eduKate Learning Manual
Science | Physical World
Understand → Teach → Learn → Memorize → Test → Go Deeper

The Ball Bearing

Why Tiny Steel Balls Let a Shaft Spin Under Load

WAIT, WHAT? The Balls Reduce Sliding—but They Still Carry Enormous Local Stress

A shaft can support a heavy load and still rotate smoothly inside a ball bearing.

The bearing reduces much of the sliding that would occur if the shaft rubbed directly against a stationary sleeve.

But the balls do not remove force. They concentrate it into small rolling contact patches where stresses can be extremely high.

A successful bearing therefore has two simultaneous jobs:

• allow low-resistance relative rotation;
• carry repeated load through millions of rolling contacts without damaging the surfaces.

shaft load → inner race → balls → outer race → housing.
rotation → balls roll through raceways → lubricant limits damaging contact → cage keeps spacing.

Big Question: How can a set of small balls carry substantial radial or axial load while allowing two machine parts to rotate relative to each other with much less friction than direct sliding contact?

Quick Answer

A typical ball bearing contains an inner ring, outer ring, balls and usually a cage.

The rings contain shaped raceways. The balls roll between those raceways while transferring load from one ring to the other.

Replacing broad sliding contact with rolling contact greatly reduces frictional losses, but friction does not become zero.

Real bearings lose energy through elastic deformation, lubricant shear, microscopic sliding, cage contact, seal drag and churning of grease or oil.

Lubricant forms a protective film between balls and raceways. Cleanliness matters because hard particles can dent the raceway and create stress concentrations.

Repeated rolling stress eventually can cause fatigue cracks and spalling, which is why bearing life depends strongly on load, lubrication, alignment and contamination.

What You Will Learn

• What the inner and outer races do.
• How balls carry load.
• Why rolling contact reduces friction.
• Why rolling contact is not frictionless.
• What the cage does.
• Why raceways are curved.
• Why ball–race contact occurs over a small patch.
• What lubrication does.
• Why contamination is dangerous.
• What preload and clearance mean.
• Why bearings can fatigue even when nothing visibly rubs.
• Why alignment matters.

Part 1 — The Bearing Creates a New Load Path

Without a rolling bearing, a rotating shaft might slide directly against a bushing or hole.

With a ball bearing, the shaft is attached to or supported by the inner ring while the outer ring is mounted in the housing.

The balls bridge the gap between them.

Load therefore travels ring → ball → ring instead of through a large sliding interface.

Part 2 — The Balls Are Rolling Elements

NSK describes rolling bearings as using balls or rollers between raceway rings.

As one ring rotates relative to the other, each ball spins about its own axis while circulating around the bearing.

The ball therefore combines local rolling with orbital motion.

Part 3 — Why Rolling Usually Loses Less Energy Than Sliding

In broad sliding contact, surface asperities continuously shear past one another across a large relative sliding distance.

In rolling contact, the surfaces enter and leave the contact patch with much smaller local relative displacement.

This greatly reduces frictional losses in many machine conditions.

That is why rolling bearings improve mechanical efficiency.

Part 4 — Rolling Does Not Mean Zero Sliding

The ball and raceway surfaces deform elastically under load.

Their curved geometries create small regions of microslip and spin within the contact.

The cage can slide against balls or rings. Seals rub. Lubricant is sheared and churned.

NSK technical literature explicitly separates rolling friction from sliding and lubricant-related friction inside bearings.

Part 5 — The Raceway Shape Keeps the Balls Captured

Deep-groove ball bearings contain curved tracks machined into the inner and outer rings.

The groove guides each ball and determines the direction of contact force.

Different bearing types use different groove shapes and contact angles to favour radial, axial or combined loads.

Part 6 — Radial and Axial Loads Are Different

A radial load acts roughly perpendicular to the shaft axis.

An axial or thrust load acts along the shaft axis.

Deep-groove ball bearings can carry radial load and moderate axial load, while angular-contact bearings use a deliberate contact angle to carry larger combined loads.

Part 7 — A Ball Does Not Touch the Raceway at a Mathematical Point

At zero load, geometry may suggest point contact.

Under real load, steel deforms elastically and the contact spreads into a small elliptical patch.

This is Hertzian contact.

The patch is small enough that contact pressure can be extremely high even when the overall bearing load seems modest.

Part 8 — Why Hardened Steel Is Used

Raceways experience repeated high contact stress every time a ball passes.

Bearing steels are hardened and manufactured to high surface quality so they can resist plastic indentation and repeated fatigue.

Smoothness and cleanliness are structural requirements, not cosmetic finishing.

Part 9 — The Cage Keeps the Balls Apart

Without a cage, neighbouring balls can crowd together and rub against each other.

