Bearings for Robotics: The Ultimate Guide

Every joint that rotates, every wheel that rolls, every spindle that spins sits on a bearing, and the bearing quietly decides how stiff, how accurate, and how long-lived that motion is. A robot arm that drifts a millimeter at the tool point, a quadruped leg that develops play after a season of impacts, a spindle that whines and heats: trace the fault back and you often land on a bearing that was the wrong family, the wrong size, or mounted with the wrong fit. The part costs a few dollars. The consequence of getting it wrong costs a rebuild.

Bearings are one of the oldest solved problems in mechanical engineering, and that maturity is exactly why they get skipped in design reviews. The catalog gives a dynamic load rating, someone picks a number bigger than the load, and the team moves on. That shortcut works until the load has a moment arm, until the duty cycle has shock in it, until the joint needs to hold sub-arcminute accuracy, or until the housing bore was reamed two hundredths oversize. Then the physics that the catalog quietly assumed (clean contact, adequate lubrication, correct fit, load through the intended path) stops holding, and the bearing tells you so.

This guide treats the bearing as a component you size and select, with the governing contact mechanics, the fatigue-life math, the families and their tradeoffs, and where each one belongs in a robot. Numbers with units. Load paths drawn explicitly. The goal is that you can look at a joint, name the loads on it, and pick a bearing that survives the duty cycle instead of one that merely looks big enough.

The take: A bearing does one job, constrain a shaft to rotate while carrying load, and the whole selection problem is matching the load type (radial, axial, moment) to a bearing family that carries that type well, then sizing it against L10 fatigue life and static safety. Deep-groove ball bearings are the default for wheels and light shafts; angular-contact and tapered rollers take combined thrust; crossed-roller and thin-section bearings own robot joints because one compact ring carries radial, axial, and moment load together. Get the fit and the lubrication right or the catalog life is fiction. Size for the cube-mean load over the real duty cycle, not the peak, and never mount a moment load on a bearing that only carries radial.

Companion reading: gearboxes (harmonic & cycloidal), linear motion systems, robot actuators, industrial robot arms, and humanoid robot hardware.

Table of contents

1. Key takeaways
2. How a bearing works: contact, load paths, and friction
3. Load types: radial, axial, and the moment that kills joints
4. The bearing families
5. Life and load: L10, dynamic C, static C0
6. Preload and stiffness
7. Where each bearing goes in a robot
8. Mounting, fits, and tolerances
9. Lubrication and sealing
10. Failure modes: brinelling, spalling, contamination
11. A selection workflow
12. Frequently asked questions

How a bearing works: contact, load paths, and friction

A rolling bearing separates two parts that move relative to each other (a shaft and a housing) with rolling elements (balls or rollers) between two hardened raceways. The rolling elements let the shaft turn with very low friction while transmitting load from the inner ring to the outer ring through the contacts.

The whole behavior of a rolling bearing lives in those tiny contacts, and they obey Hertzian contact mechanics, the theory Heinrich Hertz worked out in 1882 for two elastic bodies touching. A ball pressed against a raceway does not touch at a point; it flattens into a small elliptical contact patch a fraction of a millimeter across, carrying contact pressures that routinely reach 1 to 3 GPa. Two facts fall straight out of Hertz and drive everything downstream:

• Ball contact is a point (ellipse); roller contact is a line. A ball deflects under load following F ∝ δ^1.5, so its incremental stiffness k = dF/dδ ∝ δ^0.5 ∝ F^(1/3) climbs only as the cube root of load. A cylindrical roller spreads the same load along a line, following a nearly linear F ∝ δ^1.1. That single difference is why roller bearings are stiffer and carry more load for the same envelope, and why ball bearings run with less friction and higher speed.
• Subsurface stress sets fatigue life. The peak shear stress from a Hertzian contact sits a few tenths of a millimeter below the surface. Repeated rolling cycles that stress, and eventually a microcrack nucleates there and works its way up until a flake of raceway lifts off. That is spalling, the classic fatigue end-of-life, and its statistics are the basis of the L10 rating.

Friction in a rolling bearing is low but not zero. The friction torque comes from rolling resistance at the contacts, sliding at the cage and guiding surfaces, and churning of the lubricant. A rough model:

M_friction ≈ 0.5 × μ × P × d_bore
   μ ≈ 0.0010 to 0.0015   deep-groove ball
   μ ≈ 0.0018 to 0.0025   tapered / cylindrical roller
   μ ≈ 0.10 to 0.25       plain bushing (sliding)

The two-orders-of-magnitude gap between rolling and sliding friction is the reason rolling bearings dominate anything that spins continuously, and the reason plain bushings survive in low-speed, low-duty pivots where that friction never matters.

