# Rolling-element bearing

> Source: https://aiwiki.ai/wiki/rolling_element_bearing
> Updated: 2026-07-14
> Categories: AI Hardware, Robotics
> License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)
> From AI Wiki (https://aiwiki.ai), the free encyclopedia of artificial intelligence. Reuse freely with attribution to "AI Wiki (aiwiki.ai)".

A **rolling-element bearing** is a machine component that lets one part rotate or pivot against another while carrying load, typically a shaft turning inside a housing, by placing hardened balls or rollers between two grooved rings so the parts roll against each other instead of sliding.[1] Rolling contact produces far less friction than a plain sliding joint, which is why rolling-element bearings sit at nearly every moving joint in a modern machine, from a bicycle hub to a jet-engine shaft, and why a single [humanoid robot](/wiki/humanoid_robot) arm or leg typically contains a dozen or more of them.[1][2] The family splits into several distinct types, chosen joint by joint for how much load they carry, in which direction, and how fast and precisely the joint must move. This article surveys that family and maps each type onto the humanoid body; the increasingly important [crossed-roller bearing](/wiki/crossed_roller_bearing) gets a deeper, dedicated treatment in its own article.

## How a rolling-element bearing works

### Anatomy

Most rolling-element bearings share the same basic parts: an inner ring that mounts on the rotating shaft, an outer ring that seats in the stationary housing, a set of rolling elements, balls or rollers, that roll in matching grooves (raceways) machined into both rings, and a cage, also called a retainer or separator, that holds the rolling elements evenly spaced so they do not rub against each other.[1] Many bearings add a seal or shield on one or both faces to keep contaminants out and grease in. Cages are commonly stamped steel, machined brass, or molded polyamide; more specialized materials such as PEEK show up in bearings built for high temperature or vacuum service.[3] That four-part anatomy does not change with scale: a bearing the size of a coffee mug in a wind-turbine hub and one the size of a shirt button in a robot finger are built the same way.

### Load types

Choosing a bearing starts with the same question every time: which direction is the load coming from? Engineers sort real-world loads into three categories.[4][5]

- **Radial load**: force pushing perpendicular to the shaft, the way a swing pulls down on its pivot pin.
- **Axial (thrust) load**: force pushing along the shaft's own axis, the way leaning on a cane pushes straight down its length.
- **Moment (tilting) load**: a torque that tries to tip the rotation axis itself, the way holding a suitcase out at arm's length twists the shoulder.

A real joint, in a gearbox, a wind turbine, or a robot hip alike, almost never experiences just one of these in isolation; it experiences some mix of all three, and bearing catalogs publish combined-load formulas, 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 their ratio, so an engineer can size a single bearing against the real combined load rather than an idealized single-direction case.[5]

### Point contact versus line contact

The other defining choice is geometric: does the rolling element touch its raceway at a point or along a line? A ball is a sphere, so it touches a raceway at essentially one point; a cylindrical roller touches along the full length of its body, a line.[2] That difference in contact area drives the central tradeoff across the whole bearing family. Point contact concentrates stress into a tiny area, which limits how much load a ball can carry before the steel deforms, but it also keeps friction and running torque low, so ball bearings tend to be the smooth, fast, quiet choice.[2][6] Line contact spreads the same force across a much larger area, so a roller bearing in the same envelope can carry substantially higher radial load and hold its position more rigidly under force, at the cost of somewhat higher friction and a lower practical speed ceiling.[6][7] Bearing engineers sum it up as smooth and fast versus strong and stiff, and that single tradeoff explains most of why a humanoid robot needs several different bearing families rather than one.[2][7]

## Bearing types and where they fit in a humanoid

A humanoid's joints do not all want the same bearing. The table below summarizes the main rolling-element families, from lightest-duty to most heavily loaded, together with where each tends to show up in a bipedal robot.

