Polymer Bearings: Plastics That Replace Steel

Not every rotating joint needs a rolling bearing of hardened steel. Plastic and composite bearings — polymer bushes, plastic rolling bearings and fibre-reinforced slide bearings — solve a family of problems that steel handles badly: washdown and chemicals, weight, electricity, noise, marginal lubrication, and cost at volume. They also fail in characteristic ways when they are treated like steel, so knowing where polymers genuinely win and where they quietly lose is a material selection skill worth having.

This guide surveys the polymer options and the design rules that keep them working, from plain POM and nylon bushings to moulded rolling bearings and filament-wound composites.

The polymer families in plain terms

Three groups cover most applications. The first is plain polymer bushes and slide bearings, typically acetal (POM), nylon (PA) or high-performance plastics such as PEEK and PTFE-filled compounds, often with fillers such as glass fibre or solid lubricants, running dry or marginally lubricated against steel shafts or stainless sleeves. The second is plastic rolling bearings: complete bearings with polymer rings and glass or polymer balls, often one-piece moulded and sealed for life with food-compatible grease. The third is filament-wound or fibre-reinforced composite bearings with PTFE-based liners — the heavy-duty end, carrying heavy loads in construction machinery, agricultural equipment and marine rudder stocks where greasing is impractical.

Where polymer bearings genuinely win

• Wet and washdown duty. Polymers do not rust, and complete plastic bearings survive food-plant cleaning chemistry that consumes steel housings.
• Lubrication-free operation. Plain polymer bearings run dry for life, removing grease guns, grease nipples and the failure mode of forgotten lubrication.
• Weight and inertia. A plastic rolling bearing weighs a fraction of its steel equivalent, which matters in robotics, packaging heads and anything reciprocating fast.
• Electrical and magnetic neutrality. Plastics neither conduct nor magnetise, which simplifies sensor environments and removes one path for stray currents.
• Noise and smoothness. Polymer contacts damp vibration, which packaging and consumer machinery designers value.
• Edge tolerance. Polymer plain bearings deform locally rather than spall, forgiving misalignment and edge loads that would brinell a steel raceway.

A fifth advantage hides in plain sight: chemical inertness beyond corrosion. Polymer materials shrug off acids, alkalis and solvents that attack protective coatings as well as bare steel, which is why chemical dosing equipment adopted plastic bearings early and completely. Where the medium itself is the enemy — pumps moving aggressive fluids, electroplating lines, photographic processing — the material choice is settled before any capacity table is opened.

The honest limits of plastics

Polymer strength falls with temperature: most commodity plastics lose stiffness dramatically above 80 to 100 °C, and even PEEK, the strongest common engineering plastic, is comfortable only to roughly 250 °C — far below steel metallurgy. Thermal expansion is an order of magnitude higher than steel, so clearances designed at room temperature open when hot and close when cold. Load capacity is lower, especially in rolling designs. None of these limits is obscure, but each one invalidates a naive copy of a steel-bearing design.

Creep under standing load

A plastic that is loaded continuously slowly deforms, so a bush that fits correctly on day one can lose clearance over months of standing load. Creep is worst at high stress and high temperature, and it argues for a generous initial clearance and for a material such as POM or a composite liner in preference to a soft commodity plastic, whose dry-running surface speeds are commonly limited to about 1 m/s or less. A steel bearing does not creep in this way, which is why a direct substitution so often surprises the designer.

Moisture swelling in nylon

Moisture matters as much as heat in humid or wet duty. Nylon absorbs water from the air and can swell by 0.2 to 0.5 percent, which is enough to turn a comfortable fit into an interference seizure. Where humidity is unavoidable, a moisture-stable material such as POM or a composite is the safer choice, and the fit must be designed to tolerate the change rather than assuming a dry dimension.

Plain polymer bearings and the PV limit

Plain polymer bearings live or die by three numbers: pressure, velocity and the PV product — the combination of contact pressure and sliding speed that governs frictional heating. Manufacturers publish PV limits for each material, and staying under them, with margin at the duty temperature, is the whole selection game.

The PV product

PV matters because the heat generated at the rubbing contact rises with both the pressure and the sliding speed, and a polymer has nowhere to send that heat. A bearing that is well inside its limits at low speed can overheat purely because the surface speed has risen. Shaft finish matters enormously as well: a polished, hardened or stainless shaft can extend polymer life several-fold, while a rough machined shaft acts as a file. Running clearance is larger than for steel bearings — typically 0.5 to 1 percent of the bore or more — and length-to-diameter ratios above about 2 to 1 cause edge loading rather than better guidance. Finally, embeddability is a genuine asset: soft polymer matrices swallow small contaminant particles that would score a steel contact, which is one reason these bearings thrive in dusty agriculture.

Moulded plastic rolling bearings

Moulded plastic ball bearings — polypropylene or acetal rings, glass or polymer balls, sealed with food-safe grease — are excellent in washdown conveyors, bottling plants and chemical dosing, anywhere a steel bearing dies of its environment before its fatigue. Their limits are capacity and speed: use them where loads are light and speeds moderate, and return to steel — stainless, coated or hybrid — when either rises. The economics are compelling at volume, because a plastic bearing frequently costs less than the stainless bearing it replaces while outliving it in the same washdown position.

Filament-wound composite bearings

At the industrial end, filament-wound composite bushings with PTFE liners carry heavy, slow, oscillating loads — excavator pivots, rudder stocks, hydraulic cylinder eyes — without any lubrication for years. The wrapped-bush family is standardised — ISO 3547 covers wrapped plain bushes and their boundary dimensions — so a bore and an outside diameter identify the envelope across makers even though the liner chemistry differs. A filament-wound composite liner carries roughly 3 to 5 times the static load of a plain polymer bush of the same envelope, and it holds useful capacity across a temperature band 2 to 3 times wider. Their design language differs again: high static capacity, tolerance of edge loading and shaft deflection, and installation by press fit into machined housings. For oscillating duty, where rolling bearings suffer ball sliding and grease starvation, these composites are frequently the technically correct answer, not merely the cheap one.

Choosing polymer instead of steel

The decision sequence that works starts from the environment and the lubrication reality, not from the load. The table below is the quick filter.

If loads are high, temperatures serious or precision tight, the steel families come back — with stainless or hybrid variants bridging the middle. Where a machine mixes both, polymer plain bearings and steel rolling bearings coexist happily on adjacent positions, each doing the job it is best at.

Design rules that keep polymers alive

• Design the clearance for the whole temperature range. Polymer expansion is far higher than steel, so a close room-temperature fit can seize when hot.
• Stay well inside the PV limit at duty temperature. Heat, not load, is usually what kills a plain polymer bearing.
• Specify a suitable shaft. A hard, smooth, corrosion-resistant surface multiplies life; a rough one destroys it.
• Allow for moisture swelling. Nylon grows in humid service, so the fit must tolerate the change.
• Watch creep under standing load. A continuously loaded plastic bush relaxes and the clearance can close.
• Let the material embed debris. Softness is a feature in dusty duty, not a defect.

Material tables, PV limits and clearance guidelines belong in the design notes of the technical library, and steel-family candidates remain filterable in the bearing selector. Where a polymer bearing is chosen for a plain sliding application, the same principles govern the spherical plain range; where it replaces a small rolling bearing in washdown, the comparison is against the deep groove ball range. The wider material choice for wet and aggressive plants is covered in the harsh-environment guide, the way friction and heat trade in the friction and efficiency article, and the family-level comparison in the bearing type selection guide. House-brand stock parts, including polymer and composite variants, are listed under ANDGE, and sliding-contact designs are described in the spherical plain bearing guide.