Further Reading

For a broader understanding of motion transfer system design and the role of gears and couplings in overall system behavior, the following articles provide additional engineering insights:

• Gears & Couplings: An Engineering Guide to Precision Motion Transfer

• How to Choose the Right Coupling Without Guessing

• Common Coupling Failures and How to Prevent Them

• Gear Material Selection Guide: Strength, Wear, Corrosion & Environment – How to Choose Correctly

• Backlash Is Not a Number: Understanding What Really Determines Accuracy, Stability, and System Life

• Spur, Helical and Worm Gears – Engineering Differences and How to Choose the Right One

• Backlash in Gears – From Geometry to System Behavior: Understanding what really happens between gear teeth
• Small Spur Gears: Why Miniaturization Creates Hidden Mechanical Problems

Not All Steel Gears Are Equal

When engineers compare gear materials, the discussion often starts with torque ratings, module size, or tooth count.

But in reality, one of the most critical factors that determines whether a gear survives – or fails – is the hardening philosophy behind it.

A gear can:

• wear out,
• crack,
• deform,
• suffer pitting,
• generate backlash,
• create vibration,
• or catastrophically lose teeth,

even when the geometry itself is perfectly designed.

This is exactly why modern high-performance gear systems rarely rely on untreated steel alone.

Instead, they rely on:

• Case Hardening,
• Induction Hardening,
• Through Hardening,
• and carefully selected alloy steels such as:
  • EN36 / 655M13
  • EN24 / 817M40
  • EN8 / 080M40

The choice between these materials and hardening methods dramatically affects:

• torque capability,
• fatigue life,
• shock resistance,
• wear behavior,
• noise,
• precision retention,
• and long-term gearbox reliability.

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What Is Case Hardening?

Case Hardening is a heat-treatment process where:

• The outer surface of the gear tooth becomes extremely hard
• While the inner core remains relatively tough and ductile

The result is a gear that combines:

• hard wear-resistant teeth
• with a shock-absorbing internal structure

This is ideal for gears because gear teeth experience:

• rolling contact stress
• sliding friction
• repeated cyclic loading
• impact during backlash transitions
• shock loads
• vibration

A fully hardened gear may become brittle.

A soft gear may wear rapidly.

Case hardening creates the best compromise between both worlds.

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Why Gear Teeth Need Hard Surfaces

The contact area between two gear teeth is surprisingly small.

Under high torque, the local contact stress becomes enormous.

Without sufficient surface hardness:

• teeth wear,
• tooth profile changes,
• backlash increases,
• efficiency drops,
• and pitting begins.

Hard surfaces improve:

• wear resistance
• pitting resistance
• rolling fatigue life
• dimensional stability over time

This becomes critical in:

• robotics
• servo systems
• aerospace
• military systems
• industrial automation
• heavy-duty transmissions

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Why The Core Must Remain Tough

Here is the engineering trick many people miss:

A gear tooth does not fail only at the surface.

Very often, failure starts at the tooth root.

If the entire gear becomes too hard:

• the tooth may crack,
• fracture,
• or fail under shock loading.

A tough internal core acts like a mechanical shock absorber.

This is why Case Hardened gears are so successful:

• hard outside,
• tough inside.

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The Three Major Gear Steel Families

EN8 – 080M40

EN8 (080M40) is a medium carbon steel commonly used for:

• simple gears
• shafts
• low-cost power transmission
• moderate-duty applications

Advantages:

• inexpensive
• easy machining
• widely available
• good general-purpose strength

Disadvantages:

• not optimized for carburizing
• limited case-hardening performance
• lower fatigue resistance
• higher distortion risk after hardening
• lower torque density

EN8 is often acceptable for:

• prototypes
• light industrial systems
• low-speed drives
• non-critical gear trains

But once torque, duty cycle, or fatigue life become important, engineers usually move away from EN8.

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EN36 – 655M13

EN36 (655M13) is a dedicated carburizing steel.

This is where professional gear engineering starts.

It was specifically designed for:

• Case Hardening
• deep carburized surfaces
• high fatigue resistance
• shock-loaded gears

Advantages:

• extremely hard surface capability
• excellent fatigue resistance
• excellent tooth-root toughness
• very high wear resistance
• ideal for precision gears
• ideal for heavy-duty gearboxes

This is why EN36 is widely used in:

• aerospace gearing
• military drivetrains
• servo gearboxes
• high-load industrial reducers
• automotive transmissions

When a manufacturer says:

“We only harden EN36 and not EN8”

that usually means:

• they want predictable metallurgy,
• reliable carburizing,
• lower failure risk,
• and consistent long-term durability.

