Crossed Roller Bearing Selection Guide: Series Comparison, Mounting Methods, and Application Recommendations

What Are Crossed Roller Bearings?

Crossed roller bearings are precision rolling-element bearings in which cylindrical rollers are arranged at 90° to each other in a V-groove raceway. Unlike conventional bearings that can only support radial loads (deep groove ball bearings) or only axial loads (thrust bearings), crossed roller bearings can simultaneously handle radial loads, axial loads, and moment loads in a single compact unit. This multi-directional load capacity makes them indispensable in applications where space is limited but rigidity and accuracy are paramount.

The fundamental design principle is elegant: each roller alternates orientation relative to its neighbors, with one roller carrying load in one direction and the adjacent roller carrying load in the perpendicular direction. The rollers are retained by a cage or spacer block and preloaded through elastic deformation of the inner and outer rings. The result is a bearing with extremely high rigidity per unit volume — typically 3–5× greater than an equivalently sized angular contact ball bearing pair.

Key Technical Characteristics

Crossed roller bearings offer several inherent advantages that make them the preferred choice for precision rotary applications:

• Multi-directional load capacity: A single bearing can simultaneously carry radial, axial, and moment loads, eliminating the need for bearing pairs or combinations.
• High rotational accuracy: Standard accuracy grades achieve runout values of 2–5 μm, with ultra-precision grades reaching 0.5–1 μm. This directly translates to positioning accuracy in the application.
• High rigidity: The roller contact (line contact versus point contact for balls) provides 3–5× greater stiffness than ball bearings of comparable size, critical for machining and grinding applications.
• Compact design: A single crossed roller bearing replaces what would traditionally require two angular contact ball bearings in back-to-back or face-to-face arrangement, reducing axial space by 40–60%.
• Easy mounting: Most series feature split inner or outer rings with mounting holes, allowing direct bolt-on installation without interference fits — simplifying assembly and reducing maintenance downtime.

Series Comparison: RB, RU, RA, RE, CRBH, CRBT

Crossed roller bearings are available in several series, each optimized for specific application requirements. Understanding the differences is critical for proper selection.

RB Series — Standard Type (Split Outer Ring)

The RB series features a split outer ring with the inner ring as a solid one-piece design. The outer ring is divided into two halves, clamped together by mounting bolts. This design provides excellent rigidity and is the most widely used series for general-purpose precision rotary applications. The split outer ring allows the bearing to be mounted after the rollers are preloaded, ensuring consistent preload force. RB series bearings are available in bore diameters from 10 mm to 400 mm, with outer diameters up to 600 mm. They are the standard choice for CNC rotary tables, indexing heads, and industrial robot joints.

RU Series — Integral Inner/Outer Ring with Mounting Holes

The RU series has mounting holes in both the inner and outer rings, allowing direct bolt-on installation to both the shaft and housing. Unlike the RB series, neither ring is split — both are solid one-piece designs. This provides the highest rigidity among all crossed roller bearing series but requires slightly more care during installation because the preload is factory-set and cannot be adjusted. The RU series is preferred for applications demanding maximum stiffness and rotational accuracy, such as precision grinding machine rotary tables and high-accuracy indexing mechanisms.

RA Series — Compact Type (Thin Section)

The RA series is a thin-section version of the RB series, designed specifically for applications where radial and axial space is severely constrained. The cross-sectional dimensions are reduced while maintaining the same bore diameter range. This series trades some rigidity for compactness, making it ideal for robot wrist joints, collaborative robot joints, and compact rotary actuators where every millimeter of space matters. RA series bearings are 30–40% lighter than their RB equivalents.

RE Series — Integral Type (No Mounting Holes)

The RE series features integral (non-split) inner and outer rings without mounting holes. This design requires interference fit mounting, which provides the highest possible rotational accuracy and runout performance. The absence of bolt holes also means the raceway geometry is uninterrupted, yielding smoother rotation and lower noise. RE series bearings are the choice for ultra-precision applications such as CMM rotary stages, optical instrument turntables, and semiconductor wafer handling equipment.

CRBH Series — Separable Type (Cross-Roller Guide)

The CRBH series is a separable design where the inner ring, outer ring, rollers, and cage can be assembled separately. This allows the user to adjust preload during installation by selecting appropriate roller sizes or cage configurations. The separable design is particularly useful for custom-designed equipment where the bearing dimensions must be integrated into the machine structure. CRBH bearings offer the most flexibility in preload adjustment but require more skilled assembly.

CRBT Series — Thin-Section Separable Type

The CRBT series combines the separable design of CRBH with the thin-section profile of the RA series. This creates a bearing that is both space-efficient and adjustable. The CRBT is commonly used in medical equipment, precision measuring instruments, and lightweight robotic applications where both compactness and the ability to fine-tune preload are important.

Series	Ring Design	Mounting Holes	Preload Adjustment	Typical Application
RB	Split outer ring	Yes (outer)	Factory-set	CNC rotary tables, robot joints
RU	Integral (solid)	Yes (both rings)	Factory-set	Precision grinding, indexing
RA	Split outer ring (thin)	Yes (outer)	Factory-set	Robot wrists, compact actuators
RE	Integral (solid)	No	Interference fit	CMM stages, optical instruments
CRBH	Separable	Optional	User-adjustable	Custom machinery
CRBT	Separable (thin)	Optional	User-adjustable	Medical, measuring instruments

Accuracy Classes and Runout

Crossed roller bearings are manufactured to several accuracy classes, each defining allowable tolerances for dimensional accuracy and rotational runout. The most common classification follows JIS B 1521 (Japanese Industrial Standard), which defines five classes:

• Class 0 (Normal): Radial runout ≤ 5 μm, axial runout ≤ 5 μm. Suitable for general industrial equipment where moderate precision is acceptable.
• Class 6X: Radial runout ≤ 3 μm, axial runout ≤ 3 μm. Common for CNC rotary tables and indexing devices.
• Class 5: Radial runout ≤ 2 μm, axial runout ≤ 2 μm. Standard for precision grinding machines and high-accuracy CNC equipment.
• Class 4: Radial runout ≤ 1 μm, axial runout ≤ 1 μm. Used in semiconductor manufacturing equipment and precision measuring instruments.
• Class 2: Radial runout ≤ 0.5 μm, axial runout ≤ 0.5 μm. Ultra-precision grade for CMM rotary stages and optical equipment.