Rule of thumb: if you only remember one thing about bearing physics, remember that ball contact is a point and roller contact is a line. Point contact buys speed and low friction; line contact buys load capacity and stiffness. Every family tradeoff in this guide is a consequence of that.

Load types: radial, axial, and the moment that kills joints

A bearing sees three kinds of load, and picking the wrong family for the load present is the single most common bearing mistake in robotics.

• Radial load (Fr). Force perpendicular to the shaft axis. A wheel carrying a robot's weight, a shaft with a belt pulling sideways, a gear reaction. Almost every bearing carries radial load.
• Axial / thrust load (Fa). Force along the shaft axis. A vertical spindle carrying its own rotor weight, a bevel-gear thrust reaction, the weight of a rotary table. Some bearings carry this well (angular-contact, tapered roller, thrust bearings), some barely at all (cylindrical roller).
• Moment / tilting load (M). A couple that tries to cock the bearing, tilting the inner ring relative to the outer. This is the load that a robot joint lives on and the one a single small bearing handles worst.

The moment load deserves its own attention because it is where robot joints diverge from generic machinery. Picture a robot arm link cantilevered off a joint. The payload at the end of the link is a radial force at a long moment arm, so the joint sees a large tilting moment. A single deep-groove ball bearing resists that moment only across its narrow ball row, which is a terrible lever, so it deflects and the arm droops. The classic fix is two bearings spread apart on the shaft, converting the moment into a couple of opposing radial loads:

Bearing radial reactions from a moment M over a bearing span L:
   F_A = M / L        (one bearing pushed one way)
   F_B = M / L        (the other pushed the opposite way)
Widen the span L and the reaction forces drop linearly.

That is why a well-designed shaft puts its two bearings as far apart as the packaging allows: the moment arm you want is the bearing span, and every millimeter of span you add reduces the radial load each bearing sees. When you cannot spread two bearings apart (a compact joint, a thin pancake actuator), you reach for a single bearing that carries moment on its own: a crossed-roller, a four-point-contact, or a thin-section bearing with a large enough raceway diameter to give the moment a lever inside one ring.

Rule of thumb: name the three loads on your bearing before you open a catalog. If there is a moment and no room to spread two bearings apart, you are in crossed-roller / thin-section territory, and no amount of oversizing a deep-groove ball bearing will fix the droop.

The bearing families

Here are the families a robotics engineer meets, what each carries, and where it belongs.

Deep-groove ball. The default. Balls in a deep circular groove on both rings. Carries radial load well and moderate thrust in either direction, runs fast, cheap, sealed variants everywhere. This is the wheel bearing, the idler, the light shaft support, the fan. When in doubt and the load is mostly radial, this is the starting point.

Angular-contact ball. The raceways are offset so the contact line runs at an angle (commonly 15, 25, or 40 degrees) to the radial plane. That angle lets the bearing carry heavy thrust in one direction plus radial load. Mount them in pairs (back-to-back "O", face-to-face "X", or tandem) to take thrust both ways and to preload out play. This is the spindle bearing and the precision-shaft bearing: machine-tool spindles, robot wrist axes, anything needing stiffness under combined load.

Tapered roller. Conical rollers on conical races. Line contact plus the cone angle gives very high radial and axial capacity in one bearing. Always used in opposed pairs, adjustable preload. This is the heavy-duty combined-load workhorse: automotive wheel hubs, heavy AMR drive wheels, gearbox output shafts. Higher friction than ball, so less common at high speed.

Cylindrical roller. Straight rollers, line contact, very high radial capacity and stiffness, but they carry little or no thrust (the rollers just slide axially). Used where radial load is large and thrust is handled elsewhere: gearbox shafts, high-radial spindles.

Needle roller. Long thin rollers, very high radial capacity in a small radial envelope (thin cross-section). No thrust capacity. Used where space is tight: robot joint pivots, linkage bearings, planetary-gear planet pins.

Four-point-contact (QJ) ball. A single ball row whose groove geometry contacts at four points, so one bearing takes thrust in both directions plus moment. Compact axial-plus-moment support, common as a single-bearing joint solution and in slewing rings.

Thin-section (thin-ring) ball. Bearings with a very small cross-section (the ring is thin relative to its diameter), available in radial-contact (C), angular-contact (A), and four-point-contact (X) types, in equal cross-sections across a wide bore range (the Kaydon "Reali-Slim" concept). The point is packaging: a large-diameter, low-mass, low-height bearing that a hollow robot joint or a camera gimbal can be built around, passing wiring and optics through the bore.

