Gear Module & Pressure Angle Explained: The Complete Guide to Gear Specifications

A few months ago, a maintenance manager called me about a machine that had been down for three days. He’d ordered replacement gears — same tooth count, same outer diameter, same material. When the parts arrived, they wouldn’t mesh. The gears locked up and groaned at him like a wounded animal.

Turns out, the old gears used a 14.5° pressure angle. The new ones were 20°. They looked nearly identical. You’d need a microscope and a protractor to spot the difference. But in gear design, “nearly” doesn’t count. Those few degrees meant thousands in downtime and rush shipping.

I’ve seen this happen more times than I can count. Engineers and buyers alike assume that if a gear looks right, it is right. Gears are deceptive that way — small differences in specs that your eye can’t detect can mean the difference between smooth transmission and a smoking motor.

So here are 10 gear specs you absolutely must check before you place that next purchase order. Each one alone could save you a costly mistake — and an awkward conversation with your production manager.

1. Module (m) — The Foundation of Gear Size

If you only check one thing, make it the module. Think of module as the gear’s “scale factor” — it determines the size of each tooth, and by extension, the overall dimensions of the gear.

The formula is simple: m = Pitch Diameter ÷ Number of Teeth. A gear with a 20 mm pitch diameter and exactly 20 teeth has a module of 1. That’s why you’ll hear engineers say “module 1 gear” — it’s shorthand for the entire tooth geometry.

Standard modules run from 0.2 all the way up to 6 and beyond. The most common in precision machinery are 0.5, 1, 1.5, and 2. A module 0.5 gear has teeth half the size of a module 1 gear — even if both have exactly the same number of teeth.

Here’s a quick comparison for a 20-tooth gear to make this concrete:

Notice the pattern: the outside diameter is always two modules larger than the pitch diameter. That’s because the addendum (the tooth height above the pitch circle) equals one module, and you have teeth on both sides.

Rule of thumb for measuring an unknown gear: Measure the outside diameter with a caliper, count the teeth (N), then calculate: approximate module = OD ÷ (N + 2). Round to the nearest standard module value. If your calculation gives you m = 1.08, it’s almost certainly a module 1 gear, not a custom size.

We stock a full range of standard spur gears with common modules from 0.5 to 3, in both 1045 carbon steel and 304 stainless steel.

2. Diametral Pitch (DP) — The Inch-System Cousin

If you work with imported equipment, especially from the US or UK, you’ll run into Diametral Pitch (DP) instead of module. DP is the imperial equivalent — it tells you how many teeth exist per inch of pitch diameter.

DP = Number of Teeth ÷ Pitch Diameter (in inches)

The relationship between the two systems is straightforward: m = 25.4 ÷ DP. A DP 20 gear equals roughly module 1.27 — but never assume a direct swap. Always confirm the standard.

Common DP values include 16, 20, 24, 32, and 48. The higher the DP number, the finer the teeth. A DP 48 gear has very small teeth, typical in precision instruments and small automation.

Gotcha: A module 1.25 gear and a DP 20 gear are close in size but not identical. Close enough to almost fit — and almost fitting is how teeth get damaged.

3. Pressure Angle — The Tooth Shape Nobody Sees

Pressure angle is the spec that trips up the most people. It’s the angle of the tooth face relative to the gear’s rotation direction, and it’s invisible to the naked eye on a small gear.

Three standard pressure angles exist:

• 14.5° — An older standard. Softer tooth roots, lower load capacity. Found in legacy equipment and some imported machinery.
• 20° — The modern standard. Stronger tooth roots, higher load capacity. Used in virtually all new gear designs today.
• 25° — Heavy-duty applications. Even stronger, but also higher sliding friction. Less common in standard catalog parts.

What happens if you mix them? A 20° gear trying to mesh with a 14.5° gear will experience tooth interference — the tips of one dig into the root of the other. The result is noise, vibration, accelerated wear, and eventually broken teeth. I once watched a test rig destroy a set of module 2 gears in under 30 minutes because of this mismatch.

How to identify pressure angle without a spec sheet: Use a gear tooth caliper or a simple protractor with a known reference. Better yet, compare against a known sample — most gear shops keep reference gears of both common pressure angles. If the gear came from a machine built after 1980, it’s almost certainly 20°. Pre-1960s equipment may still use 14.5°.

All our standard spur gears use a 20° pressure angle, which covers the vast majority of modern applications.

