Gear surface hardness is the single most useful number for predicting how long a gearbox will last in a dirty, heavily loaded plant. Two units can be identical in ratio, centre distance and rated torque on paper and still differ by a factor of two in service life, because one has a 60 HRC carburised flank and the other a 300 HB through-hardened one. Hardness governs pitting resistance, wear rate and the permissible contact stress - and it does so before lubrication quality or installation care get a chance to matter.

This article explains what the hardness numbers mean in practice, how the three common hardening routes differ, why case depth is the figure buyers forget to ask for, and what evidence should accompany a quotation.

Why Hardness Decides Life Before Any Other Number

A gear tooth carries load through a contact patch a few millimetres wide. The stress underneath that patch is not uniform: it peaks a short distance below the surface, typically at a depth of a few tenths of a millimetre, and falls away steeply either side. That peak is where the first fatigue crack starts, and it is exactly the region that surface hardening is designed to strengthen.

Contact stress in a gear mesh scales with the square root of the transmitted torque, so a 20 percent overload raises the subsurface stress by roughly 10 percent - which sounds tolerable until you remember that fatigue life falls off somewhere between the sixth and the tenth power of stress in the high-cycle regime. A small stress increase consumes a large share of the available cycles. Surface hardness raises the stress the flank can tolerate, which is why the hardness specification effectively sets the ceiling on the whole unit's capacity.

There is a second, quieter mechanism. Hardness also resists plastic flow. Where a soft flank meets a hard one, the soft surface deforms, the contact patch widens, and the geometry that the designer calculated stops being the geometry in the box.

The Three Hardening Routes Used in Gearboxes

Most industrial gear units use one of three approaches, and each carries a different balance of hardness, distortion and cost.

Carburising diffuses carbon into the surface at around 900 degrees Celsius and then quenches, producing a hard case over a tough core. Because the quench moves the part, teeth are normally ground after treatment, which restores profile accuracy and removes the decarburised skin. Nitriding introduces nitrogen at a much lower temperature (around 500-560 degrees Celsius) with no quench at all, so the part keeps its shape; the trade-off is a thinner case and a somewhat lower achievable hardness. Through-hardened gears are homogeneous: cheaper, easier to machine, and adequate for moderate loads where weight and size are not critical.

Worm wheels deserve a note of their own. The usual pairing is a hardened and ground steel worm running against a bronze wheel, because bronze cannot be hardened to resist pitting in the same way. The wear mechanism there is different, and the hardness that matters most is the worm's.

Case Depth: The Number Buyers Never Ask For

Surface hardness alone is not enough. A very hard but very thin case behaves like a hard shell on a soft support: the subsurface peak stress lands in the soft core, the case cracks from below, and the flank spalls in a way that looks dramatic and is often misdiagnosed as a material defect.

The practical rule used across the industry is that effective case depth should be roughly 0.1 to 0.2 times the normal module, measured to the 550 HV limit. For a module of 6 mm that means something around 0.9 to 1.2 mm; for a module of 10 mm, 1.5 to 2.0 mm. Too little case and the core yields under the contact stress; too much case and the tooth becomes brittle at the root, where it needs to bend rather than resist indentation.

There is a second rule that runs in parallel. The core must be hard enough to support the case. A carburised tooth with a 25 HRC core will bend further under load than an identical tooth with a 35 HRC core, and that extra deflection changes the load distribution across the face width. When a quotation states "case hardened gears" without numbers, both of these values are unknown, and the guarantee is effectively about appearance rather than performance.

Pitting, Micropitting and Why the Flank Is the Battleground

Pitting is fatigue of the flank surface. It begins as a cluster of small craters on the dedendum of the driving gear, where sliding velocity is highest and the oil film is thinnest, and it progresses until the contact area is reduced enough that the remaining surface overloads and the process accelerates.

Hardness raises pitting resistance in two ways: it increases the allowable contact stress directly, and it preserves the surface finish for longer. A rough flank peaks its way through the oil film at every revolution, so a hard, finely ground surface keeps the film intact where a softer, rougher one breaks through.

Micropitting is a subtler cousin. It appears as a grey, frosted patch rather than discrete craters, and it is driven by local asperity contact under mixed-film conditions. It is common in carburised gears running on mineral oil at moderate speeds, and it is more sensitive to surface finish and lubricant additive chemistry than to bulk hardness. A unit can pass a pitting calculation comfortably and still develop micropitting, which is one reason the lubricant specification belongs in the same conversation as the hardness specification.