Crossed-roller. Cylindrical rollers arranged so that each roller is oriented 90 degrees to its neighbors, alternating, running in a V-groove. That crossed arrangement lets a single thin ring carry radial, axial (both directions), and moment load simultaneously with high stiffness. This is the robot-joint bearing: harmonic-drive outputs, rotary tables, robot-arm joints, precision indexing. THK and IKO are the reference names.

Slewing ring / turntable bearing. A large-diameter bearing (often with integral gear teeth) that carries huge moment and axial load at low speed. This is the base-joint bearing of big industrial arms, cranes, and heavy positioners.

Plain bushing (sleeve bearing). No rolling elements: a shaft sliding directly in a bushing of bronze, sintered bronze (oil-impregnated), or engineered polymer (Igus iglidur, GGB). Cheap, light, quiet, tolerant of shock and misalignment, corrosion-proof, dry-running polymers need no lubrication. The cost is high friction and a wear allowance instead of a fatigue life. Good for low-speed, low-duty, oscillating, or dirty joints.

Family	Radial	Axial	Moment	Speed	Stiffness	Typical robot use
Deep-groove ball	High	Light (both)	Poor	Very high	Medium	Wheels, idlers, light shafts, fans
Angular-contact ball	High	High (one dir)	With pair	High	High	Spindles, wrist axes, precision shafts
Tapered roller	Very high	Very high	With pair	Medium	High	Heavy wheel hubs, gearbox output
Cylindrical roller	Very high	None	Poor	High	Very high	High-radial gearbox shafts
Needle roller	High (compact)	None	Poor	Medium	High	Joint pivots, linkages, planet pins
Four-point-contact	Medium	High (both)	Yes	Medium	Medium	Compact axial+moment joints
Thin-section	Medium	Medium	Yes (X-type)	Medium	Medium	Hollow joints, gimbals, robot arms
Crossed-roller	High	High (both)	High	Low to medium	Very high	Robot joints, rotary tables, HD output
Slewing ring	High	Very high	Very high	Low	High	Big arm base joints, positioners
Plain bushing	Medium	With flange	Low	Low to medium	Low	Light pivots, washdown, oscillating

Rule of thumb: the family ladder for robotics reads: deep-groove ball for wheels and light shafts, angular-contact for spindles, tapered roller for heavy combined load, crossed-roller or thin-section for joints, plain bushing for cheap low-duty pivots. Start there and only deviate for a reason you can name.

Life and load: L10, dynamic C, static C0

Bearing fatigue life is a statistical number, and it is the number you design to. It comes from the Lundberg-Palmgren subsurface-fatigue theory (Gustaf Lundberg and Arvid Palmgren, 1947), the Weibull-distributed model of rolling-contact fatigue that underpins ISO 281. The basic rating life:

L10 = (C / P)^p × 10^6 revolutions
   C = basic dynamic load rating (from the catalog, in N or kN)
   P = equivalent dynamic bearing load (N)
   p = 3     for ball bearings (point contact)
   p = 10/3  for roller bearings (line contact)

L10 is the number of revolutions at which 10% of a large population has failed by fatigue (equivalently, 90% survive). The exponent is the whole story of why doubling the load is not a small deal: for a ball bearing, p = 3, so halving the load multiplies life by 8, and doubling the load cuts life to one eighth. Roller bearings, with p = 10/3, are even more sensitive.

To turn revolutions into hours at a running speed:

L10h = L10 / (60 × n)          hours,  n in rpm
     = (10^6 / (60 × n)) × (C / P)^p

Equivalent dynamic load. When a bearing sees both radial and axial load, you combine them into a single equivalent P using catalog factors:

P = X × Fr + Y × Fa
   X, Y from the catalog, depend on the bearing and the ratio Fa/Fr
   For pure radial load on a deep-groove ball: P ≈ Fr

Static rating and safety. Separate from fatigue, a stationary or slow bearing can be permanently dented by overload (brinelling). The static load rating C0 is the load producing a permanent deformation of 0.0001 times the rolling-element diameter at the most heavily loaded contact. The check is a static safety factor:

s0 = C0 / P0
   P0 = equivalent static load (X0 × Fr + Y0 × Fa)
   s0 ≥ 1.5 to 2   smooth, quiet duty
   s0 ≥ 2 to 4     shock, vibration, high accuracy required

Worked example: a robot drive wheel

Take an AMR drive wheel bearing. The robot plus payload puts 1,800 N of radial load on the wheel, a mild axial load of 300 N from cornering, and the wheel turns at 240 rpm at cruise. We are looking at a deep-groove ball bearing with catalog C = 25.5 kN, C0 = 15.3 kN, and for this Fa/Fr the catalog gives X = 0.56, Y = 1.4.

