Crossed-roller bearing
Last edited
Fact-checked
In review queue
Sources
25 citations
Revision
v1 · 3,059 words
Fact-checks are independent of edits: a reviewer re-verifies the article against its sources and stamps the date. How we verify
A crossed-roller bearing is a precision rolling element bearing that packs cylindrical rollers into a single V-shaped raceway, with each roller's axis turned 90 degrees from the roller next to it in a criss-cross pattern. Because neighboring rollers point in perpendicular directions, one compact bearing can carry radial load, axial (thrust) load from either direction, and a tilting moment load all at once, a combination that ball bearings and single-direction roller bearings normally need two or more separate units to cover.[1][2] That mix of high stiffness, tight rotational accuracy, and small size is why crossed-roller bearings sit at nearly every heavily loaded joint in a modern humanoid robot, from the hip and knee to the output flange of a harmonic drive reducer.
How it works
Looked at from outside, a crossed-roller bearing resembles an ordinary flat ring or a short cylinder. Inside, a 90-degree V-shaped groove is machined into both the outer face of the inner ring and the inner face of the outer ring, forming one raceway track rather than the two separate tracks a pair of mounted bearings would need.[1][3] Cylindrical rollers run in that groove, but instead of all lying the same way, each roller's axis is turned 90 degrees from the rollers on either side of it, so the rollers alternate all the way around the ring: one oriented "this way," the next "that way."[2][3]
Because the rollers touch the raceway along a line rather than at a single point, and because every other roller is oriented to catch a load the rollers next to it cannot, one full circuit of alternating rollers resists load pushing in from any direction at once.[1][5] Small spacers, often called separators or spacer retainers, sit between each roller and its neighbor to stop the perpendicular rollers from rubbing against each other, which would otherwise raise friction and let the rollers skew out of alignment.[4] Some designs drop the spacers entirely and pack the rollers edge to edge; this "full complement" style raises the number of load-carrying rollers and the peak capacity, at the cost of higher friction and heat.[2]
Most crossed-roller bearings split one of the two rings into two bolted halves so the rollers can be dropped into the groove during assembly, while the other ring stays one solid piece; a filler-slot loading method lets higher-rigidity types keep both rings solid. Exact model codes for these variants are not standardized across brands. THK's Cross-Roller Ring line, one of the oldest and best known (the company was founded in 1971 and built its early business on rolling-contact linear guides before expanding into rotary bearings), uses RA, RE, and RU type codes for roughly the split-outer-ring, split-inner-ring, and integral-ring configurations, according to industry catalogs.[1][22] HIWIN labels similar configurations CRBA, CRBB, and CRBC, while IKO uses CRBC for a split-outer-ring caged type and a separate CRBH code for its integral-ring, high-rigidity type.[2][6] A part number from one supplier does not automatically mean the same thing at another, though the bore, outside diameter, and width of many catalog sizes line up closely enough that the bearings are functionally interchangeable, which matters for how the parts get sourced at scale (see Use in humanoid robots, below). The rollers themselves are commonly sized to the DIN 5402 cylindrical-roller standard, another point of cross-brand compatibility.[4]
In brief: picture a bearing where, instead of a ring of identical rollers all lying flat, every other roller is turned on its side. Half the rollers resist a push straight down along the shaft; the other half resist a push out to the side, and together the two sets resist a twisting force that tries to tip the shaft over. Because both sets share the same ring and the same mounting bolts, the whole assembly fits in the space of one ordinary bearing instead of two stacked ones.
