What Is the Maximum Operating Temperature of a Plastic Bushing, and What Happens as It Heats Up?

What does a "maximum temperature" actually mean?

Datasheets quote more than one figure, and they mean different things.

• Continuous-use temperature. The highest temperature the bushing can withstand for a long time without a loss of tensile strength.
• Short-term maximum. The temperature, for a few minutes only, above which the material becomes so soft it can only carry low external loads.
• Minimum temperature. The temperature below which the material becomes so rigid and hard that it is too brittle for standard use.

One caution applies to every datasheet. These standardised test results have limited use, because bearings are almost always under load. The wear limit of the material at a given temperature is more instructive. So a temperature rating is a starting point. A bushing rated for a temperature in a test can still wear out fast at that temperature under your load.

A fourth figure matters in practice: the temperature above which the bushing must be secured axially. Approaching the short-term maximum, a bushing can relax in its housing even without extra load, so it must be prevented from sliding out of the bore. Across the grades in one plastic-bearing maker's published table, the temperature at which securing is needed runs from about 50 °C to 135 °C, and the short-term ceilings from about 110 °C to 315 °C. That range is quoted only to show how widely grades differ. Do not carry it over to another grade, and a different plastic has a different number.

Where does the heat come from?

From two places, and you have to add them. The thermal environment of a bearing is the ambient temperature plus the frictional heat produced in operation. The frictional heat comes from load and speed, which is the PV value (Q1-02). Heat also leaves the joint differently depending on the housing: a steel housing carries heat away much faster than a plastic one, so the same load and speed can give a different temperature in a different housing.

The history before the machine runs also counts. A bushing that has sat in hot storage or transport has seen a thermal history beyond its working temperature.

What happens as a plastic bushing heats up?

Four things, usually together.

It softens. A semi-crystalline polymer loses stiffness in its amorphous regions between its glass transition temperature and its melting point, and keeps its strength only from its crystalline regions. Operating in that range is possible with the right material, but it changes the load capacity and the wear.

It loses its grip in the bore. Operating above the glass transition can lead to a loss of press fit, which is addressed by mechanically securing the bushing (Q1-04).

It expands much more than the steel around it. Take a nylon coefficient of 9.9 × 10⁻⁵ per K and 1.17 × 10⁻⁵ per K for steel. For a 25 mm part heated by 50 K (an illustration of free expansion), nylon grows about 25 × 9.9 × 10⁻⁵ × 50 = 0.12 mm, and steel about 25 × 1.17 × 10⁻⁵ × 50 = 0.015 mm. The plastic grows about eight times as much. In a real joint the housing holds the outside of the bushing, so the extra growth goes inward and the bore closes: a pressed-in bushing gets tighter when it heats, not looser (Q1-03).

The lubricant and moisture change. High-temperature bushings often fail because the lubricant breaks down or evaporates before the bushing material reaches its limit, and heat makes the bushing and shaft expand at different rates. Moisture also matters: some plastics take up water, and polyurethane is damaged for good by steam and long humid heat above about 70 °C (Q1-07, Q1-01).

What about cold?

Cold has a limit too. Below its minimum temperature a material becomes too brittle for standard use. Some plastics keep their properties in extreme cold better than bronze, which can become brittle at very low temperatures, but nylon is not well suited to cryogenic temperatures. Check the lower limit of the grade as well as the upper one.

How should you check a temperature limit?

Start with the real operating temperature: add the ambient temperature to the heat from friction. If you cannot measure it, estimate the PV value and compare it with the grade's PV limit (Q1-02). Use the maker's limits for the exact grade: the continuous limit, the short-term limit and the temperature above which the bushing must be secured. Allow for heat that is not constant, since short peaks such as a jam or a start-up under load can exceed the continuous limit, and the short-term figure is for minutes only. Check the clearance at temperature, because the cold clearance is not the hot clearance (Q1-03). Secure the bushing with flanges, retaining rings or other axial securing if the maker says so (Q1-04). Finally, check the lubricant and the housing: the grease must suit the temperature, and a plastic housing will run hotter than a steel one (Q1-06).

If the joint is hotter than the grade allows, the answer is a different grade or a metal bushing (Q1-12).

How hot does a plastic bushing get on an off-road or race vehicle?

The ambient figure is harder to know on a vehicle. A pivot beside an exhaust, a turbo, the engine block or a brake sees radiant and conducted heat that a datasheet ambient does not include. A dark panel in the sun runs hotter than the air. When the vehicle stops, the airflow stops too, so some places peak after shut-down and not while moving. Low road speed with high engine load, such as crawling or towing, means more heat from the engine and less cooling air around the pivot; the pivot's own friction heat is small at low speed, so the temperature rise comes from the heat sources and the cooling conditions. At the other end, a winter cold start takes the bushing toward its minimum temperature (see the cold section above) while the steel around it barely changes size.

An illustration of the swing: from −20 °C to 80 °C is 100 K. A free 25 mm nylon part grows about 25 × 9.9 × 10⁻⁵ × 100 ≈ 0.25 mm, against about 0.03 mm for steel. In a bore the plastic is held by the housing, so the bore closes when hot and relaxes when cold (Q1-03).

So read the grade's continuous and short-term limits against the hottest place the part sits, not against the garage. Move a pivot away from a heat source, or change to a metal bushing, before pushing a plastic past its limit (Q1-12). For suspension bushings, see our sister site's Bushings Resource Center.

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