Why is acetal the best material for plastic gears?

Acetal (POM) combines the properties most critical for gear performance: high stiffness (3,100 MPa modulus) for accurate tooth geometry under load, natural lubricity (CoF 0.20–0.35 vs steel) for dry running, excellent dimensional stability (< 0.9% moisture absorption) for consistent tooth mesh clearance, and good fatigue resistance for cyclic tooth root stress. No other commodity engineering plastic matches this combination at POM's price.

What module should I use for an acetal plastic gear?

For light-duty acetal gears (hand tools, instruments), Module 0.5–1.0 (Diametral Pitch 50–25). For general-purpose small drives (consumer electronics, small actuators), Module 1.0–2.0 (DP 25–12). For medium-duty industrial drives, Module 2.0–4.0 (DP 12–6). Avoid very fine pitches (M < 0.5) in acetal if the gear will be CNC machined — tooth geometry becomes difficult to hold and tool deflection causes tooth form error.

Can acetal gears run against steel gears?

Yes — POM-on-hardened-steel is the standard pairing for plastic/metal gear pairs. The harder steel pinion drives the plastic gear. Key requirements: steel must be hardened (HRC 45+) and the tooth surface ground or honed to Ra 0.8–1.6 µm (32–63 microinches). Rough steel surfaces accelerate POM wear rapidly. Plastic-on-plastic pairs are also common in low-load, low-speed drives where noise reduction is important.

What is the maximum torque an acetal gear can handle?

Torque capacity depends on gear geometry (module, face width, pitch diameter) and material allowable bending stress. For a Module 2, 20-tooth acetal gear with 12 mm face width, allowable torque at the tooth root is approximately 0.5–1.5 N·m depending on speed and duty cycle. Use the Lewis bending equation with POM allowable bending stress of 20–30 MPa for design calculations. Never size a plastic gear by intuition — run the calculation.

Do acetal gears need lubrication?

Unfilled acetal gears can run dry for low-to-medium duty applications (PV < 0.08 MPa·m/s at the pitch cylinder). Grease lubrication is recommended for gears running at higher PV or elevated temperatures — it reduces CoF from 0.20–0.35 to approximately 0.05–0.10, dramatically extending tooth life. PTFE-filled acetal (Delrin AF) gears can run dry at higher PV than unfilled POM. Avoid oil bath lubrication unless the gear housing is sealed — oil contamination of plastic gear teeth accelerates softening.

Application Design Guide · 12 min read

Acetal Plastic Gears: Design Rules and Material Guide

Q: What module should I use for an acetal plastic gear?

For light-duty acetal gears (hand tools, instruments), Module 0.5–1.0 (Diametral Pitch 50–25). For general-purpose small drives (consumer electronics, small actuators), Module 1.0–2.0 (DP 25–12). For medium-duty industrial drives, Module 2.0–4.0 (DP 12–6). Avoid very fine pitches (M < 0.5) in acetal if the gear will be CNC machined — tooth geometry becomes difficult to hold and tool deflection causes tooth form error.

Q: Can acetal gears run against steel gears?

Yes — POM-on-hardened-steel is the standard pairing for plastic/metal gear pairs. The harder steel pinion drives the plastic gear. Key requirements: steel must be hardened (HRC 45+) and the tooth surface ground or honed to Ra 0.8–1.6 µm (32–63 microinches). Rough steel surfaces accelerate POM wear rapidly. Plastic-on-plastic pairs are also common in low-load, low-speed drives where noise reduction is important.

Q: What is the maximum torque an acetal gear can handle?

Torque capacity depends on gear geometry (module, face width, pitch diameter) and material allowable bending stress. For a Module 2, 20-tooth acetal gear with 12 mm face width, allowable torque at the tooth root is approximately 0.5–1.5 N·m depending on speed and duty cycle. Use the Lewis bending equation with POM allowable bending stress of 20–30 MPa for design calculations. Never size a plastic gear by intuition — run the calculation.

Q: Do acetal gears need lubrication?