Load types
| Load type | Direction of force | Everyday analogy | How the crossed-roller geometry handles it |
|---|---|---|---|
| Radial | Perpendicular to the shaft, pushing it sideways | Sitting on a porch swing | Line contact spreads the sideways push over a long stretch of raceway instead of a point |
| Axial (thrust) | Along the shaft, in either direction | Leaning your weight on a cane | Rollers angled one way react a push from one direction; the perpendicular set reacts a push from the other |
| Moment (tilting) | A torque trying to tip the rotation axis itself | Holding a loaded suitcase out at arm's length | The two roller orientations, spread across the full raceway diameter, act together as a couple that resists the tipping torque |
A single crossed-roller bearing carries all three load types in combination, and manufacturers publish a combined "equivalent radial load" formula, generally of the form P = X·Fr + Y·Fa, where Fr and Fa are the radial and axial loads and X and Y are factors set by the ratio between them, so an engineer can size one bearing for a real joint instead of stacking separate radial and thrust bearings and adding up their individual ratings.[8]
Construction variants
| Variant | Ring configuration | Typical role |
|---|---|---|
| Split outer ring | Outer ring in two bolted halves; inner ring solid | General-purpose rotary tables and robot joints |
| Split inner ring | Inner ring in two bolted halves; outer ring solid | Mountings where the outer ring bolts directly to a rigid housing |
| Integral (high-rigidity) rings | Both rings solid; rollers loaded through a filler slot | Maximum rigidity for a given envelope; common in compact actuator joints |
| Full complement | No spacers; rollers packed edge to edge | Highest load capacity and stiffness; more friction and heat |
| Spacer or caged | A separator between every roller | Lower friction, smoother and quieter rotation |
| Thin-section, large-bore | Constant cross-section held across a wide range of bore sizes, often with a large hollow center | Waist and torso rotation stages that need a hollow bore for cable and hose routing |
Sources: THK, IKO, and HIWIN product catalogs.[1][2][6]
Stiffness, rotational accuracy, and preload
Line contact is the core of what makes a crossed-roller bearing stiff. A ball touches its raceway at essentially a single point; a cylindrical roller touches it along the roller's full length, spreading the same load over a much larger contact area and producing less local deformation for a given force.[5] That lower deflection under load is what lets a crossed-roller bearing hold its position accurately even when a robot arm or a machine-tool table pushes hard against it off-center.
Bearing makers grade rotational accuracy using the ISO and JIS system of precision classes, from loosest to tightest roughly JIS class 0, 6, 5, 4, and 2, corresponding to DIN classes P0, P6, P5, P4, and P2 and loosely matching the ABEC 1, 5, 7, and 9 scale used for ball bearings.[7] Crossed-roller bearing catalogs commonly span most of that range; IKO, for example, lists its crossed-roller series across accuracy classes P6, P5, P4, and P2, the last two aimed at machine-tool spindles and precision robot joints.[9] Because both roller sets share one raceway, the accuracy holds under combined loading rather than only under a single clean load case, which is closer to what a real robot joint actually experiences.
Preload, a small built-in negative clearance that presses the rollers against the raceway before any external load arrives, is what removes play, or backlash, from the bearing. Manufacturers apply it by grinding the rings to a slight interference fit, or by clamping the bearing during installation with precision shims, sleeves, or a locknut pressed against a shoulder.[25] Heavier preload raises rigidity and rotational accuracy but also raises friction, running torque, and operating heat, and it can shorten bearing life if set too high, so catalogs offer a small menu of clearance and preload classes rather than one fixed setting.[3][25] Roller bearings also follow a steeper life-to-load relationship than ball bearings, a life exponent of 10/3 rather than 3, a consequence of line contact rather than point contact, so overloading or over-preloading a crossed-roller bearing shortens its service life faster than the same mistake would on a comparable ball bearing.[8]
Crossed-roller bearings compared with angular-contact and four-point-contact bearings
Three bearing families compete for the same job: supporting a rotating joint that sees load from more than one direction, in as little space as possible.