Fa/Fr = 300 / 1800 = 0.17   (above the bearing's e threshold, so use X,Y)
P = 0.56 × 1800 + 1.4 × 300 = 1008 + 420 = 1428 N

L10 = (25500 / 1428)^3 × 10^6
    = (17.86)^3 × 10^6
    ≈ 5697 × 10^6 revolutions

L10h = 5697e6 / (60 × 240) ≈ 396,000 hours

That is far more life than the robot will ever run, which is normal: wheel bearings are usually contamination-limited or seal-limited, not fatigue-limited, so the seal choice and the fit matter more than shaving the size. Static check:

P0 ≈ 0.6 × 1800 + 0.5 × 300 = 1080 + 150 = 1230 N
s0 = 15300 / 1230 ≈ 12.4    (huge margin, fine)

The lesson is the one from the linear-motion world: size against the cube-mean load over the real duty cycle. If the robot spends 20% of its time hitting curbs at triple load, that segment dominates the cube-mean and can slash L10 by a large factor even though it is a small fraction of the time.

Cube-mean load over a duty cycle of segments i (fraction u_i at load P_i):
   P_m = ( Σ u_i × P_i^3 )^(1/3)      (ball, p = 3)

War story: A warehouse AMR fleet started shedding drive-wheel bearings at eight months, well short of the 400,000-hour paper life. Nobody had over-loaded them. The docks had a lip the robots crossed thousands of times a shift, and each crossing was a shock load with a large momentary radial spike. The fatigue math on the cruise load looked immortal; the cube-mean including the dock-lip impacts was a fraction of it, and the impacts were also slowly brinelling the raceways at the stationary contact when robots queued. The fix was a shock-rated bearing with higher C0, a compliant wheel to soften the impact, and a ramp over the lip. Size for the worst repeated event, not the cruise.

Preload and stiffness

Many robot bearings run preloaded: assembled so the rolling elements are squeezed between the races even with no external load. Preload does two things a joint cares about: it removes internal clearance (so there is zero play and zero backlash at the bearing), and it raises stiffness (so the shaft deflects less under load).

Why preload buys stiffness is the same Hertzian story as before. A ball contact's incremental stiffness climbs as F^(1/3), so near zero load the bearing is floppy and the first bit of external load just takes up slack. Preload shoves every rolling element up its force-deflection curve to a firm operating point before any external load arrives, so the joint starts stiff instead of taking up lash. Two consequences:

• Stiffness climbs sub-linearly with preload for ball bearings (F^(1/3), so doubling preload buys only about 26% more contact stiffness), and near-linearly for roller and crossed-roller bearings (line contact), which is a second reason crossed-roller bearings dominate stiff joints.
• Preload costs friction and life. The rolling elements carry preload plus external load, so the L10 calculation must use the combined load, and the extra contact force raises the running friction torque. Heavy preload is a deliberate stiffness-versus-life trade, not a free upgrade.

Preload is set three ways in practice: by an axial adjustment (a nut that draws opposed angular-contact or tapered bearings together, measured by torque or by set distance), by matched bearing sets ground to give a defined preload when clamped ("universal" or DB/DF sets), or by an interference fit that expands the inner ring against the balls. Crossed-roller and thin-section joint bearings usually come with the preload built in by roller/raceway sizing.

Angular-contact / tapered preload rules of thumb:
   Light preload   → smoothness, high speed, lower stiffness
   Medium preload  → general precision joints and spindles
   Heavy preload   → maximum stiffness, at a real life penalty
Too much preload runs the bearing hot and can thermally run away:
heat → expansion → more preload → more heat.

Rule of thumb: preload out the play on any bearing that has to hold accuracy or position under a reversing load (joints, spindles, wrist axes). Use light-to-medium preload as the default; reserve heavy preload for a stiffness requirement you can justify, and check that the combined preload-plus-external load still hits your L10 target.

Where each bearing goes in a robot

Walk through a robot and the bearing choices become concrete.

Robot arm and joint axes. The output of a harmonic or cycloidal gearbox at each joint carries the downstream link's weight plus payload at a moment arm, so it sees radial, axial, and moment load in a compact, often hollow envelope. This is crossed-roller and thin-section territory. A crossed-roller bearing (or an integrated one built into the gearbox output) supports the whole joint in one thin ring with high moment stiffness, which is why nearly every industrial arm and cobot joint uses them. The bore passes wiring and the wave generator through.