Unfilled acetal gears can run dry for low-to-medium duty applications (PV < 0.08 MPa·m/s at the pitch cylinder). Grease lubrication is recommended for gears running at higher PV or elevated temperatures — it reduces CoF from 0.20–0.35 to approximately 0.05–0.10, dramatically extending tooth life. PTFE-filled acetal (Delrin AF) gears can run dry at higher PV than unfilled POM. Avoid oil bath lubrication unless the gear housing is sealed — oil contamination of plastic gear teeth accelerates softening.

Acetal is the default material for precision plastic gears. Module selection, PV limits, tooth profile rules, and POM-on-steel pairing — all in one engineering reference.

Module Selection Guide Get a CNC Gear Quote

Acetal Gear Quick Reference

Unfilled POM — dry running

PV limit (dry, continuous)~0.08 MPa·m/s

Max pitch line velocity~3 m/s (600 ft/min)

Allowable bending stress20–30 MPa

Pressure angle20° full-depth preferred

Min tooth root fillet radius0.3× module

Updated Mar 2026

20–30 MPa

Allowable bending stress

3 m/s

Max pitch line velocity

0.08

PV limit (MPa·m/s dry)

20°

Preferred pressure angle

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Size the Gear First. Choose the Material Second.

The most common mistake in plastic gear design is selecting the module by intuition and then checking if acetal can handle it. The correct sequence: calculate required torque → determine pitch diameter and face width from Lewis bending equation → check pitch line velocity against PV limit → confirm acetal as the material. Only then should you consider PTFE-filled grades, nylon, or metal as alternatives if the initial sizing fails.

Section 1 of 5

Why Acetal Is the Default Plastic Gear Material

If you're designing a plastic gear and haven't chosen a material yet, acetal POM should be your default — here's why. Acetal outperforms all commodity engineering plastics in the specific combination of properties required for gear service.

Stiffness

Tensile modulus 3,100 MPa — 4.5× stiffer than UHMW and 1.9× stiffer than nylon PA6 at service humidity (conditioned, 50% RH). Stiff tooth geometry transmits load efficiently and resists tip deflection under load. Deflection changes effective pressure angle and increases sliding velocity — both reduce gear life.

Natural Lubricity

Dry CoF against steel 0.20–0.35 without added lubricant. POM develops a thin transfer film on the mating surface that further reduces friction over time. This allows dry-running gear trains in sealed enclosures where periodic lubrication is impractical.

Dimensional Stability

Less than 0.9% moisture absorption ensures tooth pitch and mesh center distance hold their designed values in variable-humidity environments. Nylon gears in humid environments can swell sufficiently to eliminate backlash — causing binding and overload.

White acetal spur gear meshed with a hardened steel pinion on parallel shafts on a clean machine shop bench.

Figure 1. Acetal is commonly paired with a hardened steel pinion when the plastic gear needs low friction, dimensional stability, and quiet running.

Section 2 of 5

Module Selection for Acetal Gears

Module (metric) or Diametral Pitch (imperial) selection drives tooth size, bending strength, and contact stress. Use this table as a starting point — always verify with the Lewis bending equation for your specific torque and speed.

Worked Example: Sizing a Module 2 Acetal Gear

Given: Module (m) = 2 mm, Number of teeth (N) = 20, Face width (b) = 12 mm, Pressure angle = 20°

Step 1 — Pitch diameter: d = m × N = 2 × 20 = 40 mm (1.575 in.)

Step 2 — Lewis form factor: Y ≈ 0.32 for 20 teeth at 20° pressure angle (from AGMA 218.01 tables)

Step 3 — Allowable tooth load:

W_t = σ × b × m × Y
W_t = 25 MPa × 12 mm × 2 mm × 0.32 = 192 N

Step 4 — Torque at pitch circle:

T = W_t × d / 2 = 192 × 0.020 m = 3.84 N·m (raw)

Step 5 — Apply safety factor and velocity derating: At 500 RPM, pitch line velocity = π × 0.040 × 500 / 60 = 1.05 m/s. Barth velocity factor K_v = 3.05 / (3.05 + 1.05) = 0.74. With 2× safety factor:

T_allowable = 3.84 × 0.74 / 2 ≈ 1.4 N·m

Result: A Module 2, 20-tooth, 12 mm face width acetal gear can safely transmit approximately 1.4 N·m at 500 RPM — enough for a small conveyor indexer or a light-duty rotary actuator. At lower speeds (100 RPM), the same gear handles ~1.9 N·m. Scale face width proportionally for higher torque requirements.