| Bearing type | Contact | Loads carried by one unit | Axial footprint | Speed and friction | Common robotics role |
|---|---|---|---|---|---|
| Single-row angular-contact ball bearing | Point, one contact angle | Radial plus axial from one direction only; a matched pair (back-to-back or face-to-face) is needed for axial load in both directions and for a moment load | A matched pair takes roughly twice the axial space of a single bearing | Lowest friction, highest speed of the three | High-speed spindles; lighter joints, usually paired |
| Four-point-contact ball bearing | Point, four contact points from a split (Gothic-arch) raceway | Moderate radial load, axial load from both directions, and some moment, all in a single row | Compact; one row replaces a paired angular-contact arrangement | Lower friction and higher speed than a crossed-roller bearing | Compact rotary tables and lighter-duty robot joints |
| Crossed-roller bearing | Line, alternating 90-degree rollers | Radial, axial (both directions), and moment, all at high magnitude, in a single row | Most compact for a given stiffness; replaces two bearing positions with one | More friction and a lower speed ceiling than either ball design | Heavily loaded joints: robot hips, knees, shoulders, wrists, and reducer output flanges |
The pattern across all three is a straightforward tradeoff between contact type and duty. Point contact, as in ball bearings, favors speed and low friction; line contact, as in crossed rollers, favors stiffness and load capacity; the four-point design sits in between as a lighter-duty, single-row alternative that still saves axial space over a paired arrangement.[10][11] Schaeffler and other bearing makers describe the crossed-roller bearing's main systems-design advantage in similar terms: it lets an engineer collapse what would otherwise be two separate bearing positions, such as a back-to-back angular-contact pair, into one.[3]
Use in humanoid robots
A humanoid robot concentrates most of its structural load into a small number of joints that each resist several kinds of force from an off-center limb at once: a hip supporting body weight while the leg swings forward, a knee flexing under that same body weight, a shoulder holding an outstretched arm, a wrist twisting under a gripped tool. Every one of those is, mechanically, a radial load plus an axial load plus a moment load arriving together, exactly the load case a crossed-roller bearing is built for.[1][2] That is also why crossed-roller bearings show up wherever a robot builder wants to package a motor, gearbox, sensors, and bearing into one self-contained actuator unit rather than bolting several separate mechanical stages together.
Unitree specifies "industrial grade crossed roller bearings (high precision, high load capacity)" as the joint output bearing across its G1 humanoid line, a direct, named example of the component in a commercially sold humanoid robot rather than just a generic design pattern.[12] Industry coverage of Boston Dynamics' all-electric Atlas humanoid describes a similar architecture: strain-wave (harmonic drive) gearing paired with frameless torque motors and cross-roller bearings across most of the robot's joints, with the bearings absorbing the load swings of highly dynamic, acrobatic motion.[14][17] IKO, which sells crossed-roller bearings into the humanoid market, estimates that a typical full-size humanoid uses on the order of 14 to 20 of them across its hips, shoulders, elbows, and wrists.[13]
Reducer makers increasingly build the bearing into the gearbox rather than leaving it for the robot builder to add separately. Harmonic Drive Systems' CSD and SHD component-set lines, marketed in part for humanoid and exoskeleton joints, build a cross-roller bearing directly into the output side of the unit, with the bearing's inner race doing double duty as the output mounting flange.[15] That keeps a gearbox, its output bearing, and the mounting interface to the next link in the kinematic chain in one part, instead of a gearbox plus a separately mounted bearing plus a coupling. Nabtesco's competing RV-series cycloidal drive reducers, long used in industrial robot joints and increasingly pitched at humanoid builders, follow the same integration logic with a large main bearing built into the reducer housing; Nabtesco's own literature describes that main bearing as an angular-contact design rather than a crossed-roller one, but the underlying idea, a single large bearing carrying the reducer's output loads directly, is the one crossed-roller bearings popularized across the reducer industry.[16][24]
Thin-section, large-bore crossed-roller bearings also do the less visible job of supporting waist and torso rotation, where a robot needs a wide hollow center for cables and hoses running between the upper and lower body, something a small-bore joint bearing cannot provide.[13]
The bearings themselves have become considerably easier to source than they were even a decade ago. The basic X-arranged roller geometry is not new: patent filings for bi-angular roller bearings date to at least the early 1950s, when Timken engineer Dennis Ashton patented a bearing using tapered rollers with alternating axes, an early precursor to the cylindrical, 90-degree-groove design now standard across the industry.[21] What has changed more recently is standardization and supply. Crossed-roller bearings from THK, HIWIN, Schaeffler's INA brand, IKO, and a growing number of Chinese manufacturers are built to largely interchangeable bore, outside-diameter, and width series, letting a robot builder qualify more than one supplier for the same joint.[7] In China specifically, Luoyang Hongyuan Bearing Technology, founded in 2005 and based in Luoyang, Henan, says its cross-roller and turntable bearings hold more than 80 percent of the domestic market and its robot bearings more than 90 percent, and reported an order book strong enough in early 2026 to justify a new production line.[18][19] A similar buildout has happened in the reducers that crossed-roller bearings are so often bolted to: Leaderdrive, a Chinese harmonic-reducer maker founded in 2011, is estimated by J.P. Morgan to hold 30 to 40 percent of China's harmonic-reducer market and counts humanoid builders including Agibot and UBTECH among its customers.[20] Standard catalog dimensions plus a wider supplier base, more than any single breakthrough in the bearing's design, is what turned the crossed-roller bearing from a specialized machine-tool component into a commodity item cheap and available enough to put a dozen or more of them into a single mass-produced humanoid robot.
