A press fit plastic clearance copied from a steel drawing will usually seize, spin or split once an acetal or nylon part sees service temperature and moisture. Steel and aluminium behave predictably because their thermal expansion is modest and they do not absorb water; acetal and nylon do both, and they also creep under sustained load. A fit that feels correct on a dry bench can loosen within a week in a humid plant, and a fit that is too tight can split the hub during assembly.
This article sets out the movements you have to allow for, the interference values that usually work, and how to prove an assembly before you commit to tooling. Every grade mentioned is a standard stock shape, cut and finished to order, in materials such as POM-C acetal and PA66 nylon.
Why plastic press fits behave differently
In metal, a press fit is mostly a machining problem: choose an interference, hold the tolerances, press the parts together. In plastic the interference is not fixed once the parts are made. The hub and the shaft move relative to each other whenever the temperature changes, whenever one material absorbs moisture and the other does not, and whenever the plastic relaxes under stress. All three effects are larger in plastics than in metals, because plastics have higher coefficients of thermal expansion, several of them take up measurable moisture, and they lose contact pressure over time through creep.
The practical result is that you cannot specify a single figure and walk away. A press fit plastic clearance is a range: it has to stay positive at the coldest, driest point in the part’s life and must not overstress the plastic at the hottest, wettest point. That range, not a single number, is what belongs on the drawing, and the rest of this article shows how each term in it is calculated.
Thermal expansion: the numbers that matter
As a rule of thumb, unfilled acetal expands at roughly 100 to 120 ppm/K, and unfilled PA66 at about 80 to 100 ppm/K. A steel shaft is close to 11 ppm/K and an aluminium housing around 23 ppm/K. Over a 40 C swing, a 50 mm acetal bore grows by about 0.2 mm while the steel shaft inside it grows by roughly 0.02 mm. That difference alone can consume most or all of the interference you intended.
The design response is straightforward: calculate the fit at both temperature extremes rather than at room temperature, and make sure the plastic is the element that grows. Where the housing and the shaft see the same temperature, expansion largely cancels and you are left with moisture and creep as the governing effects.
Moisture growth in nylon and acetal
PA66 is hygroscopic. In a humid plant it can reach 2 to 3 per cent moisture by weight, and each per cent adds roughly 0.2 to 0.3 per cent to its linear dimensions. On a 50 mm bore that is up to 0.3 mm of growth, easily more than a typical interference. Acetal takes up far less, typically 0.2 to 0.8 per cent in service, so its dimensional change is correspondingly smaller.
This is why nylon parts often need a wider clearance allowance, or deliberate conditioning to a known moisture content before final machining. A nylon bushing machined dry and then installed in a wash-down area will swell and tighten. The same bushing machined after conditioning keeps the fit close to what you designed.
Creep and the loss of interference over time
Interference is a stress, and plastic under sustained stress creeps. The contact pressure that holds a press fit together falls over weeks and months, and it falls fastest when the part runs warm. A fit that starts with a comfortable grip can become a loose, rattling joint long before the part wears out. Creep is worse in unfilled nylon and acetal and better in glass-filled grades, which resist both creep and thermal expansion.
The mitigation is to design for that relaxation: start with a little more interference than the room-temperature calculation suggests, keep the working temperature as low as the application allows, and where the load is high consider a metal-backed insert or a keyed design so the joint does not depend on friction alone. Once a press fit plastic clearance has relaxed, no amount of re-torquing restores it. We cover the geometry in our bushing tolerance notes.
Practical interference values by material
The figures below are starting points from our experience, not guarantees. They assume a metal shaft in a plastic hub, a steady service temperature under 25 C and moderate humidity. Tighten or loosen them with the corrections above, and confirm the result on a prototype before releasing the drawing.
| Материал | Interference per 25 mm | Expansion | Moisture growth | Notes |
|---|---|---|---|---|
| POM-C acetal | 0.02-0.05 mm | ~110 ppm/K | low | Stable and easy to machine |
| POM-H acetal | 0.02-0.05 mm | ~110 ppm/K | low | Stiffer and tougher than copolymer |
| PA66 unfilled | 0.03-0.06 mm | ~90 ppm/K | high | Dry-machined parts tighten in humid service |
| PA66 GF30 | 0.02-0.04 mm | ~40 ppm/K | moderate | Less creep, more abrasive to machine |
| PEEK unfilled | 0.03-0.06 mm | ~50 ppm/K | very low | Hot and dry duty, higher cost |
All of these grades are held as cut-to-size stock shapes. If you send the drawing and the service conditions, we will suggest a starting range and machine the prototype to it; the contact page is the quickest route.
Questions buyers ask about press fits in acetal and nylon
How much interference should a plastic press fit use?
For a metal shaft in a plastic hub, a diametral interference of about 0.02 to 0.06 mm per 25 mm of diameter is a common starting point, then corrected for temperature and moisture. Nylon usually needs the wider end of that range and often a little more, because it swells in service. Verify by pressing a prototype before releasing the drawing.
Should a plastic bushing be machined wet or dry?
For nylon, machine it at the moisture content it will see in service, or allow for the swell in the tolerance. Acetal moves little and is forgiving. In a wash-down area, condition the nylon first, cut to the conditioned size and press the metal shaft cold rather than hot.
When does a press fit need a threaded insert or a key?
Creep, not machining, is the usual reason a plastic press fit loosens. If the joint carries torque, runs above about 40 C, or cannot be inspected after assembly, a machined-in metal insert, a key or a mechanical shoulder will hold where friction alone will not.
Can acetal and nylon be press fitted to each other?
Yes, but the fit is sensitive to moisture because the two absorb water at different rates. Acetal is the more stable of the two, so use it for the hub and allow a wider clearance on the nylon. Do not assume the joint stays tight through repeated humidity cycling.
Нужна рекомендация по материалу?
Пришлите задачу — и мы ответим двумя-тремя подходящими марками и ценой.
