Acetal Bushings and Bearings: Design Guide (2026)

Q: Does an acetal bushing need a press fit in the housing?

Light press fit or interference fit is recommended for non-rotating bushings (shaft rotating in bushing bore). A press fit prevents the bushing from spinning in the housing under startup torque. Typical housing interference: 0.010–0.030 mm for a 25 mm OD bushing. Too much interference can crack the bushing on installation or close the bore excessively. For housings in soft materials (aluminum, plastic), reduce interference to 0.005–0.015 mm.

Q: How long will an acetal bushing last?

Bushing life depends entirely on actual PV and the ratio of operating PV to the material PV limit. At 50% of the PV limit with good shaft finish and correct clearance, an acetal bushing can provide tens of thousands of hours of service. At 90% of PV limit, expect accelerated wear and thermal failure within hundreds of hours. The most reliable predictor is the specific wear rate multiplied by actual sliding distance — calculate expected wear at service PV before committing to unfilled acetal.

Most Acetal Bushing Failures Are PV Failures, Not Design Failures.

The most common reason an acetal bushing fails prematurely is that the designer used the material's tensile strength to size the bearing wall, without checking the PV (pressure × velocity) thermal limit that actually governs sliding bearing life. This guide gives you the tools to size the bearing correctly — starting with PV, then clearance, then shaft finish.

Section 1 of 5

PV Limit Fundamentals

If you size an acetal bushing by wall stress alone, you'll miss the PV thermal limit that actually governs bearing life. The PV (Pressure × Velocity) parameter is the single most important number in sliding bearing design. It determines whether the bearing interface will maintain a stable temperature or heat to failure.

How to Calculate Bearing PV

P = F / (d × L)

P = bearing pressure (MPa)
F = radial load (N)
d = bore diameter (mm)
L = bearing length (mm)

V = π × d × n / 60,000

V = sliding velocity (m/s)
d = shaft diameter (mm)
n = shaft speed (RPM)

PV = P × V

Must be below material PV limit for continuous operation

PV Limits by Acetal Grade

Unfilled POM (dry)

Default bearing grade

~0.10 MPa·m/s

Unfilled POM (with grease)

Periodic relubrication possible

~0.20 MPa·m/s

PTFE-filled (AF grade, dry)

Self-lubricating upgrade

~0.20 MPa·m/s

Carbon-filled (CF grade, dry)

CF self-lubricating at higher P

~0.15 MPa·m/s

Glass-filled 25% (dry)

High friction limits PV — avoid for bearings

~0.06 MPa·m/s

Worked Example: Is Your Bushing Within PV Limits?

Given: 25 mm (1 in.) bore acetal bushing, 20 mm (0.79 in.) bearing length, 50 N radial load, shaft speed 500 RPM.

Step 1 — Bearing pressure:

P = F / (d × L) = 50 N / (25 mm × 20 mm) = 0.10 MPa

Step 2 — Sliding velocity:

V = π × d × n / 60,000 = π × 25 × 500 / 60,000 = 0.654 m/s

Step 3 — PV product:

PV = 0.10 × 0.654 = 0.065 MPa·m/s — 65% of unfilled POM limit (0.10)

Verdict: Within the 50–70% design target. This bushing will run reliably in dry service with good shaft finish. If your load doubles to 100 N, PV hits 0.13 MPa·m/s — above the limit. You'd need PTFE-filled acetal (PV limit ~0.20 MPa·m/s) or add lubrication.

Design to 50–70% of PV Limit — Not 100%

Published PV limits assume clean operation, good shaft finish, and correct clearance. Real applications have load surges, contamination, and temperature variation. Apply a safety factor of 1.5–2× — operate at 50–70% of the published PV limit. A bushing operating at 0.05 MPa·m/s (50% of unfilled POM limit) will provide far longer service than one squeezed to the limit.

Section 2 of 5

Clearance Fit Recommendations

If your bushing bore clearance is wrong, the bearing either seizes under thermal expansion or wobbles under load — both kill service life. Clearance must balance running clearance (too tight seizes; too loose wobbles) against acetal's thermal expansion mismatch with steel shafts.