| Type | Contact | Loads carried | Typical humanoid location |
|---|---|---|---|
| Deep-groove ball | Point | Radial, plus light-to-moderate axial in both directions | Wrists, fingers, hands, lighter elbow and shoulder axes |
| Angular-contact / four-point-contact ball | Point (angled, or four points per ball) | Radial plus axial, one direction per row (angular-contact) or both directions at once (four-point) | Shoulder and wrist joints; gearbox input stages |
| Cylindrical roller | Line | High radial | Weight-bearing hips, knees, ankles |
| Needle roller | Line, very thin rollers | High radial in a shallow cross-section | Compact rotary joints, finger and hand hinges |
| Thrust (ball or roller) | Point or line, loaded parallel to the shaft | Axial, one or both directions | Ankles, neck and head yaw, other vertical rotation axes |
| Spherical roller | Line, self-aligning | High radial plus axial, tolerant of shaft misalignment | Rare at humanoid scale; the misalignment-tolerant role is usually filled by a spherical plain bearing instead (see below) |
| Crossed-roller | Line, rollers alternating 90 degrees | Radial, axial (both directions), and moment, all at once | Hips, knees, shoulders, wrists, reducer output flanges |

### Deep-groove ball bearings

A single-row deep-groove ball bearing runs its ball in a groove whose cross-section is a slightly larger circular arc than the ball itself, which lets one part take radial load and moderate axial load from either direction.[8] That versatility, low cost, and low friction make it the default wherever a joint needs to spin quickly and often without carrying much body weight: wrists, fingers, hands, and the lighter axes of the elbow and shoulder. [NSK](/wiki/nsk), one of the type's largest makers, describes deep-groove ball bearings as roughly 70 percent of what it sells; independent market-research estimates of the type's share of total bearing production are more conservative, generally clustered in the 30 to 45 percent range, a reminder that these figures shift depending on whether they measure one company's catalog or the whole world market.[8][11]

### Angular-contact and four-point-contact ball bearings

Where a joint needs both radial support and real axial capacity in a compact package, designers reach for angular-contact ball bearings, whose raceways are ground at an angle so each ball's line of contact points partly along the shaft axis; mounting two of them back-to-back or face-to-face shares a heavy combined load and removes play from the joint, a standard configuration in humanoid shoulder and wrist joints.[9] A four-point-contact ball bearing achieves something similar in a single row: its raceway is split into a Gothic-arch profile so one ball touches four points at once, carrying axial load from both directions without needing a second bearing.[10] [Harmonic drive](/wiki/harmonic_drive) reducers lean on a related idea: their flexible, thin-walled wave generator rides on a specialized flexible ball bearing built to deform slightly on every rotation, and a four-point-contact bearing commonly sits at the reducer's input stage.[9]

### Cylindrical and needle roller bearings

A cylindrical roller bearing swaps the ball for a short cylinder, turning point contact into line contact and, for a given envelope, substantially raising how much radial load the bearing can carry before it deflects.[6][7] That is the profile a bipedal robot's weight-bearing joints need: hips, knees, and ankles carry the robot's full body weight on every step, a load a ball bearing of the same size would not survive for long.[12] Needle roller bearings apply the same line-contact idea to rollers stretched thin, typically with a length-to-diameter ratio of roughly 4:1 to 10:1, which packs meaningful radial capacity into a very shallow cross-section, sometimes just a few millimeters.[13] That makes needle bearings the default wherever a humanoid joint is tight on diameter rather than short on load: compact rotary actuators, finger and hand hinges, and other dense assemblies where a standard roller bearing simply will not fit.[13]

### Thrust bearings

Radial and cylindrical-roller bearings resist sideways force well but carry little load pushing straight along the shaft; that job belongs to thrust bearings, which orient their balls or rollers to roll between two flat washers rather than two concentric rings.[14] Ball thrust bearings suit lighter, faster axial loads; roller and needle thrust bearings, thanks to their line contact, carry several times more axial load in the same diameter.[14][15] In a humanoid, thrust bearings belong wherever gravity pushes straight through a rotating axis, such as an ankle carrying body weight down through the leg or a neck and head yaw joint carrying head weight, and industry design guidance also places small thrust bearings at elbow and shoulder modules to absorb axial load swings during arm motion.[12]

