Polyoxymethylene homopolymer and acetal copolymer are selected for dry sliding parts only after load, speed, and duty cycle have been separated numerically. The tribological response is commonly expressed as the product of apparent bearing pressure P and surface speed V, where P is calculated from the projected contact area in newtons per square millimetre and V is measured in metres per second. The limiting P·V figures published for wear-resistant POM grades are not universal ratings; they are screening values obtained at 20 °C ambient temperature against hardened steel with a counterface roughness of 0.1–0.2 μm Ra and steady-state interface temperatures below the heat-distortion region. Wear-rate comparisons are made under ASTM D3702-94(2019) thrust-washer geometry or ISO 6601:2002 sliding-wear identification because wear factor K in mm³/N·m is geometry-dependent. Unfilled acetal homopolymer grades typically produce K values from 2×10⁻⁶ mm³/N·m to 6×10⁻⁶ mm³/N·m at P·V between 0.02 MPa·m/s and 0.08 MPa·m/s; PTFE-modified POM grades frequently shift K to 10⁻⁷ mm³/N·m magnitude. Acetal copolymer differs from homopolymer in thermal endurance and wear morphology because the comonomer units interrupt the oxymethylene sequence, reducing unzipping degradation but also altering crystallinity.

Is Glass Fibre Reinforcement Justified When Load Dominates the Sliding Interface?

At surface speeds below 0.1 m/s and apparent pressures above 0.05 MPa, the dominant failure mechanisms are creep, loss of bore clearance, and abrasive scoring rather than frictional melting. Load-dominated selection is not based on tensile strength alone. The apparent bearing pressure should be calculated from the projected contact area, not from the entire washer face. For a thrust washer with outside diameter 40 mm and inside diameter 16 mm, the contact area is π/4×(40²−16²) = 1.06×10⁻³ m². A thrust load of 200 N gives P = 0.19 MPa; at 60 min⁻¹ the surface speed at the mean diameter of 28 mm is 0.088 m/s, giving P·V = 0.017 MPa·m/s, within the unfilled POM envelope but creep still governs over long cycles. A 25 wt% short-glass-filled POM raises tensile modulus from the unfilled range of 2,800–3,200 MPa to 8,500–10,000 MPa when measured by ISO 527-2, and flexural strength increases correspondingly under ISO 178. This stiffness delays bore closure in heavily loaded bushings. However, glass reinforcement is not a universal wear solution. Exposed fibre ends can produce a transfer film on the shaft that becomes abrasive if the counterface hardness is below 50 HRC; hardened AISI 52100 or nitrided steel counterfaces are typically specified to avoid accelerated scoring. In production, glass-filled POM compounds are processed on twin-screw extruders with barrel-temperature profiles constrained between 170 °C and 205 °C. Melt excursions above 215 °C are known to liberate formaldehyde from the oxymethylene backbone, producing silver streaks and a drop in molecular weight; the processing window narrows to approximately ±5 °C at glass loadings above 25 wt% because of viscous heating in mixing zones. Published data for the tribological benefit of glass fibre under continuous sliding is less consistent than for PTFE or carbon-fibre modification.

Surface speed, not apparent bearing pressure, governs the transition from mild abrasion to thermal failure in unfilled acetal. For a 20 mm diameter journal rotating at 1,500 min⁻¹, V = π × 0.020 m × 1,500 min⁻¹ / 60 s/min = 1.57 m/s. If the radial load is 60 N and the projected area from a 20 mm diameter by 20 mm length bushing is 400 mm², the apparent pressure is 0.15 MPa. The P·V product becomes 0.24 MPa·m/s, which exceeds the continuous dry-sliding rating of many unfilled POM grades at 20 °C. The same geometry with a higher shaft speed or lower load can cause interface temperatures above the heat-deflection threshold because frictional power per unit area is proportional to μ·P·V. Dry POM against steel typically exhibits dynamic friction coefficients between 0.20 and 0.35; at 1.57 m/s, local flash temperatures can exceed material limits even when the calculated P·V is moderate. Internally lubricated grades containing PTFE or ultrahigh-molecular-weight silicone reduce dynamic friction to 0.08–0.18 and move the thermal boundary outward. For high-speed low-load conditions, silicone-oil concentrate grades are selected preferentially when oscillation amplitude is small and external lubrication cannot reach the interface. Their wear mechanism is controlled by migration of the internal lubricant; the rate of migration depends on cycle time, interface temperature, and the free volume of the copolymer matrix.