Shaft Diameter	ISO Fit	Diametral Clearance	Application
6–10 mm	H7/f6	0.013–0.040 mm	Precision rotating shaft — instrument or servo
10–18 mm	H7/f6	0.016–0.050 mm	Light-duty rotating shaft — robotics, small actuators
18–30 mm	H7/f6	0.020–0.065 mm	Standard rotating shaft — most common design range
30–50 mm	H7/f6	0.025–0.080 mm	Medium shaft — industrial drives, conveyor applications
50–80 mm	H8/f7	0.040–0.110 mm	Larger shaft — increased clearance for thermal expansion
Any size (elevated temp >60°C)	Add 0.001 in / 10°C above ambient	—	Acetal CTE (110 µm/m·°C) >> steel (12 µm/m·°C) — bore tightens with temperature
Axial sliding (linear)	H9/d9	0.065–0.250 mm	Linear motion — larger clearance for smooth axial travel

Section 3 of 5

Mating Shaft Requirements

Your shaft surface finish and hardness determine bushing wear rate as much as the acetal grade you select. The shaft surface is the other half of the tribological system. Shaft quality has as much impact on bushing life as the bushing material itself.

Surface Finish

Ra 32–63 µin (0.8–1.6 µm): optimal range for dry POM sliding
Ra < 16 µin: too smooth — POM transfer film does not adhere, higher friction
Ra > 63 µin: too rough — abrades POM surface, dramatically increases wear
Ground finish preferred over turned finish for consistent bearing zone
Hard chrome plating acceptable: provides smooth, hard surface compatible with POM

Shaft Hardness

Minimum: HRC 30 for steel shafts in continuous rotation
Recommended: HRC 45+ for hardened steel (4140, 4340 H&T, 17-4 H900)
Hard chrome over mild steel: acceptable if chrome layer is ≥ 0.0005 in
Avoid soft steel (< HRC 20): POM wear debris scratches shaft surface, accelerating further wear
Stainless steel 316L (HRB 79–95): marginal — use only for light PV applications

Shaft Geometry

Roundness: ≤ 0.0005 in (0.013 mm) total variation in bearing zone
Cylindricity: ≤ 0.001 in per inch (0.025 mm/25 mm) of bearing length
Chamfer at bearing entry (30° × 0.5 mm minimum): prevents edge loading on bushing
No sharp steps in bearing zone — edge loading concentrates stress on bushing end
Shaft straightness: ≤ 0.001 in per foot for long shafts with multiple bearing supports

Incompatible Shaft Materials

Aluminum 6061/7075 (anodized or bare): too soft — acetal scratches anodize surface
Brass: too soft for sustained contact under load
Plastic shafts: plastic-on-plastic at similar hardness generates heat rapidly
Corroded or pitted steel: surface peaks abrade POM rapidly — replace shaft before fitting new bushing
Case-hardened steel with chipped or worn hard layer: soft substrate exposed; replace shaft

Section 4 of 5

Grade Selection for Bearing Applications

Match the acetal grade to the actual PV and duty cycle of your application.

Application	Recommended Grade	Rationale
Low PV (< 0.05 MPa·m/s), intermittent, light duty	Unfilled POM-H (Delrin 150)	Cost-effective; dimensionally stable; holds tight bore tolerance
Medium PV (0.05–0.08 MPa·m/s), continuous, dry	Unfilled POM-H or POM-C	Both grades suitable; POM-C if bore is machined from large rod stock (> 25 mm)
High PV (0.08–0.15 MPa·m/s), dry, no lubrication possible	PTFE-filled (Delrin AF / AF blend)	PTFE filler doubles dry PV limit; lower CoF extends thermal margin
Elevated temperature (60–90°C ambient)	PTFE-filled (AF) or POM-C	PTFE-filled provides more thermal margin; POM-C more thermally stable long-term
Chemical environment (acids, alkalis)	POM-C copolymer	Better chemical resistance than POM-H; avoid glass or carbon-filled in acid contact
High dimensional precision required (bore tolerance H7)	Unfilled POM-H or POM-C	Filled grades can cause bore form error during machining; unfilled better for H7 tolerance
Structural load on bushing wall (thin-wall, high radial load)	Glass-filled 25% GF	Higher compressive strength and creep resistance; not for high-PV or acid exposure
ESD/semiconductor environment	Carbon-filled CF acetal	ESD dissipative; self-lubricating CF also reduces dry friction

Precision Acetal Bushings — CNC Machined to H7 Bore

MakerStage machines precision acetal bushings to H7 bore tolerances (±0.001 in) in unfilled POM-H, POM-C, and PTFE-filled AF grade. Free DFM review on every order — we flag clearance, press-fit, and bore tolerance concerns before machining.