Hardness, Core Toughness and the Compromise at the Root

Bending fatigue at the tooth root and pitting on the flank pull in opposite directions. A harder case improves flank capacity but reduces the toughness available to absorb root strain; a tougher core improves bending behaviour but may not support a thin case.

The conventional answer is a gradient: a hard case of the right depth over a core of 30-40 HRC, with a smooth transition zone rather than an abrupt change. Residual compressive stress in the case, a by-product of carburising and grinding done correctly, adds further resistance to crack initiation and is one reason a properly treated gear tolerates stress cycles that a through-hardened gear of similar bulk hardness cannot.

Grinding deserves a mention here because it can undo the benefit. Excessive grinding heat produces untempered martensite in a thin surface layer - a hard, brittle skin prone to cracking. It is invisible in a hardness test taken at the wrong depth and appears later as a network of fine cracks on the flank. This is why gear grinding parameters, not just the furnace cycle, belong in a supplier's process control.

How Running-In and Lubrication Interact with Hardness

A new gear set runs in during its first tens of hours. Asperity peaks are smoothed, the contact pattern spreads across the face, and the residual stress state settles. A hard, well-ground set runs in quickly and predictably; a rough set runs in by wearing material away, which contaminates the oil and loads the filter.

Three practical consequences follow for the plant, not the factory:

• Change the oil after the first running-in period as the supplier instructs, because the initial fill carries the wear debris from that process.
• Hold the oil at the specified viscosity for the actual sump temperature. Hardness protects against contact stress; it does not compensate for a film that is too thin at operating temperature.
• Keep water out. Moisture attacks the case boundary, promotes hydrogen-assisted cracking in hardened steels and turns a pitting problem into a spalling problem within a season.

Where a gearbox runs at very low output speed, the oil film is renewed slowly and the number of stress cycles per hour is small; the limiting factor shifts from fatigue to wear and to the possibility of standstill corrosion on the flanks. In that regime, hardness still matters, but the breather, the oil level and the storage conditions matter just as much.

What Hardness Data Should Come with a Quotation

A quotation that says only "hardened and ground gears" leaves the buyer unable to compare offers. Five items turn the statement into evidence:

1. Surface hardness range and the method used to measure it - HRC on the flank, or HV converted, with the test location described.
2. Effective case depth, quoted to the 550 HV limit, along with the normal module it was calculated against.
3. Core hardness range.
4. Material grade for gear and pinion, and whether pinion and wheel use the same grade or a deliberate pair.
5. The finishing operation - ground after heat treatment, or cut after nitriding - and the achievable profile accuracy.

Ask for these on the first enquiry, and suppliers who actually control their heat treatment will answer without hesitation. For a unit that will run continuously for two decades, such as a heavy right-angle drive like the MC3RLSF06 bevel-helical gear unit, this data is the difference between a specification and a wish. Smaller helical-worm units follow the same logic at a different scale - the S67 helical-worm gearbox lives or dies by its worm hardness and wheel bronze quality rather than by its case depth.

If you would rather check a duty point against the catalogue before writing the enquiry, the online selection tool returns a frame, ratio and service factor in a few steps; the hardness questions above then go into the message that follows.

FAQ

Is a harder gear always a longer-lasting gear?

No. Beyond a point, extra hardness without matching case depth and core toughness moves the failure mode from pitting to case cracking or root fracture. The objective is a matched gradient, not a maximum number.

What surface hardness should I specify for a general industrial gearbox?

For carburised and ground helical or bevel gears, 58-62 HRC on the flank is the normal band, with a core of 30-40 HRC. Nitrided gears typically reach 550-700 HV. Through-hardened gears at 250-330 HB are adequate for light and moderate duties.

Does hardness change how I run the gearbox in?

It shortens the running-in period for well-ground gears, but it does not remove the need to change the initial oil charge on schedule. Hardened surfaces generate less wear debris, so a clean first oil change is both easier and more important to document.

Can I check hardness on a gearbox already in service?

Yes, with portable rebound or ultrasonic testers, but readings are affected by surface condition and curvature, so they are best used as a trend rather than an absolute verdict. Destructive testing of a sacrificial tooth is the only reliable way to confirm case depth.

Does a higher hardness allow a smaller gearbox for the same torque?

Often yes on flank capacity, which is why modern units are compact. The limit is usually bending at the root and thermal capacity of the housing, so the reduction in size is real but bounded - a smaller case with a very hard gear set still has to shed the same heat.