Robot wrists and gimbals. Low load, high accuracy, tight packaging, and usually a need to route cables or optics through the center. Thin-section bearings (angular-contact or four-point X-type) shine here: large bore, low mass, moment capacity, minimal height.

Base / slewing joints of large arms. A big arm's first axis carries the entire arm's weight and moment. A slewing ring bearing, often with integral gear teeth for the drive, takes the huge moment and axial load at low speed.

Drive wheels (AMRs, AGVs, mobile robots). Radial load from the robot's weight plus cornering thrust plus curb-crossing shock. Deep-groove ball for light robots, tapered roller pairs or a sealed hub unit for heavy ones. Sealing against floor debris matters more than raw fatigue life here. See the mobile-robots guide.

Spindles and tool axes. High speed, needing stiffness and running accuracy under combined cutting or grinding load. Angular-contact ball pairs (back-to-back for stiffness) preloaded, or a matched spindle-bearing set. High-speed variants use ceramic (silicon-nitride) balls to cut inertia and heat.

Leg and drivetrain of legged robots. Impact-heavy, reversing, compact. Crossed-roller at the joints for moment stiffness, needle rollers in the linkages where radial space is tight, and shock-rated static capacity because the foot strike is a repeated static overload. See the legged/quadruped hardware guide and the humanoid hardware guide.

Linear axes. The recirculating-ball blocks and ball screws in a linear motion system are bearings too, governed by the same L10 math with the linear-motion ISO 14728 standard instead of ISO 281.

Rule of thumb: joints want one thin ring that carries everything (crossed-roller or thin-section); shafts and spindles want two bearings spread apart (a "locating / non-locating" pair); wheels want a sealed radial bearing sized for shock, not cruise. Match the bearing to the joint's job, not to a generic load number.

Mounting, fits, and tolerances

A correctly chosen bearing installed with the wrong fit fails early, and this is where a lot of field trouble actually starts. The bearing's internal clearance, its stiffness, and whether its rings creep are all set by how tightly it is pressed onto the shaft and into the housing.

The governing principle: the ring that rotates relative to the load direction gets an interference (press) fit; the ring that is stationary relative to the load gets a looser (transition or clearance) fit. For a normal robot shaft where the inner ring turns and the load points a fixed way (gravity on a wheel), that means an interference fit on the inner ring and a slightly loose fit in the housing.

Why: a ring that is loose where the load rotates around it will "creep"
(slowly rotate in its seat), fretting and wearing the fit surface.
An interference fit on the rotating-load ring locks it and prevents creep.

Fits are specified as ISO 286 tolerance bands, not as "a light press." A typical robot-shaft example:

• Shaft (inner ring, rotating): k5 or m5 for a normal load, js5/j6 for a lighter interference. The letter sets where the tolerance band sits relative to nominal; the number sets its width (IT grade).
• Housing (outer ring, stationary): H7 or J7, a transition-to-clearance fit that lets the outer ring settle without creeping under a fixed load.

Two failure traps live in the fit:

• Over-interference crushes internal clearance. A bearing ships with a small radial internal clearance (classes C2, CN/Normal, C3, C4, increasing). Pressing the inner ring onto an oversize shaft expands it and eats that clearance. Too much interference on a bearing with normal clearance can drive it to negative clearance (preload it unintentionally), which runs it hot and kills it. High-interference or high-temperature installs use a C3 (looser) bearing to leave clearance after mounting.
• Thermal expansion changes the fit. A shaft that runs hotter than the housing grows and tightens the fit further. Locating/non-locating bearing arrangements exist precisely to let one bearing float axially so thermal growth of the shaft does not jam the pair.

Robot-specific caveats: hollow joint shafts and thin-section bearings are far more sensitive to housing roundness and flatness than a chunky machine shaft, because a thin ring conforms to a distorted housing and loses its accuracy. Thin-section and crossed-roller bearings specify tight housing squareness and surface flatness for exactly this reason, and they are often clamped between ground faces rather than press-fit.

War story: A team building a compact cobot joint reamed the housing bore to the middle of the tolerance and pressed a normal-clearance thin-section bearing in with a healthy interference "to be safe." The joint ran hot within minutes and developed drag. The interference had driven the thin ring to near-zero clearance, and the slightly out-of-round bore made it worse on one side. Swapping to a C3 bearing and holding the bore to a tighter roundness spec fixed it. On thin rings, the housing is part of the bearing.