Three white acetal spur gears with visibly different tooth pitch arranged beside a steel shaft blank on a metal workbench.

Figure 2. Module selection changes tooth size, pitch diameter, and available tooth-root area before material strength is even checked.

Module (m)	Diametral Pitch	Tooth Height (approx.)	Typical Application	Acetal Manufacturing
M0.5	DP 50	1.1 mm	Instruments, watches, micro drives	Hobbing or injection molding preferred over CNC — tooth too fine for reliable end milling
M0.8	DP 32	1.8 mm	Small electronics, precision instruments	Gear hobbing; end milling marginal — requires 1.5 mm or smaller end mill
M1.0	DP 25	2.25 mm	Light-duty drives, consumer electronics	CNC gear milling feasible; hobbing preferred for production
M1.5	DP 17	3.4 mm	Small actuators, general-purpose light drives	CNC gear milling standard practice; good tolerance control
M2.0	DP 12	4.5 mm	General purpose — most common small plastic gear	CNC gear milling excellent; standard for prototype and low-volume production
M3.0	DP 8	6.75 mm	Medium-duty drives, small power transmission	CNC machining excellent; hobbing or form milling for production
M4.0	DP 6	9.0 mm	Higher-load drives; larger pitch diameters	CNC machining straightforward; confirm Lewis stress at design loads
M5.0+	DP 5 and below	11.25 mm+	Heavy-duty — consider metal at this module	CNC or hobbing; re-evaluate if metal gear is more appropriate at M5+

Section 3 of 5

Tooth Profile Design Rules for Acetal

If you apply metal gear profile rules to your acetal gear, the teeth will fail prematurely at the root fillet. Plastic gear tooth geometry must accommodate the lower modulus and fatigue resistance of POM versus metal. These rules reduce tooth root stress and improve service life.

Pressure Angle: 20° full-depth involute

20° PA is the correct default for acetal gears. 14.5° stub tooth has lower tooth height and is not recommended for plastic — it produces higher Hertzian contact stress. 25° PA (high-pressure angle) can be used for higher load but reduces contact ratio and increases noise.

Use 20° PA, full-depth

Tooth Root Fillet Radius: Minimum 0.3× module

A generous root fillet reduces stress concentration at the critical cross-section where bending failure initiates. For CNC machined gears, the root fillet radius is limited by the cutting tool tip radius. Specify minimum root radius explicitly — do not leave it to the machine shop default. For M2 gear: minimum root fillet 0.6 mm.

r_root ≥ 0.3 × module

Face Width: 8–12× module

Face width in this range balances load capacity against manufacturing tolerance requirements. Very wide gears (> 15× module) require precise shaft parallelism that is difficult to achieve in plastic housings — concentrated edge loading increases actual stress beyond Lewis calculation predictions.

b = 8–12 × module

Minimum Number of Teeth: 12–14

Below 12 teeth, undercutting of the tooth root becomes severe in standard involute geometry, reducing effective tooth strength. For acetal, 14 minimum teeth is a conservative starting point that avoids significant undercut. Stub-tooth or long-addendum corrections can allow fewer teeth if needed.

N_min = 14 teeth

Contact Ratio: ≥ 1.4

Contact ratio is the average number of tooth pairs in simultaneous contact. Higher contact ratio means load is shared across multiple teeth — reducing peak tooth stress. Full-depth involute gears with adequate tooth count typically achieve 1.5–1.7 contact ratio. Never design below 1.4 in plastic gears.

