series does not rely on external flanges or housings to maintain ring integrity, the act of bolting the bearing into place does not alter its internal clearance or preload. In many other bearing types, improper mounting can distort the rings, leading to loss of precision, increased torque, or premature failure. With the RU series, performance is inherently stable – the bearing’s factory‑set precision remains unchanged after installation.
3.6. Adjustable Clearance and Preload (Available in Special Designs)
While the standard RU series is supplied with optimized internal clearance, ONA can also provide versions with adjustable clearance by incorporating a split ring design on one of the rings (custom option). This allows field adjustment of preload to fine‑tune rigidity versus speed characteristics for specific applications. Even when preload is applied, the high‑precision rotational motion is maintained.
3.7. Material – Carburized Steel for Durability
RU series bearings are manufactured from high‑quality carburized bearing steel. This material provides:
Excellent impact resistance – Withstands shock loads common in industrial robots and heavy machinery
Superior surface wear resistance – Extends service life in contaminated or poorly lubricated environments
High rolling contact fatigue strength – Supports long‑term, high‑duty operation
4. Application Fields
Thanks to its combination of compact dimensions, high load capacity, high rigidity, and precision, the RU series is suitable for a wide range of industries and equipment:
| Application Field | Specific Equipment / Use | Why RU Series Is Suitable |
|---|---|---|
| Industrial Robots | Joints (shoulder, elbow, wrist), swiveling units, rotating actuators | High rigidity under dynamic loads; compact size fits limited space; supports moment loads from robot arms |
| Machine Tools | Rotary tables (4th/5th axes) of machining centers, vertical/horizontal milling machines, grinding machines, gear cutting machines | P4/P2 precision ensures accurate indexing; handles combined cutting forces; mounting holes simplify table design |
| Precision Rotary Tables | Indexing tables, dividing heads, direct‑drive motor (DD motor) rotors | Stable torque and low starting friction; adjustable preload for optimized stiffness |
| Medical Equipment | CT scanner gantries, surgical robot joints, dental milling spindles, diagnostic device positioning stages | Low noise and vibration; high reliability; clean‑room compatible (sealed versions available) |
| Measuring Instruments | Coordinate measuring machines (CMMs), optical inspection systems, gear testers, roundness testers | Exceptional rotational accuracy (P4/P2) ensures measurement repeatability; extremely low starting torque prevents measurement hysteresis |
| Semiconductor / IC Manufacturing | Wafer handling robots, steppers, inspection stages, dicing equipment | Clean operation; smooth motion; minimal particle generation (special grease and seals available) |
| Military & Aerospace | Radar turrets, weapon aiming systems, optical telescope mounts, actuator mechanisms | High reliability under extreme conditions; wide temperature range; can be supplied with special coatings |
| General Automation | Rotary indexers, cam indexers, assembly turntables, pick‑and‑place units | Long maintenance‑free life; easy bolt‑on installation reduces design time and cost |
5. Available Models and Dimensional Data
The RU series is available in a comprehensive range of sizes.
6. Comparison with Other ONA Crossed Roller Bearing Series
| Series | Ring Structure | Mounting Holes | Installation Method | Best Suited For |
|---|---|---|---|---|
| RU | One‑piece inner + one‑piece outer | Yes (both rings) | Direct bolt‑on – no housing required | Universal rotation (inner or outer); compact designs; ease of installation |
| RB | Split outer, one‑piece inner | No (requires flanges) | Clamped between flanges | Inner ring rotation; maximum rigidity; general precision applications |
| RE | Split inner, one‑piece outer | No (requires flanges) | Clamped between flanges | Outer ring rotation; same dimensions as RB |
| RA | Split outer (thinner than RB) | No | Clamped between flanges | Ultra‑compact; space‑limited inner ring rotation |
| ORU | One‑piece inner + one‑piece outer | Yes | Direct bolt‑on – same as RU | Identical to RU (ONA model designation) |
The RU series is the easiest to install among all crossed roller types because it does not require any clamping flanges or special housings – simply bolt it onto prepared surfaces and the bearing is ready for operation.