4. Tooth Count — Strategic Choices Beyond the Ratio

Everyone knows tooth count determines the gear ratio. But there’s a hidden consideration: the minimum number of teeth to avoid undercutting.

For a 20° pressure angle gear, the minimum recommended tooth count is 18. Below that, you start getting undercut at the tooth root — a weakened section that’s prone to breakage. For 14.5° gears, the minimum is even higher, around 32 teeth.

If you need a ratio that forces fewer teeth, consider profile shifting (also called “addendum modification”). It’s a common technique to strengthen low-tooth-count gears without changing the module or center distance.

One more thing: For smooth operation, avoid gear pairs where both tooth counts share a common factor larger than 1. For example, a 20-tooth driving a 40-tooth gear means the same teeth always contact each other. A 21-tooth driving a 40-tooth distributes wear more evenly over time.

5. Material — Steel, Stainless, Brass, or Plastic?

Gear material is a trade-off between strength, wear resistance, corrosion resistance, noise, and cost.

Carbon steel (1045 or S45C) is the workhorse. It’s strong, affordable, and can be surface-hardened or black-oxided for extra wear life. Most standard spur gears come in this material.

Stainless steel (304 or 316) is your pick for food processing, medical devices, or any environment where corrosion is a concern. It won’t rust, but it’s more expensive and can be trickier to machine precisely.

Brass and bronze gears run quietly and resist corrosion. They’re often used in light-duty applications where noise matters — think printers, small instruments, or decorative mechanisms.

Plastic (POM, nylon, PEEK) gears are lightweight and quiet. They don’t need lubrication and work well in low-load, high-speed applications. PEEK can even handle sterilisation temperatures, making it popular in medical equipment.

A common mistake: mixing materials in a gear pair. A steel gear running against a steel gear is fine — they wear at similar rates. But a plastic gear running against a stainless gear can cause accelerated plastic wear because the hard stainless surface acts like a file against the softer plastic tooth. If you use plastic, pair it with the same plastic or a compatible material.

My take: If you’re unsure, carbon steel with black oxide is the safest default. It handles most industrial environments well, it keeps costs under control, and it’s available off the shelf in most standard sizes.

6. Precision Grade — AGMA, JIS, and DIN Demystified

Two gears can have identical module, tooth count, and material — and one can cost twice as much. The difference is precision grade.

Gear precision is classified by standards bodies:

A higher AGMA number (like Q12) means tighter tolerances, smoother running, and less noise. Lower numbers (like Q7) are fine for slow-speed applications like hand-cranked mechanisms.

For most industrial automation, AGMA Q8–Q10 (or JIS 4–6 / DIN 7–9) is sufficient. That covers conveyors, packaging equipment, and general-purpose machine tools. For robotics, semiconductor equipment, or high-speed spindles, you’ll want Q12 or better.

What does “Q8” actually mean in practice? It means the gear’s tooth-to-tooth spacing error is held within roughly 10–15 microns for a module 1 gear. That’s about one-fifth the thickness of a human hair. Q12 cuts that error in half — and roughly doubles the machining cost.

Practical tip: Don’t overspecify precision. Specifying AGMA Q12 when Q9 would do adds cost without real benefit — sometimes 40–60% more. Match the grade to the actual speed and load requirements. A hand-cranked adjustment mechanism doesn’t need aerospace-grade tolerances.

7. Backlash — Why a Little Slack Is a Good Thing

Backlash is the intentional gap between mating teeth. Without it, gears would bind as they heat up and expand during operation.

Too little backlash → overheating, noise, premature wear, and possible seizure.
Too much backlash → positioning errors, vibration, and impact loading on teeth.

Standard backlash for module 1 gears is roughly 0.05 to 0.15 mm for commercial grades. Finer modules get proportionally tighter clearance — a module 0.5 gear might have only 0.02–0.08 mm of backlash. Your application dictates the sweet spot:

• Power transmission — Moderate backlash (0.1–0.2 mm for m1). Minimises binding without sacrificing efficiency. Most catalog gears fall into this category.
• Positioning/indexing — Minimal backlash. Consider anti-backlash gears or spring-loaded split gears if you’re building a rotary table or a tool changer.
• High-speed operation — Extra backlash to accommodate thermal expansion. Gears heat up during running, and steel expands. Without enough clearance, they’ll seize.

How to measure backlash: Mount a dial indicator against a tooth flank of the driven gear, lock the driving gear, and rock the driven gear gently. The reading is your backlash. A feeler gauge between teeth can give you a rough estimate, but a dial indicator is far more reliable.