### Spherical roller bearings and the plain-bearing alternative

A spherical roller bearing uses barrel-shaped rollers running in a single spherical outer raceway, which lets the inner ring and roller set tilt a degree or two relative to the outer ring and keep working even when the shaft and housing are not perfectly aligned.[16] That self-aligning tolerance is valuable in the large industrial gearboxes, motors, and conveyor pillow blocks that much robot-actuator engineering descends from, but true spherical roller bearings are uncommon at humanoid scale specifically. The misalignment-tolerant role inside a humanoid's linkages, hip and leg push-rods, dog-bone connectors, and camera or sensor gimbals, is more often filled by a spherical plain bearing instead: a sliding bushing with a ball-and-socket inner geometry rather than rolling elements, commonly built into a rod end at each end of a linkage.[9] Industry coverage of humanoid ankle and foot design describes the same part doing duty there, letting the foot turn, rotate, or lean without a rigid bearing fighting it.[17] The distinction is worth keeping precise: the two parts solve the same misalignment problem through different mechanisms, rolling contact versus sliding contact, and only the spherical roller bearing belongs to the rolling-element family this article covers.

### Crossed-roller bearings

A crossed-roller bearing packs cylindrical rollers into a single V-shaped raceway with each roller's axis turned 90 degrees from its neighbor, so one compact ring resists radial load, axial load from both directions, and a tilting moment load all at once, a combination that would otherwise take two or more separate bearings stacked together.[18] That combination of high stiffness and small size has made the type the default output bearing for a humanoid's most heavily loaded joints. A teardown of a production [Unitree](/wiki/unitree) [G1](/wiki/unitree_g1) identified an ultra-thin crossed-roller bearing, supplied by the Chinese manufacturer Luoyang Baina Bearing, at the leg joints.[19] [Boston Dynamics](/wiki/boston_dynamics)' all-electric Atlas is reported to pair the same type of bearing with strain-wave gearing and frameless torque motors across most of its joints.[12] IKO, a maker of the type with a stated interest in the humanoid market, estimates that a typical full-size humanoid carries 14 to 20 crossed-roller bearings across its hips, shoulders, elbows, and wrists.[17] The type's internal construction, precision grading, and supplier landscape are covered in full in the dedicated crossed-roller bearing article.

## Load ratings, life, and materials

**In brief:** a bearing's basic dynamic load rating, C, works like the weight limit printed on a climbing rope: it is not the load at which the part snaps, but the load at which it wears out on a predictable schedule. L10 life takes that rating, the load actually applied, and the bearing's operating conditions, and predicts how long 9 out of 10 identical bearings will run before their rolling surfaces begin to fatigue, so an engineer can pick a bearing that will outlast the machine around it instead of guessing.

### The formula

Bearing catalogs define the basic dynamic load rating, C, as the constant load, radial for most bearings, axial for a thrust bearing, at which a bearing has a 90 percent statistical chance of reaching one million revolutions before the first sign of rolling-contact fatigue.[20] From that rating, the ISO 281 standard (matched in the United States by American Bearing Manufacturers Association practice) defines the basic rating life, L10, in millions of revolutions, as L10 = (C/P)^p, where P is the actual equivalent load on the bearing and p is a life exponent set by contact geometry: 3 for point-contact ball bearings and 10/3, about 3.33, for line-contact roller bearings.[20][21] The steeper exponent for roller bearings is the fatigue-life expression of the same point-versus-line tradeoff that governs speed and stiffness: because line contact already spreads stress over a larger area, small increases in load raise a roller's contact stress, and therefore shorten its life, more sharply than the same increase would for a ball.[20][21] The 2007 revision of ISO 281 also added a life-modification factor for lubrication quality and contamination, since a bearing running on marginal or dirty lubricant can fail well before its catalog L10 life regardless of how light its load is.[21]

One wrinkle matters specifically for a walking robot: L10 is defined in revolutions, but a humanoid's hip or knee does not complete revolutions in normal use, it oscillates back and forth through a few tens of degrees, thousands of times an hour. Bearing engineers handle this with a published oscillation factor that converts a catalog's continuous-rotation life rating into an equivalent oscillating-duty life; the technique originated with wind-turbine pitch and yaw bearings, which see a very similar back-and-forth duty cycle, and the same math applies directly to a bipedal joint.[22][23]

