Nylon vs Acetal for Bearings and Gears: Which Engineering Plastic Wins?
Quick Comparison: Nylon vs Acetal for Mechanical Components
| Property | Nylon 6/6 (PA66) | Acetal (POM) |
|---|---|---|
| Tensile Strength | 82 MPa | 69 MPa |
| Coefficient of Friction | 0.20-0.30 | 0.10-0.20 |
| Moisture Absorption | 1.5-2.5% (equilibrium) | 0.2-0.3% |
| PV Limit (unlubricated) | 3,000 | 5,000 |
| Dimensional Stability | Variable (moisture) | Excellent |
| Fatigue Endurance | Good | Excellent |
| Continuous Service Temp | 80-120°C | 85-105°C |
| Relative Cost | $$ | $$ |
Acetal (POM): The Precision Bearing and Gear Material
Acetal (polyoxymethylene, POM), commercially known as Delrin® (homopolymer) or Celcon®/Hostaform® (copolymer), is the first-choice material for unlubricated bearings, gears, and precision mechanical components. Its combination of low friction (0.10-0.20), high PV limit (5,000 psi-fpm unlubricated), excellent fatigue endurance, and outstanding dimensional stability makes it the benchmark against which all other bearing-grade plastics are measured.
The critical advantage of acetal over nylon in precision mechanical applications is dimensional stability. POM absorbs only 0.2-0.3% moisture at equilibrium versus 1.5-2.5% for PA66. This means acetal gears and bearings maintain their designed geometry, clearances, and tolerances regardless of ambient humidity — essential for precision gear trains where a 0.5% dimensional change from moisture absorption would alter gear mesh geometry and increase noise. In a bearing application, moisture-induced swelling of nylon changes the running clearance, potentially causing either excessive play (dry condition after humid conditioning) or seizure (humid condition after dry conditioning).
Acetal's fatigue endurance limit is approximately 35 MPa at 10⁷ cycles — significantly higher than nylon's 20-25 MPa. For gear teeth experiencing cyclic bending loads, this translates to 50-75% longer fatigue life. Acetal gears in business machine and appliance applications routinely achieve 10⁷+ cycles without fatigue failure. See our full POM vs PA66 comparison for detailed mechanical data.
Nylon (PA66): The Tough, High-Temperature Alternative
Nylon 6/6 (PA66) offers higher tensile strength (82 vs 69 MPa), higher temperature capability (120°C dry vs 105°C), and superior toughness and impact resistance compared to acetal. These properties make nylon the preferred material for heavy-duty gears and bearings where shock loads, high temperatures, or extreme mechanical stress are expected.
In gear applications, nylon's higher toughness provides better resistance to shock loads and impact — a single tooth overload that would crack an acetal gear tooth is absorbed by nylon's ductility. This makes nylon the standard for gears in power transmission applications subject to occasional shock loads: conveyor drives, agricultural equipment, and industrial machinery with intermittent peak loads.
Nylon's moisture absorption, while a liability for precision components, becomes an advantage in unlubricated bearing applications. Absorbed moisture acts as a plasticizer, reducing friction and improving the material's bearing characteristics. A nylon bearing in a humid environment has lower friction and wear rate than the same bearing in dry conditions — the absorbed water provides boundary lubrication. This self-lubricating effect is unique to nylon and is not available in acetal.
However, the moisture-driven dimensional change is the fundamental limitation. A PA66 bearing bore will change by 0.5-2% depending on humidity, requiring the designer to specify either: (a) oversized bores that shrink to the correct dimension in service humidity, (b) moisture-conditioned blanks machined after conditioning, or (c) filled grades (MoS₂, glass) that reduce absorption by 40-60%. Compare POM vs PA66 for specific application guidance.
Application-Specific Recommendations
Precision Gear Trains (Business Machines, Appliance Timers)
Choose Acetal (POM). Dimensional stability ensures consistent gear mesh and quiet operation over the product lifetime. Fatigue endurance provides 10⁷+ cycle life. The low and consistent friction coefficient eliminates the need for lubrication in lightly loaded gear trains.
Power Transmission Gears (Conveyors, Industrial Machinery)
Choose Nylon PA66 (dry or MoS₂-filled). Higher toughness absorbs shock loads. Higher temperature capability handles motor heat and friction heating. The moisture-related dimensional change is manageable in these less-precision applications.
Unlubricated Sleeve Bearings
Choose Acetal for precision bearings where clearance must be maintained. Choose Nylon for bearings in humid environments where the self-lubricating effect of absorbed moisture is beneficial. Both materials are acceptable; the choice depends on whether dimensional stability (POM) or self-lubrication (PA66) is the higher priority.
Conveyor Rollers and Wear Strips
Choose Acetal for dimensional stability and consistent low friction. Nylon's moisture absorption causes roller diameter changes that alter line speed and tracking. In food processing conveyors, acetal copolymer is preferred for its FDA compliance and superior creep resistance under sustained load.
Frequently Asked Questions
Why is Delrin (acetal) preferred over nylon for precision gears?
Dimensional stability. Acetal absorbs only 0.2-0.3% moisture versus 1.5-2.5% for nylon 6/6. In a precision gear train, even 0.5% dimensional change from moisture absorption alters gear mesh geometry, increasing backlash, noise, and wear. Acetal's consistent dimensions ensure stable gear mesh and quiet operation over the product lifetime, making it the standard for business machine and appliance gear trains.
Does moisture absorption make nylon bearings better or worse?
It depends on the application. Moisture absorption is a disadvantage for precision bearings where clearance must be maintained — the dimensional change (0.5-2%) alters running clearance unpredictably. However, in unlubricated sleeve bearings operating in humid environments, absorbed moisture acts as a plasticizer that reduces friction and wear rate, effectively providing boundary lubrication. This self-lubricating effect is unique to nylon and beneficial in appropriate applications.
What is the PV limit and why does it matter for bearings?
The PV limit (pressure × velocity) defines the maximum bearing load a material can sustain in continuous unlubricated operation. Acetal's PV limit of 5,000 psi-fpm is 67% higher than nylon's 3,000, meaning acetal bearings can handle higher loads, higher speeds, or both simultaneously without failure. Exceeding the PV limit causes rapid wear, excessive heat generation, and eventual bearing seizure. PV limit is the primary selection criterion for unlubricated bearing design.
Can nylon and acetal be used together in the same gear train?
Yes, and this is common practice. Nylon-acetal gear pairs are widely used because the combination leverages each material's strengths: acetal's dimensional stability for the driven gear, and nylon's toughness for the driving gear subject to motor torque fluctuations. The slightly different friction characteristics also promote smooth mesh. Avoid using the same material for both meshing gears — identical materials tend to adhesive wear (galling) under load.
Should I use filled grades for bearing applications?
For most applications, unfilled acetal (POM) is optimal for bearings and gears. Fillers improve specific properties but reduce others: MoS₂-filled acetal has lower friction but reduced strength and fatigue life; glass-filled nylon has higher strength and stiffness but reduced toughness and increased bearing wear on the mating surface. Use filled grades only when the specific property improvement is required and the trade-offs are acceptable for your application.
References & Industry Standards
- ASTM International. Standard Specifications for Engineering Plastics & Thermoplastics. astm.org
- ISO. ISO 1043 — Plastics — Symbols and Abbreviated Terms. iso.org
- UL Prospector. Plastics & Elastomers Material Database. ulprospector.com
- MatWeb — Material Property Data. matweb.com