Suppliers and market landscape
The crossed-roller bearing market spans long-established Japanese and German precision-bearing makers, a Taiwanese motion-control specialist, and a fast-growing set of Chinese manufacturers supplying the domestic humanoid-robot boom.
| Company | Headquarters | Notes |
|---|---|---|
| THK | Japan | Cross-Roller Ring product line (RA/RE/RU-style type codes); also a major supplier of the linear guides used elsewhere in robot arms and gantries [1] |
| IKO International (Nippon Thompson) | Japan | CRBC and CRBH crossed-roller series; publishes humanoid-specific application guidance estimating 14 to 20 bearings per robot [2][13] |
| Schaeffler (INA brand) | Germany | XSU and SX crossed-roller series; markets the bearing explicitly as a way to collapse two bearing positions into one [3] |
| NSK | Japan | N-series crossed-roller bearings alongside NSK's angular-contact and four-point-contact lines [10] |
| SKF | Sweden | Four-point-contact and crossed-roller bearings for compact rotary joints [11] |
| HIWIN | Taiwan | CRBA, CRBB, CRBC, CRBD, CRBE, and CRBX crossed-roller series alongside HIWIN's linear-motion and robot-arm business [6] |
| Timken | United States | Crossed-roller precision bearings; the company's 1953 tapered-roller bi-angular patent is an early forerunner of the modern design [21][23] |
| Harmonic Drive Systems | Japan | Builds a cross-roller output bearing directly into its CSD and SHD strain-wave gear units for humanoid and exoskeleton joints [15] |
| Nabtesco | Japan | Integrates a large main bearing, described in company literature as angular-contact rather than crossed-roller, into the output of its RV cycloidal reducers [16][24] |
| Luoyang Hongyuan Bearing Technology | China | Says it holds more than 90 percent of China's domestic robot-bearing market; supplies humanoid builders domestically [18][19] |
| Leaderdrive | China | Harmonic-reducer maker, not a bearing specialist, whose growth illustrates the same Chinese supply-chain buildout around integrated reducer-and-bearing units [20] |
See also
References
- THK Co., Ltd. "Cross-Roller Ring." Product information. https://www.thk.com/us/en/products/cross_roller_ring/ ↩
- IKO International. "Crossed Roller Bearings." https://ikont.com/rotary-bearings/crossed-roller-bearings/ ↩
- Schaeffler Group USA. "Crossed Roller Bearings." https://www.schaeffler.us/us/products-and-solutions/industrial/product-portfolio/rolling_and_plain_bearings/crossed_roller_bearings/ ↩
- PIB Sales. "Crossed Roller Bearings used in Robotics." https://pibsales.com/robotics/crossed-roller-bearings-used-in-robotics/ ↩
- NTN Bearing Wizard. "Point and line contact." https://bearingwizard.com/introduction-to-rolling-bearing-technology/point-and-line-contact/ ↩
- HIWIN Corporation. "Crossed Roller Bearings." https://hiwin.com/products/crossed-roller-bearings/ ↩
- LKPB Bearing. "SKF/THK/IKO/INA/HIWIN/LKPB Cross Roller Bearings Standard Models and Specifications." https://lkpbearing.com/thk-iko-ina-hiwin-lkpb-crossed-roller-bearing-models-and-specifications-most-complete-guide/ ↩