Get a CNC Acetal Bushing Quote with Free DFM Review

Section 5 of 5

Bushing Design Rules Checklist

Check these before releasing your bearing design to manufacturing.

Sizing

Calculated PV at rated load and speed ≤ 50–70% of grade PV limit
L/d ratio (length-to-bore): 0.5–1.5 for most applications
Wall thickness: minimum 10% of bore diameter; minimum 1.5 mm absolute
Lewis stress check for thin-wall press-fit: wall can crack during installation if too thin

Tolerances

Bore specified as H7 for precision running fits
OD specified as tolerance band compatible with housing press fit
Housing interference: 0.010–0.030 mm for metals; 0.005–0.015 mm for aluminum or plastic housing
Thermal expansion noted in design record if operating > 60°C

Shaft Drawing Notes

Shaft OD tolerance specified (f6 for H7 bore)
Surface finish Ra 32–63 µin called out in bearing zone
Minimum shaft hardness noted (HRC 30 min)
Chamfer at bushing entry (0.5 × 30° or 0.5 × 45°)

Material Callout

POM-H or POM-C specified explicitly (not just "acetal")
PTFE-filled grade called out as "PTFE-filled acetal (AF grade)" if required
Natural color specified for FDA/food applications
Material grade confirmed available from multiple suppliers

Frequently Asked Questions

What is the PV limit for acetal bushings?

Unfilled acetal (POM) has a dry continuous PV limit of approximately 0.10 MPa·m/s (P in MPa bearing pressure, V in m/s sliding velocity). For a bushing with 10 mm bore, 10 mm length, and 100 N radial load: P = 100 N / (10 mm × 10 mm) = 1.0 MPa. Maximum shaft speed for this bushing: V = PV_limit / P = 0.10 / 1.0 = 0.10 m/s (about 190 RPM on a 10 mm shaft). Exceeding this limit causes bearing surface to heat, soften, and fail.

What clearance should I design between an acetal bushing bore and a steel shaft?

For precision rotating fits, use H7/f6 clearance fit: approximately 0.020–0.065 mm clearance for a 25 mm bore/shaft. This provides running clearance without excessive play. Add thermal expansion clearance if operating temperature exceeds 60°C — acetal CTE (110 µm/m·°C) is much higher than steel (12 µm/m·°C). At 40°C above design temperature, a 25 mm bushing expands approximately 0.110 mm on the ID, potentially tightening the fit.

What shaft surface finish do I need for acetal bushings?

Shaft surface finish has a significant impact on acetal bushing wear rate. For dry sliding: Ra 32–63 µin (0.8–1.6 µm) ground or hard-turned finish on the shaft. Too rough (> Ra 63 µin) abrades the acetal surface rapidly. Too smooth (< Ra 8 µin) does not develop the POM transfer film that lubricates the interface. Shaft hardness: minimum HRC 30 for steel shafts; harder is better. Soft shafts (< HRC 25) may be scratched by POM abrasion products.

Does an acetal bushing need a press fit in the housing?

Light press fit or interference fit is recommended for non-rotating bushings (shaft rotating in bushing bore). A press fit prevents the bushing from spinning in the housing under startup torque. Typical housing interference: 0.010–0.030 mm for a 25 mm OD bushing. Too much interference can crack the bushing on installation or close the bore excessively. For housings in soft materials (aluminum, plastic), reduce interference to 0.005–0.015 mm.

How long will an acetal bushing last?

Bushing life depends entirely on actual PV and the ratio of operating PV to the material PV limit. At 50% of the PV limit with good shaft finish and correct clearance, an acetal bushing can provide tens of thousands of hours of service. At 90% of PV limit, expect accelerated wear and thermal failure within hundreds of hours. The most reliable predictor is the specific wear rate multiplied by actual sliding distance — calculate expected wear at service PV before committing to unfilled acetal.