8. Center Distance — The Geometry You Can’t Ignore

Center distance is the distance between the centers of two mating gears. If you get this wrong, the gears won’t mesh correctly — even if every other spec is right.

The formula for standard (unshifted) gears is:
C = m × (N₁ + N₂) ÷ 2

For example, a module 1 gear with 20 teeth driving a 40-tooth gear:
C = 1 × (20 + 40) ÷ 2 = 30 mm

Machine housings are designed around specific center distances. When replacing gears, always measure the existing center distance to confirm your new pair will fit. A caliper measurement between shaft centers (plus half each shaft diameter) is quick and reliable.

If you need to adjust center distance slightly, profile-shifted gears can help — but that’s a topic for another article.

9. Bore Size and Hub — How It Attaches to the Shaft

This sounds obvious, but I’ve seen engineers nail every gear spec only to realise their bore is 8 mm and their shaft is 10 mm. Or worse — the gear has a keyway but the shaft uses a set screw, or vice versa.

Common configurations include:

• Straight bore — Simple round hole. Secured with a set screw through the hub.
• Bore + tap — Threaded hole in the hub for a set screw without needing a nut.
• Keyway — A slot for a parallel key. Handles higher torque than a set screw alone.
• Keyway + tap — Both. Best for high-torque applications that also need axial positioning.

Most standard spur gears, including ours at GUNRI, offer all four configurations so you can match the existing shaft setup.

10. Surface Treatment — Black Oxide, Electroless Nickel, or None?

Surface treatment is the last spec on the list, but it directly affects gear lifespan.

Black oxide is a thin conversion coating that provides mild corrosion resistance and reduces glare. It’s cost-effective and doesn’t affect dimensional tolerances. Most carbon steel gears come with black oxide as standard.

Electroless nickel plating offers much better corrosion resistance and a hard, uniform surface. It’s a good choice for food-grade or wet environments — but it adds cost and can affect tolerances on very fine-pitch gears.

No treatment (as-machined) works fine in dry, controlled environments, especially with stainless steel gears that don’t need protection. Many plastic gears also run without treatment.

My rule: Carbon steel in a normal factory → black oxide. Stainless steel or plastic → no treatment needed. Wet or corrosive environment → electroless nickel or upgrade to stainless.

Bonus: Profile Shift — The Trick Engineers Use to Fix Tight Spots

Here’s something most gear guides don’t cover. Profile shift (also called “addendum modification” or “X-factor”) is a technique where the gear cutter is offset from centre during hobbing, changing the tooth shape without changing the module.

Why would you do this? Three common reasons:

• Avoid undercutting on low-tooth-count gears. A 12-tooth gear will typically have undercut roots. A positive profile shift (+0.3 to +0.5 module) pushes the cutter outward, strengthening the root.
• Adjust center distance. If your shaft centres are fixed and your calculated center distance is slightly off, profile-shifted gears can fudge the difference without changing the module.
• Balance strength between pinion and gear. The smaller gear (pinion) wears faster. A positive shift on the pinion and a negative shift on the gear can balance their service lives.

Profile shift is marked with an “X” value on engineering drawings. X = 0 means standard teeth. X = +0.5 means the teeth are 0.5 × module thicker at the root. Most standard catalog gears have X = 0 unless otherwise specified.

If you’re ordering custom gears and your application involves fewer than 18 teeth or a non-standard center distance, ask about profile shift. Your gear supplier should be able to calculate the optimum X value for your operating conditions.

Quick Reference: 3 Steps to Spec a Gear Correctly

Before you place your next gear order, run through this checklist:

1. Confirm the module or DP. Measure the outside diameter and count the teeth. If in doubt, use a gear pitch gauge.
2. Identify the pressure angle. Check the existing gear’s documentation, or compare against known 14.5° and 20° samples. A gear shop can confirm this in seconds.
3. Match mounting, material, and grade. Bore size, hub style, precision grade, and surface treatment should match the application — not necessarily the cheapest option.

Getting a gear spec wrong can cost days of downtime and thousands in expedited shipping. But with these 10 specs checked off, you’ll know exactly what you need — no guesswork, no returns, no headaches.

Need standard spur gears in a hurry? Browse our range of Spur Gears — Pressure Angle 20° with modules from 0.5 to 3, available in carbon steel and stainless steel.