### Materials and precision

The great majority of rolling-element bearings, including nearly all of the ones inside a humanoid robot, use a through-hardening chromium steel such as AISI/SAE 52100 (also standardized as 100Cr6 or SUJ2), which reaches roughly 60 to 67 HRC after heat treatment and resists the surface fatigue that ultimately limits bearing life.[24][26] Where weight or speed matters more than cost, some robotics and aerospace-grade bearings substitute silicon-nitride ceramic balls in a hybrid design that keeps steel rings but swaps in ceramic rolling elements roughly 40 percent lighter than steel, cutting both centrifugal loading and friction at high speed.[25] Bearing catalogs also grade rotational accuracy into precision classes, from loosest to tightest, roughly ISO and JIS class 0 up through class 2, corresponding to the ABEC 1 through ABEC 9 scale used for ball bearings; robot-joint and machine-tool-spindle bearings typically specify the P4 or P5 grades in the middle of that range for minimal runout.[26]

## Lubrication

Every rolling-element bearing needs a lubricant film between the rolling elements and the raceway to prevent metal-to-metal contact, carry away frictional heat, and help keep out contaminants. Grease is the lubricant in the large majority of rolling bearings because it stays in place, doubles as a seal against dust and moisture, and needs far less supporting infrastructure than a circulating oil system; oil is reserved mainly for bearings running at very high speed or temperature, where grease cannot carry heat away fast enough, conditions closer to a jet-engine main shaft than a robot joint.[27] Most of the small bearings inside a humanoid actuator are sealed for life: packed with a measured charge of grease at the factory, fitted with rubber or steel seals, and never intended to be relubricated over the life of the part, the same approach used in small electric-motor bearings.[27] Getting the lubricant right matters more than almost any other single variable in a bearing's service life; SKF's own analysis attributes over 36 percent of premature bearing failures to insufficient or incorrect lubricant, ahead of contamination, misalignment, or fatigue from correctly applied loads.[28]

## Use in humanoid robots

A humanoid's rotary joint module typically bolts together a frameless [brushless DC motor](/wiki/brushless_dc_motor) or [servo motor](/wiki/servo_motor), a harmonic-drive or [planetary gear train](/wiki/planetary_gear_train) reducer, and several of the bearing types described above, commonly a four-point-contact or angular-contact bearing supporting the reducer's input stage and a crossed-roller or thin-section bearing carrying the output flange, all inside one housing rather than as separate mechanical stages bolted together.[29] Two [rotary encoders](/wiki/rotary_encoder), one on the motor shaft and one on the load side of the reducer, typically flank that bearing stack; the angular difference between the two readings, combined with the transmission's known stiffness, lets the controller estimate joint torque without a dedicated strain-gauge [force-torque sensor](/wiki/force_torque_sensor).[40] None of that works if the output bearing has play in it: backlash or radial slop there shows up directly as position error that the motor's control loop cannot see or correct, which is why humanoid actuator designs lean on bearing types, crossed-roller especially, that are inherently stiff and low-clearance rather than tuned to be so after the fact.

Estimates of how many bearings a full humanoid carries vary by source and by what gets counted. IKO puts the crossed-roller count alone at 14 to 20 per robot, across the hips, shoulders, elbows, and wrists, before adding the smaller ball and needle bearings in the hands and fingers.[17] PIB Sales, a bearing distributor, separately estimates that a robot built on [Tesla Optimus](/wiki/tesla_optimus)' general architecture incorporates more than 40 precision bearing sets in total, each matched to its own joint's motion profile.[12] Both figures are industry estimates rather than a manufacturer-disclosed bill of materials and are best read as orders of magnitude rather than exact counts.

The supply base for these bearings mirrors the broader humanoid-hardware buildout: long-established Japanese, Swedish, German, and American precision-bearing makers compete with a fast-growing set of Chinese manufacturers supplying the domestic humanoid boom, the same dynamic already visible in Unitree's G1, whose output bearing traces to the Chinese supplier Luoyang Baina Bearing rather than one of the traditional Japanese or European brands.[19]

## Suppliers and market landscape

The global market for rolling bearings of all types, industrial and consumer alike, is large: recent industry estimates put it at roughly 140 billion to 160 billion dollars for 2025 to 2026, with several independent market-research firms converging in that range and citing robotics, electric vehicles, and renewable energy as growth drivers alongside the much larger established automotive and general-industrial-machinery base.[30][31] A handful of companies, most of them over a century old, supply most of that market and, with it, most of the bearings in humanoid robot joints.