7. Installation and Handling Advantages
7.1. Simplified Mounting Procedure
Installation steps for the RU series:
Machine the mounting surfaces on the shaft and housing to specified flatness and parallelism
Clean the mounting surfaces and the bearing
Align the bearing’s mounting holes with the corresponding holes on the shaft/housing
Insert bolts (not included) and tighten to the recommended torque
No additional pressing or clamping is required
This simplicity reduces assembly time and eliminates errors associated with flange clamping (e.g., uneven clamping force, distortion of bearing rings).
7.2. No Need for Presser Flanges or Housings
Because the bearing rings are inherently rigid and the mounting holes transfer the bolt load directly into the ring body, external flanges are completely unnecessary. This has several benefits:
Lower overall system cost – No extra machined flanges to purchase or manufacture
Reduced axial space – The bearing’s own width is the only axial requirement
Lighter machine design – No additional structural components
7.3. Reliable Torque and Accuracy Retention
Unlike bearings that rely on external clamping to maintain preload and clearance, the RU series’ internal geometry is fixed at the factory. Even if the mounting bolts are slightly over‑torqued or uneven, the bearing’s rotational accuracy and torque stability remain unaffected – a major advantage over clamped designs where improper mounting can ruin performance.
7.4. Can Be Used for Both Inner Ring and Outer Ring Rotation
The integrated ring construction, combined with symmetrical mounting hole patterns, allows the RU series to be used in two configurations:
Inner ring rotation – Inner ring bolted to rotating shaft; outer ring fixed to housing
Outer ring rotation – Outer ring bolted to rotating table; inner ring fixed to base
This versatility reduces inventory – one bearing type can serve both design needs.
8. Quality Assurance and Customization
8.1. Manufacturing Standards
ONA RU series bearings are produced under strict quality control systems:
ISO 9001:2015 certified facility
100% inspection of critical dimensions (bore, OD, width, runout, torque)
Material certification – Carburized bearing steel conforming to international standards
Traceability – Each bearing is marked with a unique serial number
8.2. Customization Options
For applications not covered by standard models, ONA offers comprehensive customization:
Non‑standard dimensions – Special bore sizes, outer diameters, or widths
Alternative materials – Stainless steel (440C / SUS440C) for corrosion resistance
Special precision grades – P2 and higher for ultra‑critical applications
Special grease – High‑temperature, low‑outgassing, or food‑grade lubricants
Sealing options – Contact seals (RU‑UU) or non‑contact seals (RU‑ZZ)
Mounting hole patterns – Custom bolt circles, threaded vs. through holes
Special coatings – Black oxide, DLC, or other surface treatments
9. Summary of Core Advantages
| Advantage | Description |
|---|---|
| One‑piece rings | Highest rigidity; no split‑line weakness; simpler handling |
| Integrated mounting holes | Direct bolt‑on – no flanges or housings required |
| Minimal mounting influence | Performance unchanged after installation – reliable and predictable |
| Versatile rotation | Suitable for inner ring rotation or outer ring rotation |
| High load capacity | Simultaneously supports radial, axial and overturning loads |
| High rigidity | 3.4 times stiffer than conventional bearing arrangements |
| High precision | P5, P4 and P2 grades available |
| Stable rotation torque | Isolation blocks eliminate roller friction and locking |
| Ultra‑thin cross‑section | Saves valuable axial and radial space |
| Wide size range | From RU42 (20mm bore) to custom bearings up to 2000mm bore |
| Direct interchangeability | Replaces THK RU, IKO CRBS/CRB, NSK NRXT, and other brands |
10. Conclusion
The ONA RU Series is the ideal choice for design engineers who need a compact, high‑rigidity crossed roller bearing that is easy to install and delivers stable precision regardless of mounting conditions. By eliminating the need for presser flanges and housings, the RU series reduces machine complexity, lowers assembly costs, and ensures reliable performance in demanding applications – from industrial robots and machine tools to medical devices and semiconductor manufacturing equipment.
For complete dimensional tables, load ratings, and custom engineering support, please contact ONA’s sales team.
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