Lets Talk About Gears: A Basic Industrial Component Explained Simply

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  Key takeaways
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  Lets Talk About Gears: The Two Numbers That Define Everything
• 3
  Pressure Angle, Addendum and Why Tooth Shape Matters
• 4
  Backlash, Center Distance and the Tolerance Stack
• 5
  Materials, Heat Treatment and Machining Sequence
• 6
  When a Gear Is the Wrong Answer
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  Step by Step: From Gear Drawing to Finished Part
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  Gear Type Comparison for Common Drive Needs

Lets talk about gears starting from the two numbers you need before anything else: tooth count and module. Module is the pitch diameter in millimeters divided by the number of teeth. A 40-tooth gear with a 2 mm module has an 80 mm pitch diameter. Two gears only mesh if they share the same module, so the first thing we check on any drawing is that both halves of a pair match.

American drawings often use diametral pitch instead, which is tooth count divided by pitch diameter in inches. Module and DP are reciprocals of each other, so a 2 mm module equals 12.7 DP. Mixing the two systems is the single most common mistake we see on incoming drawings, and it always shows up as a center distance that will not close.

Once tooth count and module are fixed, center distance is simple: add the two pitch diameters and divide by two. A 40-tooth gear at module 2 and a 20-tooth gear at module 2 need a center distance of 60 mm. If your drawing says 58 mm, something is wrong before any metal gets cut.

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  Module (metric)Pitch diameter in mm ÷ tooth count
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  Diametral pitch (inch)Tooth count ÷ pitch diameter in inches
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  Center distance(Pitch dia. 1 + pitch dia. 2) ÷ 2

The pressure angle sets how steeply the tooth flank pushes against its partner. A 20° pressure angle is the industrial default because it carries more load with less undercut at the root. A 14.5° angle runs smoother and quieter but the teeth are weaker, so it mostly appears in light instrument drives and old machinery.

The addendum is the height of the tooth above the pitch circle, normally one module on a standard gear. Addendum and dedendum together give the whole depth. If you shorten the addendum, you get a stubbier tooth that resists bending better but reduces contact ratio, which makes the drive noisier and less smooth at speed.

Undercutting is the trap. Below a certain tooth count, the cutting tool removes material from the root and weakens the gear. With a 20° pressure angle the practical minimum is around 17 teeth. Go below that and you either accept the undercut or add profile shift, which moves the tooth outward to protect the root.

For most CNC-machined gears we are not cutting a generated involute at all. We mill the profile from a CAD model and then inspect it on a coordinate measuring machine. That approach works well down to module 1 or so, and it lets you build corrected profiles that a standard hob cannot produce.

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  20° pressure angleStandard for industrial drives; minimum ~17 teeth without undercut
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  14.5° pressure angleQuieter, weaker, used in light instrument drives
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  Full-depth toothAddendum equals one module; the usual default
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  Stub toothShorter addendum, stronger root, lower contact ratio

Backlash is the play between meshing teeth. Without it, two gears will bind as they heat up and the drive will seize. For general industrial drives we aim for 0.05–0.15 mm of circumferential backlash, and for a positioning axis we tighten that to 0.02–0.05 mm. Below 0.02 mm you need a controlled environment and matched pairs.

Backlash is set by center distance, tooth thickness or both. Cutting the tooth slightly thinner is the easier route on a machined gear because you can adjust the CAD model without touching the housing. Moving the center distance is cheaper on paper but it changes the contact pattern and the pressure angle at the mesh.

Tolerance stack matters here. If the housing bore is ±0.02 mm and the gear bore is ±0.01 mm, the actual center distance can wander by 0.06 mm across a batch. That is already the whole backlash budget. This is why we ask for the housing drawing as well as the gear drawing before quoting a gear set.

Thermal growth is the other half. A steel gear running at 80 °C grows about 0.001 mm per 10 mm of diameter for every 10 °C rise. On a 100 mm gear that is roughly 0.07 mm, which eats most of a tight backlash allowance. Hot drives need more clearance, not less.

Material choice follows the load and the speed. Acetal and nylon gears run quietly and need no lubrication, but they carry light loads and creep under sustained torque. 1045 and 4140 steel cover most industrial drives. Bronze is the usual worm wheel material because it slides well against a hardened steel worm.

Heat treatment is where gear machining gets its sequence right or wrong. If you case harden a gear after cutting the teeth, the part grows and distorts, so the teeth must be ground afterward. That means leaving 0.2–0.4 mm of stock on the flanks. If the gear is only through-hardened, the distortion is smaller but still enough to matter on a tight backlash callout.

For softer materials we cut to final size and stop. 6061-T6 aluminum gears work well in low-load automation and prototype builds, and they machine fast. They wear quickly against steel, so pair aluminum with aluminum, plastic or a bronze bushing rather than a hardened steel pinion.

The machining sequence we use is simple: turn the blank and bore, cut the teeth, then finish the bore and any keyway in the same setup where possible. Cutting teeth before the final bore keeps the tooth profile concentric with the shaft, which matters more than most drawings admit.

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  Acetal / nylonLight load, quiet, no lubrication, watch creep
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  1045 / 4140 steelGeneral industrial drives, through-hardened or case hardened
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  BronzeStandard worm wheel material against a hardened steel worm
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  6061-T6 aluminumPrototypes and light automation; never pair with hardened steel

A gear pair is not always the right transmission. If the two shafts are far apart, a belt or chain is cheaper and absorbs shock. If you need a fixed ratio at high speed with low noise, a helical gear beats a spur gear because more teeth share the load at any instant.

If the load is light and the ratio is high, a worm drive gives a large reduction in one stage, but it loses efficiency to sliding friction. Expect 40–90% efficiency depending on lead angle, and plan for heat. A planetary set is better when you need high torque in a small envelope, at the cost of more parts and tighter tolerances.

For a one-off or a prototype, cutting a custom gear profile on a 5-axis machine is often faster than ordering a hob and a gear cutter. For a run of 10,000 parts, the economics flip and a generated profile with a dedicated cutter wins. That decision is worth making before the design is frozen, not after.

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  Far shaft spacingUse a belt or chain instead of a gear train
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  High speed, low noiseHelical gears share load across more teeth
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  High ratio, one stageWorm drive, but plan for efficiency loss and heat
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  High torque, small spacePlanetary set, at the cost of part count and tolerance