Lubrication and sealing

The catalog L10 is a clean-and-lubricated number. What actually keeps a rolling element off its raceway is an elastohydrodynamic (EHL) film only tenths of a micron thick, generated because the immense Hertzian contact pressure spikes the lubricant's viscosity by orders of magnitude right at the contact. Whether that film separates the metal is governed by the specific film thickness:

Λ = h_min / sqrt(Rq_element² + Rq_race²)
   Λ > 3   full separation, catalog life
   Λ ≈ 1   asperities touch, boundary lubrication, life falls off a cliff

Under-spec the grease viscosity for your speed and temperature and you have quietly designed a boundary-lubrication bearing no matter how big the C rating.

Grease vs oil. Most robot bearings are grease-lubricated: sealed for life or with a relube path, clean, no plumbing. Grease is an oil held in a soap thickener (lithium, lithium-complex, urea, polyurea) at NLGI grade 2 for most bearings, softer (0 to 1) for low temperature or crossed-roller joints. Oil (bath, circulating, or oil-air mist) goes to high-speed spindles and high-temperature or high-load bearings where grease would churn and overheat.

Grease life and quantity. A rolling bearing is filled roughly 30% of its free volume with grease, over-packing churns and overheats it. Grease has a finite life (oxidation and mechanical breakdown), specified in hours as a function of speed, temperature, and bearing size. Every 10 to 15 degrees C over the grease's rated temperature roughly halves its life, the same Arrhenius rule that governs insulation.

Sealing. The seal is what stands between the catalog life and reality. Shielded bearings (2Z, a metal shield with a gap) keep out coarse debris and let the bearing run faster; contact-sealed bearings (2RS, a rubber lip) keep out fine dust and moisture at the cost of some friction. For dirty robot environments (floor debris, machining swarf, outdoor grit) the seal choice, plus an external labyrinth or a wiper, matters more than the fatigue rating.

Environment → sealing strategy
   Clean indoor, high speed   → shielded (2Z) or open with external seal
   Floor debris / dust        → contact seal (2RS) + labyrinth
   Washdown / food            → stainless or coated, food-grade grease, or plain polymer
   Vacuum / cleanroom         → low-outgas grease or dry-film, special seals

Rule of thumb: the field life of a robot bearing is the catalog L10 multiplied by how seriously you took sealing and relube. On a sealed wheel bearing in grit, spec the seal first and the fatigue rating second, because the bearing will die of contamination long before it dies of fatigue.

Failure modes: brinelling, spalling, contamination

Bearings fail in a handful of recognizable ways, and reading the signature tells you what actually went wrong, which is usually not "the bearing was too small."

• Spalling (fatigue). The expected end-of-life: subsurface fatigue cracks reach the surface and flake off raceway material, leaving pits that grow into rough, noisy, vibrating patches. If a bearing reaches its L10 travel and spalls, the design worked. If it spalls early, the real load was higher than assumed (check the cube-mean), or the film was too thin (lubrication), or the internal clearance was wrong (fit).

• Brinelling (static overload). True brinelling is permanent dents in the raceway from a static overload greater than C0: a dropped assembly, an e-stop shock, a press force applied through the balls during mounting (never press a bearing on by pushing on the wrong ring, the force goes through the rolling elements and dents both races). The dents show up as evenly spaced marks at the ball spacing and cause vibration forever after.

• False brinelling / fretting (vibration at standstill). A bearing that is not rotating but is vibrated (a robot idling on a shaking floor, a machine shipped by truck, a joint dithering under a holding current) wears small depressions at the contact points because the tiny oscillation wipes out the lubricant film and lets metal fret. It looks like brinelling but comes from vibration, not overload. The fix is to avoid standstill vibration, or to slowly rotate stored/idle bearings, or to use a grease with anti-fretting additives.

• Contamination pitting and abrasive wear. Hard particles (grit, machining chips, wear debris) get rolled into the raceway, denting it and then abrading everything. This is the number-one real-world killer of robot bearings, and it traces to a failed or absent seal. The signature is dull, matte, scratched raceways rather than clean fatigue flakes.

• Corrosion. Moisture (washdown, condensation, a robot left in a humid warehouse) rusts the raceways; the rust then acts as an abrasive. Stainless bearings, coatings, or dry polymer bushings solve it.

• Electrical erosion (fluting). In servo-driven robots, common-mode voltage from the motor drive can push current through the bearing, arcing at the contacts and leaving a washboard "fluting" pattern. It shows up on motor and gearbox bearings on PWM-driven axes. Fixes: a shaft grounding ring, an insulated or ceramic-ball (hybrid) bearing, or a bonded low-impedance ground path.

• Overheating and lubricant failure. Too much preload, too much grease, too high a speed, or an ambient too hot cooks the grease, the film collapses, and the bearing wears and seizes. Discoloration (straw to blue temper colors) on the races is the tell.