| Company | Headquarters | Founded | Notes |
|---|---|---|---|
| [SKF](/wiki/skf) | Gothenburg, Sweden | 1907 | Founded by Sven Wingquist around his invention of the self-aligning ball bearing; widely cited as the world's largest bearing manufacturer by sales[32] |
| [NSK](/wiki/nsk) | Tokyo, Japan | 1916 | Japan's first ball-bearing manufacturer; deep-groove ball bearings are one of its largest product lines by volume[33] |
| [Schaeffler](/wiki/schaeffler) (INA and FAG brands) | Herzogenaurach, Germany | 1946 (INA); FAG traces to 1883 | Wilhelm and Georg Schaeffler founded INA in 1946; FAG descends from Friedrich Fischer's 1883 ball-grinding breakthrough in Schweinfurt and joined Schaeffler after a 2001 takeover[34][35] |
| NTN | Osaka, Japan | 1918 | Name combines founders Niwa and Nishizono with backer Tomoe Trading Co.; by the company's own account, the world's third-largest bearing manufacturer[36] |
| Timken | Canton, Ohio, United States | 1899 | Built around Henry Timken's 1898 tapered-roller-bearing patent; still the reference name in tapered roller bearings specifically[37] |
| [THK](/wiki/thk) | Tokyo, Japan | 1971 | Founded by Hiroshi Teramachi around the rolling-contact linear guide; later expanded into rotary and crossed-roller bearings widely used in robot joints[38] |
| IKO (Nippon Thompson) | Tokyo, Japan | 1950 | Became Japan's first needle-roller-bearing maker in 1959; also a major linear-motion and crossed-roller supplier to the humanoid market[39] |

Rankings by exact market share vary between research firms, but SKF, Schaeffler, NSK, NTN, and Timken are routinely named among the small group of companies that dominate global bearing revenue, alongside Japan's JTEKT (Koyo) and NACHI, neither a focus of this article.[30][32] For humanoid-robot joints specifically, the more practical story is standardized, largely interchangeable catalog dimensions: a robot builder can often qualify two or three suppliers, spanning Japan, Europe, and increasingly China, for the same bearing position, the same dynamic already visible in the crossed-roller bearing supply chain.[18][19]

## See also

- [Crossed-roller bearing](/wiki/crossed_roller_bearing)
- [Harmonic drive](/wiki/harmonic_drive)
- [Actuator](/wiki/actuator)
- [Degrees of freedom](/wiki/degrees_of_freedom)
- [Ball screw](/wiki/ball_screw)

## References

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24. Auburn Bearing & Manufacturing. "Quality Bearing Material: 52100 Chrome Steel." https://auburnbearing.com/auburn-blog/quality-bearing-material-52100-chrome-steel/
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28. SKF. "Poor lubrication accounts for over 36% of premature bearing failures." https://cdn.skfmediahub.skf.com/api/public/0901d196802103bc/pdf_preview_medium/0901d196802103bc_pdf_preview_medium.pdf
29. RoboticsTomorrow. "Joint actuators: The fundamental component for humanoid robots' power and dexterity." https://www.roboticstomorrow.com/story/2025/06/joint-actuators-the-fundamental-component-for-humanoid-robots-power-and-dexterity/24924/
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33. NSK Ltd. "History." NSK Global. https://www.nsk.com/company/about-us/history/
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35. Wikipedia. "Friedrich Fischer." https://en.wikipedia.org/wiki/Friedrich_Fischer
36. NTN Corporation. "NTN Group History." https://www.ntnglobal.com/en/corporate/history/history.html
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39. IKO Nippon Thompson Europe B.V. "History." https://www.ikont.eu/en/history/
40. Honpine. "A Complete Guide to Choosing Single-Encoder vs. Dual-Encoder Robot Joint Actuators." https://www.honpine.com/news/Industry-News/A-Complete-Guide-to-Choosing-Single-Encoder-vs--Dual-Encoder-Robot-Joint-Actuators.html