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Help CenterONA ORA 20013 Ultra-Thin Crossed Roller Bearing P4/P2 High Rigidity Compact Design for Robot Joints
Product Spotlights
Compact footprint – Saves valuable installation space without compromising performance.
Split outer ring – Same proven design as ORB series – easy installation and adjustable clearance.
High rigidity – Crossed roller arrangement delivers exceptional stiffness despite the slim profile.
Precision grades P5, P4, P2 – Meets demanding accuracy requirements for
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ONA ORA Series – Ultra‑Thin Crossed Cylindrical Roller Bearings
(Integrated Inner Ring, Split Outer Ring)
1. Product Overview
The ONA ORA Series is an ultra‑thin, lightweight crossed cylindrical roller bearing designed for applications where space is extremely limited and weight reduction is critical. As a compact evolution of the ORB series, the ORA series reduces the inner and outer ring thickness to the absolute minimum while maintaining the same fundamental structure – an integrated (one‑piece) inner ring and a split outer ring.
This design philosophy makes the ORA series the optimal choice for precision rotary systems that demand both compactness and high performance, particularly in robotics, automation, and lightweight mechanical arms.
As part of Luoyang ONA Bearing Co., Ltd.’s high‑precision product portfolio, the ORA series is manufactured to the highest quality standards. ONA specializes in thin‑walled crossed roller bearings, high‑precision YRT series rotary table bearings, precision four‑point contact ball bearings, and harmonic drive bearings, with precision grades reaching P5, P4, and P2, and size ranges covering outer diameters from 20mm to 2500mm.
2. Structural Design
2.1. Ultra‑Thin, Ultra‑Lightweight Construction
The defining characteristic of the ORA series is its extremely thin cross‑section. Compared to the ORB series, the ORA series minimizes the thickness of both the inner and outer rings to the practical limit. This ultra‑thin profile provides:
Significant weight reduction – Critical for robotic arms and moving components where inertia must be minimized
Space savings – Allows more compact machine designs without compromising bearing performance
Lower rotational inertia – Enables faster acceleration and deceleration in dynamic applications
2.2. Integrated Inner Ring, Split Outer Ring
The ORA series shares the same fundamental structure as the ORB series:
Inner ring – One‑piece integrated construction providing maximum rigidity and stability
Outer ring – Split into two parts (two‑piece design) for easy assembly and adjustable clearance
The split outer ring design serves two important purposes:
Simplified installation – After inserting the rollers and spacer retainers into the raceway, the split outer ring is fixed into position, preventing the bearing from separating during handling and operation
Adjustable clearance and preload – Because the outer ring is separable, bearing clearance can be precisely adjusted. Even when preload is applied, high‑precision rotational motion is achievable
2.3. Crossed Roller Arrangement
Inside the ORA bearing, cylindrical rollers are arranged perpendicularly to each other on the rolling surfaces of 90° V‑shaped grooves. The rollers are separated by spacer retainers (isolation blocks) that maintain proper spacing and orientation.
This arrangement ensures that:
Rollers do not tilt or skew during operation
Rollers do not rub against each other, eliminating friction between rolling elements
Friction torque is low and uniform across the entire speed range
No locking or jamming occurs, even under heavy or varying loads
2.4. Spacer Retainer Design
The spacer retainers between adjacent rollers serve multiple critical functions:
Prevent roller‑to‑roller contact – Eliminates direct friction between rollers
Maintain roller orientation – Prevents tilting and skewing
Ensure stable rotation torque – Provides consistent, predictable torque characteristics
Reduce rotational torque – Compared to steel plate retainer designs, the spacer retainer eliminates the phenomenon of rollers contacting each other on one side or locking
3. Key Performance Characteristics
3.1. High Rigidity – Exceptional Stiffness in an Ultra‑Thin Profile
Despite its ultra‑thin cross‑section, the ORA series delivers exceptional rigidity. The crossed roller geometry maximizes the contact area between rollers and raceways, resulting in:
Minimal elastic deformation under heavy or varying loads
Excellent machining stability in precision applications
Superior moment load resistance – Critical for robotic joints and rotary tables
The rigidity is achieved through:
The 90° orthogonal roller arrangement
The preloaded design (when specified)
The one‑piece inner ring construction that eliminates split‑line weakness
3.2. High Load Capacity – Radial, Axial and Overturning Moments
Because the rollers are arranged perpendicularly on V‑shaped raceways, a single ORA 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 applications where the load is offset from the bearing center
This combined load capability eliminates the need for multiple bearings arranged in complex configurations, saving space, reducing weight, and simplifying assembly.