NSK describes the cage as maintaining even spacing, preventing ball-to-ball contact and helping rolling elements move smoothly.

The cage carries little of the main external load in a normal bearing. Its primary role is organising motion.

Part 10 — Lubrication Creates a Protective Film

Oil or grease enters the rolling contacts.

Under pressure and motion, a thin elastohydrodynamic lubricant film can separate much of the metal surface roughness.

This reduces wear and surface fatigue.

Lubricant also reduces cage and seal friction and helps protect against corrosion.

Part 11 — Too Much or Too Little Lubricant Can Both Be Bad

Too little lubricant allows damaging metal-to-metal contact.

Too much grease can increase churning resistance and temperature at high speed.

Correct lubricant type, amount and viscosity depend on speed, temperature, load and bearing design.

Part 12 — One Grain of Dirt Can Matter

A hard contaminant particle entering a loaded contact can be pressed into a raceway.

The resulting dent creates a local geometric stress raiser.

SKF and NSK both identify contamination as a major cause of shortened rolling-fatigue life because dents and disturbed lubricant films increase local stress.

Part 13 — Bearing Fatigue Is Repeated-Stress Damage

Every ball passage loads a region of raceway, unloads it, then loads it again on a later revolution.

After enough stress cycles, microscopic cracks can initiate beneath or near the surface.

Those cracks can grow until small flakes detach from the raceway.

This is rolling-contact fatigue or spalling.

Part 14 — Load Has a Powerful Effect on Life

NSK’s basic rating relation for ball bearings uses:

L₁₀ ∝ (C/P)³

C is the dynamic load rating and P the equivalent load.

The cubic dependence means modest increases in load can greatly shorten calculated fatigue life.

This is a higher-resolution engineering model, not a Primary-level calculation requirement.

Part 15 — Clearance and Preload Matter

Bearings need the correct internal geometry when installed.

Too much clearance can reduce accuracy and increase vibration.

Too little clearance or excessive preload can raise contact forces, friction and temperature.

Fit and temperature changes can alter internal clearance even after assembly.

Part 16 — Misalignment Changes Which Balls Carry Load

If the shaft and housing axes are misaligned, the load distribution around the bearing becomes uneven.

Some balls and raceway regions then carry more than their intended share.

NSK damage guides identify misalignment and poor mounting as causes of abnormal running traces and shortened life.

Follow One Ball Through One Revolution

1. The shaft loads the inner ring.
2. The inner raceway presses on several nearby balls.
3. Each loaded ball deforms slightly at its contact patch.
4. Load passes through the ball to the outer raceway.
5. The inner ring rotates.
6. The ball spins and circulates through the bearing.
7. A lubricant film separates much of the surface roughness.
8. The ball leaves the heavily loaded zone.
9. Another ball enters and carries part of the load.
10. The cage preserves spacing.
11. The same raceway regions experience repeated cyclic stress.
12. Over millions of cycles, material quality, lubrication and load determine whether fatigue damage develops.

A Text Diagram You Can Draw Anywhere

HOUSING
[ OUTER RACEWAY ]
    ○   ○   ○   ← balls / rolling elements
[ INNER RACEWAY ]
      SHAFT

LOAD PATH:
shaft → inner ring → ball contacts → outer ring → housing

CAGE: keeps ○ evenly spaced
LUBRICANT: thin film at rolling contacts

Think Like a Scientist — Rolling vs Sliding

Use a smooth board, a small safe block and several identical round pencils or dowels as rollers.

1. Pull the block directly across the board and feel the sliding resistance.
2. Place several round rollers underneath.
3. Pull again while keeping the rollers aligned.
4. Compare the force needed qualitatively.
5. Observe that the rollers move relative to both surfaces.
6. Discuss why the demonstration resembles a rolling bearing but lacks raceways, cage, lubrication and concentrated contact stress.

How Do We Know the Naive “Ball Bearings Remove Friction” Model Fails?

• NSK describes rolling bearings as reducing friction, not eliminating it;
• bearing friction includes rolling deformation, sliding, lubricant shear, cage interaction and seals;
• bearings warm during operation when frictional power is dissipated;
• lubrication changes friction and fatigue life dramatically;
• contaminants can damage raceways despite the balls still rolling;
• rolling-fatigue life exists precisely because loaded rolling contact creates repeated internal stress.

Observation vs Inference

• Observation: a bearing lets a loaded shaft rotate more freely than direct sliding contact.
• Observation: balls circulate between shaped raceways.
• Observation: cage spacing keeps balls separated.
• Observation: poor lubrication or contamination causes noise, heat and damage.
• Inference: low-friction rotation depends on controlled rolling contact, precise load distribution and protective lubrication.