ε_α ≥ 1.4

Tip Relief: Required at medium-to-high speed

Under load, acetal gear teeth deflect. Without tip relief, the incoming tooth tip contacts the root of the mating tooth prematurely, causing impact loading and accelerated wear. Specify tip relief of approximately 0.3–0.5× tooth deflection under rated load, starting 5–10° before the end of approach.

Add tip relief above 1 m/s pitch line velocity

Figure 3. Tooth profile details such as root fillet radius and face width matter more in acetal than in metal because POM teeth deflect under load.

Section 4 of 5

PV Limits and Pitch Line Velocity

If you skip the PV check on your acetal gear design, thermal softening will destroy the tooth surface before mechanical wear becomes an issue. PV limit (Pressure × Velocity) at the pitch cylinder determines whether the gear will thermally fail before mechanical failure. Exceeding the PV limit causes tooth surface to soften and deform — the leading cause of acetal gear failure.

Acetal Grade	Max Pitch Line Velocity	Max PV (MPa·m/s)	Lubrication	Notes
Unfilled POM (Delrin 150)	3 m/s (600 ft/min)	~0.08 MPa·m/s	Dry	Conservative baseline — surface temperature stays below 140 F (60 C)
Unfilled POM with grease	5 m/s (1,000 ft/min)	~0.20 MPa·m/s	Grease lubricated	Grease significantly extends operating envelope
PTFE-filled (Delrin AF)	4 m/s (800 ft/min)	~0.15–0.20 MPa·m/s	Dry	Lower CoF extends dry PV limit by ~2×
Glass-filled acetal (25% GF)	2 m/s (400 ft/min)	~0.06 MPa·m/s	Dry	Higher friction CoF limits PV — but higher tooth strength for given module
Carbon-filled acetal (20% CF)	3.5 m/s (700 ft/min)	~0.12 MPa·m/s	Dry	CF self-lubricates — better dry PV than unfilled; stiffer tooth

White acetal gear and steel mating gear mounted on two parallel shafts in a small open test fixture.

Figure 4. PV and pitch-line velocity are operating checks; the photo gives the gear-mesh context, while the table carries the actual limits.

CNC Machined Acetal Gears with Quote Review

MakerStage CNC machines acetal gears in unfilled POM-H and POM-C from module 1.0 and larger. Upload your drawing or CAD file so the quote review can flag undercut, insufficient root radius, and face width concerns before production.

Get an Acetal Gear Quote

Section 5 of 5

Gear Material Pairing Guide

Your choice of mating gear material controls wear rate and noise as much as the acetal grade itself. The mating material choice affects wear rate, noise, and service life as much as the acetal grade itself.

White acetal gear, machined steel gear, and dark filled acetal gear arranged as separate loose parts on a metal workbench.

Figure 5. Material pairing is a wear decision: acetal, steel, and filled grades behave differently even when the gear geometry looks similar.

Acetal Gear Paired With	Wear Performance	Noise	Notes
Hardened steel (HRC 45+, ground Ra 0.8–1.6 µm / 32–63 microinches)	Excellent	Moderate	Standard pairing for power transmission. Hard, smooth steel allows POM transfer film to form. Rough or soft steel accelerates POM wear.
Stainless steel 17-4 PH (H900)	Good to Excellent	Moderate	Good for food/medical applications. Hardness HRC 40+ adequate with smooth surface.
Acetal POM (plastic-on-plastic)	Good at low PV	Low	Both gears generate low heat; quiet meshing. Service life lower than POM-on-steel for equal load.
Nylon PA6/6 (mating gear)	Good	Very Low	Quiet — dissimilar plastic pairs run with low noise. Nylon absorbs moisture — center distance may change.
Aluminum (6061, 7075)	Poor — avoid	High	Acetal wears aluminum rapidly. Aluminum surface too soft for sustained POM contact. Use anodized or Ni-plated Al as minimum.
Brass	Fair	Low	Soft — brass wears under sustained contact. Use only for very light loads or intermittent service.
Cast iron	Good	Moderate to Low	Cast iron surface is self-lubricating (graphite inclusions) — compatible with acetal. Confirm surface hardness.