3.3. High Precision – P5, P4 and P2 Grades
ONA ORA series bearings are manufactured to the highest precision standards:
| Precision Grade | Equivalent Standard | Typical Application |
|---|---|---|
| P5 | ISO Class 5 | General precision robotics and automation |
| P4 | ISO Class 4 | High‑precision machine tool rotary tables |
| P2 | ISO Class 2 | Ultra‑high‑precision measuring equipment and aerospace |
Each bearing undergoes rigorous inspection for dimensional accuracy, running accuracy (radial and axial runout), and torque consistency.
3.4. Smooth, Stable Rotation with Low Friction Torque
The spacer retainer design ensures smooth, stable rotation:
Low starting torque – Easy to initiate rotation, critical for precise positioning
Uniform running torque – Consistent torque throughout the entire speed range
No torque spikes – Eliminates sudden increases in friction
No locking phenomena – Reliable operation even under varying loads
This stable rotational behavior is essential for applications requiring smooth, jitter‑free motion, such as robotic arm control and precision indexing.
3.5. Excellent Rotational Accuracy
The combination of precision‑ground raceways, high‑quality rollers, and the crossed arrangement delivers exceptional rotational accuracy:
Low radial runout – Ensures concentric rotation
Low axial runout – Maintains face alignment
High repeatability – Consistent positioning cycle after cycle
3.6. Easy Installation
The split outer ring design simplifies the installation process:
No complex assembly procedures – The outer ring halves are fixed after the rollers and retainers are installed
No special tools required – Standard assembly practices apply
Adjustable clearance – Allows on‑site fine‑tuning for specific application requirements
Reduced assembly time – Compared to bearings with integrated rings that require careful pressing
3.7. Premium Material – Carburized Steel
ORA series bearings are manufactured from high‑quality carburized bearing steel (GCr15 equivalent), providing:
Excellent impact resistance – Withstands shock loads common in robotics and heavy machinery
Superior surface wear resistance – Extends service life in demanding environments
High rolling contact fatigue strength – Supports long‑term, high‑duty operation
Hardened raceways (60–62 HRC) – Provides optimal wear resistance
3.8. Optimized Preload Capability
Because the outer ring is split, the ORA series supports adjustable preload:
Eliminates internal clearance – When preloaded, the bearing operates with zero clearance
Increases system rigidity – Preload enhances stiffness under load
Improves rotational accuracy – Preload reduces runout and improves positioning repeatability
4. Design Philosophy: Ultra‑Thin Without Compromise
The ORA series represents the extreme of miniaturization in crossed roller bearing design. While the ORB series already offers a thin profile, the ORA series pushes the boundaries further:
| Comparison | ORB Series | ORA Series |
|---|---|---|
| Ring thickness | Standard thin | Minimized to limit |
| Weight | Lightweight | Ultra‑lightweight |
| Cross‑section | Compact | Ultra‑compact |
| Structure | Split outer, integrated inner | Same structure, thinner rings |
| Rigidity | High | High (maintained despite thinner section) |
The ORA series achieves this extreme thinness while maintaining high rigidity – a direct result of the crossed roller geometry and precision manufacturing.
