Guides / Design
Designing Acetal Bushings and Plain Bearings That Do Not Seize
September 21, 2026
An acetal bushing is a simple part with two design traps: the clearance moves with temperature far more than people expect, and the mounting relaxes over time. Get those two right and the rest is straightforward.
Sizing the running clearance
Start with the fact that matters. Acetal expands near 110 micrometres per metre per kelvin. Steel is near 12.
Picture a 25 mm bore acetal bushing pressed into a steel housing with a steel shaft running in it. As the assembly warms, the steel shaft grows a little, the steel housing grows a little, and the acetal bushing grows a lot. Because the housing constrains the outside of the bushing, most of that growth goes inward and the bore closes.
That is the direction that catches people out. A bushing sized with a comfortable clearance at 20 degrees C can be tight at 60.
The practical method:
- Calculate the bore at the low end of the service temperature range
- Calculate the bore at the high end, accounting for the constraint of the housing
- Check that the clearance is acceptable at both
- Add a moisture allowance, small for acetal but not zero if the part is submerged
If a single clearance cannot satisfy both ends of the range, either narrow the range, change the housing material or use a different bearing material.
PV and what it is good for
PV is contact pressure multiplied by surface speed. Published limiting values for unfilled acetal running dry are commonly cited around 3,000 psi feet per minute.
Use it as a screening tool. If your design sits well under it, acetal is plausible. If it sits above it, acetal is not the answer dry and you need lubrication, a filled grade or a different material.
What PV does not do is predict life. Two designs at the same PV can behave completely differently depending on whether the load is steady or intermittent, whether heat can escape and how good the counterface is. Treat a calculation near the limit as a reason to test, not a reason to release.
Two practical notes:
- Intermittent duty is much more forgiving than continuous. Heat has time to leave between cycles.
- Heat removal is usually the real limit. A bushing in a thin plastic housing has nowhere to send heat. The same bushing in a steel housing survives far more.
The counterface
Acetal against steel is the proven pairing. The important variables on the steel side are hardness and finish.
- A harder shaft wears less and abrades the bushing less
- A very rough shaft acts as a file
- A very smooth shaft holds no lubricant, which matters if the bushing is greased
A ground shaft in the normal industrial finish range is the usual target. What you want to avoid is an as turned shaft with a coarse lay running dry.
Mounting
Press fit alone is a trap. Acetal creeps, so an interference that generates good retention on assembly relaxes over months and the bushing starts to spin in the housing.
Reliable approaches:
- A flange on one end, so the bushing cannot walk axially
- A retaining lip or circlip in the housing
- An anti rotation feature, a flat, a pin or a key
- Bonding, which works but is awkward on acetal because of its low surface energy, so surface preparation matters
Combine a light press fit with one mechanical feature and the bushing stays where you put it.
Wall thickness
A thin walled bushing conforms to the housing bore, which helps it seat but means the bore shape follows any housing error. A thick walled bushing is more dimensionally independent but insulates better, which traps heat.
There is no universal answer. For a precision location, favour a thicker wall and a well machined housing. For a low load guide in a rough housing, a thinner wall is more forgiving.
When to move off acetal
- Above roughly 90 degrees C continuous: acetal is out of range
- Heavy compressive load with shock: nylon takes it better
- Abrasive contamination: UHMW survives what acetal does not
- Dry running with stick slip or squeak: PTFE filled acetal, or a different bearing family
- Aggressive chemistry: check the chemical resistance before anything else
On the drawing
Bore tolerance and the fit it is for, outside diameter tolerance and the housing fit, flange or retention feature, service temperature range, shaft material and finish, whether it runs dry or lubricated, and the inspection temperature.
That last one again. An acetal bore measured warm is not the bore you designed.
Sizing a bushing and unsure about the clearance? Send your RFQ with the shaft size, housing material and temperature range. The shops we match will comment before quoting. Reply within 24 business hours.
Frequently asked questions
How much running clearance does an acetal bushing need?
More than a bronze one, and the figure depends on the temperature range rather than on a single rule. Acetal grows about ten times faster than the steel shaft and housing around it, so calculate the clearance hot and cold and take the worse case.
What is a PV limit and how should I use it?
PV is contact pressure multiplied by surface speed, and published limiting values for acetal sit around 3,000 psi ft per minute dry. Treat it as a screening figure. It tells you whether a design is obviously out of range, not whether a marginal one will work.
Will a press fit hold an acetal bushing in place?
For a while. Acetal creeps, so the interference relaxes over months. For anything that must not rotate in its housing, add a mechanical feature such as a flange, a pin, a flat or a retaining lip.
Need a quote for this part?
Send the drawing. We match you with up to 3 shops that machine acetal and reply within 24 business hours.
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