Rule of thumb: before you conclude a bearing was undersized, read the failure. Matte scratched races mean contamination (fix the seal). Evenly spaced dents mean brinelling (fix the mounting or the shock load). Washboard fluting means bearing current (fix the grounding). Clean fatigue flakes at roughly the calculated life mean the size was right and the bearing simply reached the end of its L10.

A selection workflow

Put it together into a repeatable order. Do not start by picking a bearing size.

1. Name the loads. Resolve the worst-case operating condition into radial (Fr), axial (Fa), and moment (M) at the bearing, including dynamic and shock components. Decide whether a moment is present and whether you can spread two bearings apart to carry it as a couple.

2. Pick the family from the families table by load type: mostly radial and fast, deep-groove ball; heavy combined, angular-contact or tapered roller in a pair; moment in a compact ring, crossed-roller or thin-section; big low-speed moment, slewing ring; light low-duty pivot, plain bushing.

3. Choose the arrangement. Two bearings on a shaft: one locating (takes axial), one non-locating (floats for thermal growth). Or one integrated joint bearing. Set the bearing span to give the moment a lever.

4. Size against L10. Compute the equivalent dynamic load P over the cube-mean of the duty cycle, pick a bearing whose (C/P)^p × 10^6 revolutions clears your required life with margin. Convert to hours at your running speed.

5. Check static safety. Compute the equivalent static load P0 at the worst shock and confirm s0 = C0 / P0 meets 1.5 to 2 for smooth duty, 2 to 4 for shock. On impact-heavy robots (legs, curb-crossing wheels) this often sizes the bearing, not fatigue.

6. Set preload and clearance. Preload out play on accuracy-critical or reversing joints (light-to-medium default). Choose the internal clearance class (C2/CN/C3/C4) so that after the interference fit and thermal rise, the running clearance lands where you want it.

7. Specify the fits. Interference on the rotating-load ring (shaft k5/m5), transition/clearance on the stationary ring (housing H7/J7). Tighten housing roundness and squareness for thin-section and crossed-roller bearings.

8. Choose lubrication and sealing for the environment: grease NLGI 2 default, contact seals for dirt, stainless or polymer for wet, hybrid/insulated for PWM-driven axes. Write the relube interval into the maintenance plan.

9. Verify on the real assembly. Check running temperature, noise, and play after mounting. A bearing that runs hot on install is over-preloaded or over-interfered; one with play was not preloaded or the fit is loose.

A quick selection table

Robot location	Loads present	Bearing pick	Why
Arm / cobot joint	Radial + axial + moment, hollow	Crossed-roller or thin-section	One thin ring carries all three, high moment stiffness
Wrist / gimbal	Light, precise, cable-through	Thin-section (X-type)	Large bore, low mass, moment capacity
Big arm base axis	Huge moment + axial, slow	Slewing ring	Massive moment at low speed, integral gear
Spindle / tool axis	Combined, high speed, stiff	Angular-contact pair, preloaded	Stiffness and running accuracy under combined load
Drive wheel (light)	Radial + light thrust + shock	Deep-groove ball, sealed	Cheap, fast, seal-limited not fatigue-limited
Drive wheel (heavy)	High radial + axial + shock	Tapered roller pair / hub unit	Combined capacity and shock margin
Leg linkage	Radial, tight space, impact	Needle roller	Max radial in a thin envelope
Compact axial joint	Axial both ways + moment	Four-point-contact	One row takes bidirectional thrust and moment
Light / washdown pivot	Low load, dirty or wet	Plain polymer bushing	Dry, corrosion-proof, cheap, shock-tolerant

Rule of thumb: the smallest bearing that clears L10 is rarely the right one. Leave margin for the shock loads you did not fully characterize, the contamination you cannot fully seal out, and the preload you will add. A bearing that "just fits" on the fatigue math runs at the edge of every other limit.

Frequently asked questions

What is L10 life and why not L50 or a guaranteed number? L10 is the travel (in revolutions or hours) at which 10% of a large population of identical bearings has failed by fatigue, so 90% survive. Rolling-contact fatigue is statistical (Weibull-distributed), so there is no single guaranteed life, only a probability. L10 is the industry standard because it is conservative enough to design to; L50 (median life) is roughly five times longer but half the population has already failed by then, which is not a design basis. Compute L10 against the cube-mean load over your real duty cycle.

How do I choose between a ball bearing and a roller bearing? Ball bearings make point contact: lower friction, higher speed, lower cost, moderate load and stiffness. Roller bearings make line contact: higher load capacity and stiffness for the same envelope, but more friction and lower speed limits. Use ball bearings for fast, lightly-to-moderately loaded shafts and wheels; use roller bearings (cylindrical, tapered, crossed) where load and stiffness dominate and speed is modest, which describes most robot joints.