Common Misconceptions and How to Repair Them

Misconception	Better model
Ball bearings eliminate friction.	They greatly reduce many sliding losses but still contain rolling, sliding and lubricant friction.
The balls merely keep the shaft centred.	They are the primary load-transfer elements between inner and outer rings.
The cage carries the machine load.	The cage mainly spaces and guides rolling elements.
Lubrication only makes the bearing feel smoother.	It forms protective films that reduce wear and surface fatigue.
Dirt is too small to matter.	Particles can dent raceways and create severe local stress concentrations.
If a bearing still turns, it is healthy.	Fatigue, contamination or clearance problems can progress before complete seizure.

Checkpoint Questions

1. What are the four main bearing components?
2. What do the balls do?
3. Why is rolling usually lower-friction than sliding?
4. Why is bearing friction not zero?
5. What does the cage do?
6. Why are raceways shaped?
7. What is Hertzian contact?
8. Why is lubrication important?
9. Why can one contaminant particle matter?
10. What is rolling-contact fatigue?

Apply It — Diagnose the Gritty Bearing

A small bearing still rotates, but it now feels gritty and produces more noise after being exposed to dust. No ball is visibly missing.

Which mechanism should be investigated first?

Answer Key

Open after attempting the transfer

Contamination and lubrication condition. Dust can enter the rolling contacts, disrupt the lubricant film and indent or abrade raceways. The bearing can still rotate while surface damage and friction rise.

Can You Explain WHY?

• Why can tiny balls carry large overall load?
• Why does local contact stress become high?
• Why does a cage reduce unwanted friction?
• Why does oil film thickness matter?
• Why does bearing life depend strongly on load?
• Why is “rolling means frictionless” a failed model?

Singapore Everyday Connection

Bearings are hidden inside bicycles, fans, wheels, tools, appliances, lifts and industrial machinery.

Singapore’s humid environment also makes sealing and corrosion protection relevant, especially where bearings face water, dust or outdoor exposure.

Primary Science / PSLE Bridge

• friction can be reduced by changing the type of contact;
• forces can be transferred through solid objects;
• rolling and sliding are different motions;
• lubricants change surface interactions;
• repeated forces can damage materials;
• cleanliness and alignment affect machine performance.

Go Beyond Primary Science

Primary idea	Higher-resolution science
Balls reduce sliding	Rolling-contact tribology
Ball carries load	Hertzian contact stress
Oil separates surfaces	Elastohydrodynamic lubrication
Cage keeps spacing	Bearing kinematics
Repeated load damages raceway	Rolling-contact fatigue
Load changes life	Statistical bearing-life models

Deep Science Window — Point Contact Becomes an Elastic Contact Patch

A perfectly rigid sphere on a perfectly rigid surface would touch geometrically at a point.

Real bearing steel deforms elastically.

The ball and raceway flatten slightly under load, producing a finite contact ellipse with high pressure.

That elastic deformation is essential to understanding both load capacity and fatigue.

Evidence Boundaries

• Rolling bearings reduce friction ≠ they are frictionless.
• Balls transfer load through small patches ≠ contact is a mathematical point under real load.
• Cages organise rolling elements ≠ every bearing requires the same cage design.
• Lubrication protects contacts ≠ more lubricant is always better.
• Fatigue life can be modelled statistically ≠ every bearing fails at the calculated L₁₀ revolution count.
• Hands-on demonstrations with loose rollers are useful ≠ installed bearings in safety-critical equipment should be dismantled casually.

Manual Summary — KNOW → CONNECT → EXPLAIN → APPLY → CHECK

KNOW: inner ring, outer ring, raceway, ball, cage, rolling contact, lubrication, clearance, fatigue and contamination.

CONNECT: shaft loads inner ring → balls carry force to outer race → balls roll during relative rotation → lubricant protects concentrated contacts → cage maintains spacing → repeated stress governs fatigue life.

EXPLAIN: a ball bearing allows low-resistance rotation because precision rolling contacts replace much of a large sliding interface while still carrying load through highly stressed elastic contact patches.

APPLY: wheels, fans, motors, bicycles, machine tools and rotating shafts.

CHECK: separate friction reduction, load transfer, lubrication and fatigue.

Where to Go Next

• The Physical World
• The Bicycle Brake
• The Spinning Top

Research Sources and Further Reading

• NSK — Bearing Basics, Components and Cage Function
• NSK — Dynamic Load Ratings and Rolling Fatigue Life
• NSK — Bearing Damage, Lubrication, Misalignment and Contamination
• SKF Evolution — Wear and Surface Fatigue in Rolling Bearings
• Timken — Lubrication, Contamination and Bearing Failure Modes

---

eduKate Learning Manuals are written so that a learner can begin simply, a parent can teach confidently, and both can keep going until the simple school model opens into real Science.