- Motion Control Tips. "How to calculate crossed roller bearing life (with various load types)." https://www.motioncontroltips.com/how-to-calculate-crossed-roller-bearing-life-with-various-load-types/ ↩
- IKO International. "Special Selection Crossed Roller Bearings," catalog CAT-57151UK. https://www.ikont.eu/files/Documents/Catalogues/CAT-57151UKv3_IKO_CRB.pdf ↩
- NSK Ltd. "Angular Contact Ball Bearings." NSK Global. https://www.nsk.com/tools-resources/abc-bearings/angular-contact-ball-bearings/ ↩
- SKF Evolution. "Four-point contact ball bearings, two in one." https://evolution.skf.com/four-point-contact-ball-bearings-two-in-one/ ↩
- Unitree Robotics. "G1" humanoid robot, mechanical and joint specifications. https://www.unitree.com/g1/ ↩
- IKO International. "Humanoids Serve Their Purpose With Advanced Motion Components." Technology blog. https://ikont.com/technical-resources/technology-blog/humanoids-serve-their-purpose-with-advanced-motion-components/ ↩
- PIB Sales. "Precision in Motion: The Core Components Powering Humanoid Robots." https://pibsales.com/robotics/precision-in-motion-the-core-components-powering-humanoid-robots/ ↩
- Harmonic Drive. "CSD and SHD Series" catalog. https://www.harmonicdrive.net/_hd/content/catalogs/pdf/csd-shd-catalog.pdf ↩
- Nabtesco / D.P. Brown Technology. "RV-N Standard Inline Cycloidal Gear Reducer." https://dpbrowntech.com/products/nabtesco/rv-n.php ↩
- Interesting Engineering. "Where China leads and lags in humanoid joint architecture." 2026. https://interestingengineering.com/ai-robotics/china-humanoid-robots-actuators ↩
- Luoyang Hongyuan Bearing Technology Co., Ltd. "About Us." https://en.honb.com/about.html ↩
- Uniquebearing.com (Lasting Bearing). "Precision Bearings Help Domestic Robots Move Silky, And Luoyang Hongyuan Becomes The Industry Leader." 2026. https://www.uniquebearing.com/news/precision-bearings-help-domestic-robots-move-s-84949839.html ↩
- VnExpress International. "Chinese brothers behind robot joint maker Leaderdrive become billionaires as humanoid demand surges." 2026. https://e.vnexpress.net/news/tech/personalities/chinese-brothers-behind-robot-joint-maker-leaderdrive-become-billionaires-as-humanoid-demand-surges-5066573.html ↩
- Ashton, Dennis (assignee: The Timken Roller Bearing Company). U.S. Patent 2,628,137, "Roller Bearing," filed January 6, 1951, granted February 10, 1953. Google Patents. https://patents.google.com/patent/US2628137A/en ↩
- Wikipedia. "THK Co., Ltd." https://en.wikipedia.org/wiki/THK_Co.,_Ltd. ↩
- The Timken Company. "Crossed Roller Precision Bearings." https://www.timken.com/resources/timken-crossed-roller-precision-bearing-sell-sheet_5494/ ↩
- Nabtesco. "RV cycloidal gearbox with solid shaft." https://www.nabtesco.de/en/products/series/rv ↩
- Nippon Bearing Corporation. "The Basics of Crossed Roller Bearings." https://www.nbcorporation.com/basics-crossed-roller-bearings/ ↩
Improve this article
Add missing citations, update stale details, or suggest a clearer explanation. Every suggestion is reviewed for sourcing before it goes live.