Internally Lubricated Acetal Copolymer Grades for High-Frequency Oscillation

PTFE-filled homopolymer and silicone-oil copolymer grades address different high-frequency sliding regimes. A 20 wt% PTFE-filled acetal homopolymer typically lowers wear factor K by 50–80% relative to the unfilled parent material under ASTM D3702-94(2019) thrust-washer conditions, but the accompanied reduction in tensile modulus from approximately 3,000 MPa to 2,200 MPa means the bush must be rechecked for hoop stress and press-fit retention. In oscillating bearings with angular amplitude below 5°, stick-slip is measured by static-to-dynamic friction ratio rather than by absolute wear factor alone. Silicone-modified acetal copolymer grades can maintain a static-to-dynamic friction ratio below 1.5 against coarse-ground steel; specification sheets for commercial products vary, and ASTM G77 block-on-ring tests are commonly used because the test rotates continuously and does not capture reversal-induced wear. A lower friction base can still fail by fretting if the oscillation is not accompanied by enough lubrication migration to rebuild the transfer film. Data for acrylic-modified, MoS₂-filled, and graphite-filled POM grades exist, but published direct comparisons under identical duty cycles are limited. MoS₂ and graphite are more often used when a conductive path or reduced white-powder wear debris is required; their effect on acetal tribology is strongly counterface-dependent, and they can increase wear if the counterface roughness exceeds 0.4 μm Ra because the lamellar solid has to fill deeper valleys before a continuous transfer layer develops.

When the duty cycle is less than 30% and the dwell period allows heat dissipation from the housing, the apparent P·V calculated from the maximum dynamic load may exceed the continuous P·V rating without immediate failure. No ISO or ASTM standard defines a universal intermittent-duty multiplier for polyoxymethylene, because the permissible increment depends on shaft material, wall thickness, ambient air movement, and the time constant of the housing. Validation requires back-wall thermocouple measurement; a continuous interface temperature greater than 90 °C on unfilled homopolymer bearings indicates that the projected duty rating has been exceeded. Reversing duty is another boundary condition. Unidirectional thrust-washer data from ASTM D3702-94(2019) should not be transferred to a crank-rocker or reciprocating carriage without qualification. Under reversing rotation, the transfer film is peeled and rebuilt with every start; the initial wear rate may increase until a stable layer is re-established. Protocols based on ASTM G133 linearly reciprocating ball-on-flat or block-on-ring with reverse-motion steps provide more relevant comparison data for such components. Production feedback from injection-moulding lines indicates that batch-to-batch wear variation can increase when condensation on undried POM pellets raises surface moisture; drying at 80 °C for 3–4 h is applied only where visible condensation or unheated silo storage has been identified. In food-contact conveying equipment, acetal copolymer wear grades used in start-stop star wheels and chain guides must also meet the extraction limits of FDA 21 CFR 177.2470 or the relevant migration conditions of Regulation (EU) No 10/2011, which impose additional restrictions on internal lubricant packages and colourants.

PV Ceiling Is Not a Fixed Material Constant

A P·V value quoted at 20 °C is a screening result, not an intrinsic property. Unfilled acetal homopolymer may show a continuous P·V ceiling near 0.07 MPa·m/s at 20 °C, but the same grade can lose 50–70% of that ceiling when ambient temperature is raised to 80 °C because the heat-deflection temperature under 1.8 MPa is approximately 95–110 °C when measured by ISO 75-2. The limiting P·V is also affected by the heat-sink geometry: a thin-walled steel seat extracts friction heat more effectively than a 6 mm continuous polymer wall, which acts as an insulator and shifts the failure point to lower speeds and loads. The load axis and speed axis are not symmetrical. At low speed, load is limited by creep and compressive strain; at high speed, the same product of P·V may be unattainable because frictional heating is concentrated in the contact layer.

Chemical compatibility places a separate boundary on wear-grade POM. Acetal resins are not recommended for continuous exposure to strong acids, chlorine, or oxidizers; hydrolytic attack in hot water above 60 °C can generate formaldehyde and reduce molecular weight at the surface. The use of silicone-oil lubricated grades in alcoholic or strongly alkaline process environments can also alter lubricant migration and should be validated by immersion ageing according to ISO 22088-1 before deployment. For parts operating below 0.05 MPa and below 0.5 m/s, unfilled acetal copolymer often satisfies the duty without filler complication; above these values, the selection shifts first to PTFE-modified homopolymer for low friction and then to glass- or carbon-reinforced grades only when creep and dimensional stability dominate over interface abrasion.