5. Application Fields
The ORA series is specifically designed for applications requiring lightweight, compact designs.
| Application Area | Specific Equipment / Use | Why ORA Series Is Suitable |
|---|---|---|
| Industrial Robots | Joints (shoulder, elbow, wrist), swiveling units, rotating actuators | Ultra‑lightweight reduces arm inertia; high rigidity under dynamic loads; compact size fits limited joint space |
| Mechanical Arms / Manipulators | Rotating sections, articulated arms, pick‑and‑place units | Weight reduction critical for fast movement; low friction enables smooth operation |
| Machine Tools | Rotary tables (4th/5th axes), indexing heads, turret mechanisms | P4/P2 precision ensures accurate positioning; handles combined cutting forces |
| Medical Equipment | Surgical robots, diagnostic device positioning stages, CT scanner components | Low noise and vibration; high reliability; clean‑room compatible |
| Semiconductor / IC Manufacturing | Wafer handling robots, inspection stages, dicing equipment | Clean operation; smooth motion; minimal particle generation |
| Measuring Instruments | CMMs, optical inspection systems, precision test fixtures | Exceptional rotational accuracy (P4/P2); extremely low starting torque |
| Aerospace & Defense | Actuator mechanisms, antenna positioning systems, optical mounts | High reliability; lightweight construction; wide temperature range capability |
| General Automation | Rotary indexers, assembly turntables, cam indexers | Long maintenance‑free life; easy installation |
6. Available Models and Naming Convention
The ORA series follows a naming convention based on bore diameter × outer diameter. Standard models are available in a range of sizes. Below are representative examples:
Model Code Breakdown:
ORA – Series designation (ultra‑thin, split outer ring)
First digits – Bore diameter in mm
Last digits – Outer diameter in mm
Available Variations:
| Suffix | Meaning | Description |
|---|---|---|
| ORA (Standard) | Open type | Basic open bearing without seals |
| ORA‑UU | Double‑lip sealed type | Two sides sealed with rubber lip seals – prevents contamination and retains grease |
Note: For detailed dimensional data, load ratings, and custom sizes (including non‑standard bore diameters and widths), please contact ONA’s engineering team.
7. Quality Assurance and Manufacturing Standards
ONA ORA 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
Precision grinding – Raceways ground to micron‑level accuracy
Traceability – Each bearing is marked with a unique serial number
8. Comparison with Other ONA Crossed Roller Bearing Series
| Series | Ring Structure | Cross‑Section | Best Suited For |
|---|---|---|---|
| ORA | Split outer, integrated inner | Ultra‑thin (minimum) | Lightweight, compact designs; robots; mechanical arms |
| ORB | Split outer, integrated inner | Thin | General precision applications; maximum rigidity in thin profile |
| ORE | Split inner, integrated outer | Thin | Outer ring rotation applications; same dimensions as ORB |
| ORU | One‑piece inner + one‑piece outer | Thin | Universal rotation; direct bolt‑on installation with mounting holes |
| ORAU | Ultra‑thin, split outer | Ultra‑thin (5mm width) | Extremely compact applications |
9. Summary of Core Advantages
| Advantage | Description |
|---|---|
| Ultra‑thin, ultra‑lightweight | Inner and outer ring thickness minimized to the limit – ideal for weight‑sensitive applications |
| Compact footprint | Saves valuable installation space without compromising performance |
| Split outer ring | Easy installation and adjustable clearance |
| High rigidity | Crossed roller arrangement delivers exceptional stiffness despite slim profile |
| High load capacity | Simultaneously supports radial, axial and overturning loads |
| Precision grades P5, P4, P2 | Meets demanding accuracy requirements |
| Smooth, stable rotation | Spacer retainers prevent roller contact and ensure low, uniform friction torque |
| Low starting torque | Easy to initiate rotation – critical for precise positioning |
| Premium carburized steel | Excellent impact resistance and surface wear resistance |
| Wide size range | Multiple standard models plus custom sizes available |
| Direct interchangeability | Replaces THK RA series, IKO CRBS series, and other major brands |
10. Conclusion
The ONA ORA Series is the ultimate choice for design engineers who need the thinnest possible crossed roller bearing without sacrificing rigidity, precision, or load capacity. By minimizing ring thickness to the practical limit while maintaining the proven split outer ring / integrated inner ring structure, the ORA series delivers exceptional performance in the most space‑constrained applications.
Whether you are designing a lightweight robotic arm, a compact rotary table, or a precision positioning stage, the ORA series provides the ideal combination of thinness, rigidity, and precision.
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