This is generally a sign of a serious gear manufacturer.

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EN24 – 817M40

EN24 (817M40) is a high-strength nickel chromium molybdenum steel.

It is often considered the premium step above EN8.

EN24 is famous for:

• high tensile strength
• high toughness
• shock resistance
• shaft applications
• aerospace mechanical systems

Unlike EN36:

• EN24 is more commonly induction hardened
• not traditionally deep carburized

Advantages:

• very high core strength
• excellent shock capability
• superior mechanical strength
• better through-section properties

This makes EN24 ideal for:

• shafts
• pinions
• heavy shock loading
• high torsional loads

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Case Hardening vs Induction Hardening

Case Hardening (Carburizing)

Best for:

• deep wear layer
• maximum fatigue life
• precision gears
• high-cycle systems

Characteristics:

• deep hardened layer
• excellent surface hardness
• tough internal core
• highest gear durability

Usually paired with:

• EN36 / 655M13

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Induction Hardening

Best for:

• localized hardening
• lower processing cost
• selective tooth treatment
• faster production

Characteristics:

• shallower hardening depth
• fast process
• good torque improvement
• less ideal for extreme fatigue life

Often paired with:

• EN24 / 817M40

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Real Engineering Example – Why Hardening Matters

The following torque table demonstrates something extremely important.

For the same 50-tooth gear running at 1000 RPM:

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Engineering Analysis Of The Table

The key observation:

Case Hardened EN36 nearly triples torque capability

From:

• 70 Nm
  to:
• 195 Nm

That is a massive improvement.

Why?

Because:

• surface hardness drastically improves contact stress resistance
• tooth wear decreases
• pitting resistance increases
• fatigue life improves
• tooth geometry remains stable longer

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Why EN36 Outperforms EN24 Here

Many engineers assume:

“Stronger steel automatically means stronger gear”

But gear performance is more complicated.

EN24 may have higher bulk strength,
yet EN36 carburized gears often outperform it in real gear contact applications because:

• the carburized surface is deeper,
• the tooth surface hardness is optimized,
• and the fatigue behavior is superior.

This is an extremely important distinction.

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Case Study – Module 2 Gear Engineering

Let us examine a practical Module 2 gear.

Module 2 gears are common in:

• robotics
• automation
• servo systems
• industrial drives
• motion-control platforms

Suppose:

• 50 teeth
• 1000 RPM
• continuous industrial operation

The engineer must decide:

Option A – EN8

Cheaper.
Easy to machine.
Lower durability.

Suitable for:

• light duty
• intermittent operation
• low-cost systems

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Option B – EN24 Induction Hardened

Better toughness.
Better shock capability.
Moderate torque increase.

Suitable for:

• shafts and pinions
• medium-heavy systems
• dynamic loading

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Option C – EN36 Case Hardened

Highest long-term gear durability.
Best surface fatigue behavior.
Best wear resistance.

Suitable for:

• heavy-duty gearboxes
• high-cycle systems
• precision reducers
• aerospace/military systems
• servo applications with minimal backlash growth

This is why many premium gear manufacturers standardize on:

• EN36 + Case Hardening

rather than trying to harden EN8.

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The Hidden Cost Of Cheap Gear Material

Cheap gears are not always cheap.

A lower-grade material may create:

• backlash growth
• servo instability
• noise
• resonance
• maintenance
• lubrication problems
• tooth wear
• unexpected downtime

In high-performance systems, the cost of failure is usually far higher than the cost difference between steels.

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Final Engineering Takeaway

The real goal of gear engineering is not simply:

“maximum hardness”

The goal is:

• controlled hardness,
• controlled toughness,
• controlled fatigue behavior,
• and predictable long-term mechanical performance.

That is why:

• EN36 / 655M13 dominates carburized gears
• EN24 / 817M40 dominates high-strength shafts and induction-hardened systems
• EN8 / 080M40 remains mostly for economical moderate-duty applications

And this is exactly why serious gearbox manufacturers carefully choose both:

• the steel,
• and the hardening philosophy together.