In practice, the accuracy class should match the positioning accuracy requirement of the final application. Over-specifying accuracy adds cost without benefit — a Class 0 bearing in a Class 5 application will produce unacceptable errors, but a Class 2 bearing in a Class 6X application wastes money on unnecessary precision. A general rule of thumb: the bearing accuracy class should be at least one grade higher than the application's positioning accuracy requirement.

Preload Selection

Preload is the internal load applied to the bearing before external loads are introduced. It eliminates clearance, increases rigidity, and improves rotational accuracy. For crossed roller bearings, preload is typically expressed as a percentage of the basic static load rating (C₀a).

Three standard preload levels are available:

• Light preload (C₀a × 0.02–0.05): Minimal internal load. Provides smooth rotation with low torque variation. Best for applications where low friction and smooth motion are critical — such as measuring equipment, optical turntables, and low-speed indexing.
• Medium preload (C₀a × 0.05–0.08): The most common preload level for CNC machine tool applications. Provides a good balance between rigidity and friction. Suitable for rotary tables, grinding machine workheads, and robot joints.
• Heavy preload (C₀a × 0.08–0.12): Maximum rigidity with higher friction. Reserved for applications requiring extreme stiffness under heavy cutting loads — such as large-diameter machining center rotary tables and heavy-duty grinding applications.

The preload should be verified by measuring the starting torque (breakaway torque) under no-load conditions. For a medium-preload bearing, the starting torque should typically be 0.01–0.05 N·m depending on bore size. A significantly higher value may indicate contamination or assembly error, while a lower value suggests insufficient preload.

Mounting Methods

Crossed roller bearings use several mounting approaches depending on the series and application:

Bolt-on mounting (RB, RU, RA series): The most common method. High-strength bolts (typically M4–M12 depending on bearing size) pass through the mounting holes and thread into the shaft or housing. Bolt torque must be precisely controlled — under-torquing causes the bearing to shift under load, while over-torquing can distort the raceway and increase runout. The recommended torque values are listed in the manufacturer's catalog and typically range from 1.5–25 N·m.

Interference fit mounting (RE series): The bearing is pressed onto the shaft or into the housing using a hydraulic press or thermal expansion method. The interference fit provides the highest rigidity and best runout performance but requires precise shaft and housing tolerance control — typically h5 for the shaft and H6 for the housing bore.

Adhesive mounting (some thin-section types): For very light-load applications, anaerobic adhesive can supplement or replace mechanical fastening. This method is common in medical devices and lightweight equipment where bolt holes are undesirable.

Regardless of mounting method, the mounting surface must be clean, dry, and free of burrs. The surface finish of the mounting bore should be Ra 0.8 μm or better. Any contamination between the bearing ring and mounting surface will create localized stress concentrations and significantly reduce bearing life.

Application Selection Guide

The following recommendations summarize the optimal bearing series for common CNC machine tool and automation applications:

Application	Recommended Series	Accuracy Class	Preload
CNC milling rotary table	RB or RU	Class 5	Medium
Grinding machine workhead	RU	Class 4–5	Medium–Heavy
Industrial robot joint (axis 1–2)	RB	Class 5–6X	Medium
Robot wrist joint (axis 4–6)	RA	Class 5	Light–Medium
Collaborative robot joint	RA or CRBT	Class 5	Light
CMM rotary stage	RE	Class 2–4	Light
Semiconductor wafer handler	RE or CRBT	Class 2–4	Light
Indexing table (heavy duty)	RU	Class 5	Heavy

Lubrication and Maintenance

Crossed roller bearings are typically pre-lubricated with lithium-soap grease at the factory. For most applications, relubrication intervals of 2,000–5,000 operating hours are recommended, depending on speed and environmental conditions. High-speed applications (>50 RPM continuous) may require relubrication every 1,000 hours.

When relubricating, use only the grease type specified by the bearing manufacturer — mixing grease types can cause chemical incompatibility and premature failure. The relubrication volume is typically 15–30% of the bearing's internal free volume. Over-lubrication is as harmful as under-lubrication, as excess grease causes churning, heat generation, and increased torque.

For applications in contaminated environments (metalworking, grinding), consider using bearings with seals or shields. Sealed variants add approximately 0.5–1.0 N·m of seal friction torque but significantly extend bearing life by preventing chip and coolant ingress.

Conclusion

Selecting the right crossed roller bearing requires matching the series design, accuracy class, and preload level to the specific application requirements. The RB series provides the best balance of performance and cost for most CNC rotary table and robot joint applications. The RU series delivers maximum rigidity for precision grinding. The RA and CRBT series serve space-constrained applications in robotics and medical equipment. The RE series meets ultra-precision requirements in measuring and semiconductor equipment.

Proper mounting technique, surface preparation, and lubrication management are equally important to achieving the rated performance and life of the bearing. Following the manufacturer's torque specifications and relubrication intervals will ensure reliable operation over the bearing's design life.