Why do robot joints use crossed-roller bearings instead of a pair of ball bearings? A joint sees radial, axial, and moment load in a compact, often hollow envelope, and there is usually no room to spread two bearings far apart to carry the moment as a couple. A crossed-roller bearing carries all three loads in a single thin ring with high moment stiffness because its rollers alternate at 90 degrees, so one bearing does the whole job with minimal height and a large bore for wiring. That packaging plus stiffness is why nearly every industrial arm and cobot joint uses them, frequently integrated into the gearbox output.

What does preload actually do, and can I have too much? Preload squeezes the rolling elements between the races with no external load, removing internal clearance (zero play, zero backlash) and raising stiffness by shoving each contact up its force-deflection curve to a firm operating point. Yes, you can have too much: the rolling elements then carry preload plus external load, cutting fatigue life and raising friction and heat, and excess preload can thermally run away (heat expands the parts, which adds preload, which adds heat). Use light-to-medium preload as a default and reserve heavy preload for a justified stiffness need.

How tight should the fit be on the shaft and in the housing? The ring that rotates relative to the load direction gets an interference (press) fit so it cannot creep and fret; the stationary-load ring gets a transition or clearance fit. For a typical robot shaft (inner ring turning, gravity load fixed), that is an interference fit on the shaft (ISO band like k5 or m5) and a looser fit in the housing (H7 or J7). Too much interference crushes the internal clearance and can drive the bearing into unintended preload, so high-interference installs use a looser-clearance bearing (C3) to leave running clearance after mounting.

Why did my sealed wheel bearing fail so early when the fatigue life said decades? Almost certainly contamination or shock, not fatigue. The catalog L10 assumes a clean lubricated bearing carrying the loads you specified. In grit, a compromised seal lets hard particles into the raceway that dent and abrade it (matte scratched races), and repeated shock loads (curbs, dock lips) both raise the cube-mean load far above cruise and slowly brinell the raceway. Fix the seal, soften the shock, and size the static rating for the worst impact, not the cruise load.

What is false brinelling and how is it different from real brinelling? Real brinelling is permanent dents from a single static overload above C0 (a drop, an e-stop, or pressing a bearing on through the wrong ring). False brinelling (fretting) is wear at the contact points from small vibration while the bearing is not rotating, which wipes out the lubricant film and lets the metal fret. Both leave evenly spaced marks, but false brinelling comes from vibration at standstill (shipping, an idling robot dithering under holding current), and the fix is to avoid standstill vibration or use an anti-fretting grease, not a bigger bearing.

When is a plain bushing better than a rolling bearing? When the speed is low, the duty is light or oscillating, and cost, weight, shock tolerance, corrosion resistance, or silence matter more than friction. A dry polymer bushing (Igus, GGB) needs no lubrication, shrugs off dust and washdown, tolerates misalignment and shock, and costs a fraction of a rolling bearing. The trade is much higher friction and a wear allowance instead of a fatigue life, so it suits light joints, pivots, and dirty or wet axes, not high-speed spindles or stiff precision joints.

Do I need special bearings for servo-driven axes? Sometimes. PWM motor drives create common-mode voltage that can push current through the motor and gearbox bearings, arcing at the contacts and leaving a washboard "fluting" pattern that ruins the bearing. On PWM-driven axes, especially larger motors, protect against it with a shaft grounding ring, an insulated or hybrid (ceramic-ball) bearing, or a proper low-impedance ground path. Ceramic balls also cut inertia and heat, which helps high-speed spindles independent of the current issue.

How do I convert the catalog life into something meaningful for my robot? Compute the equivalent dynamic load P (combining radial and axial with the catalog X and Y factors) using the cube-mean load over your real duty cycle, then L10 = (C/P)^p × 10^6 revolutions with p = 3 for ball or 10/3 for roller, and divide by 60 times the rpm to get hours. Then sanity-check against the static rating for shock and remember the number assumes clean lubrication: derate hard for contamination and marginal sealing, which is where most robot bearings actually die.

Ball, tapered, or angular-contact for a bearing that must carry thrust? Deep-groove ball carries only light thrust in either direction, fine for incidental axial load. Angular-contact ball carries heavy thrust in one direction (mount a pair for both directions) with high speed and stiffness, ideal for spindles and precision shafts. Tapered roller carries the heaviest combined radial-plus-axial load in one bearing (always in opposed pairs, adjustable preload) but with more friction and lower speed, ideal for heavy wheel hubs and gearbox outputs. Pick by how much thrust, how much combined radial, and how fast.