What is each good at?
Each of the four wins on one property and pays for it on another. The numbers are typical values for unfilled grades, not ratings.
| Material | Strong points | Watch for |
|---|---|---|
| Acetal (Delrin, POM) | Stiff, resists creep, holds tight tolerances, low friction, takes up very little water (about 0.2% at saturation). Dry PV limit about 10,000 psi·ft/min | Less tolerant of shock than nylon |
| UHMW-PE | Very high impact strength and abrasion resistance, does not absorb water, resists most chemicals, nearly self-lubricating | Soft, so lower stiffness, lower PV limit and a lower continuous-service temperature than acetal |
| Nylon | Tough under impact, inexpensive, available in large sizes, damps vibration | Absorbs water (about 1.5 to 3.0% by weight at equilibrium in ambient humidity, more when soaked) and swells, so a bore sized dry can close up wet |
| Polyurethane | Gives and damps vibration, carries load | Size and stiffness change with temperature; limited heat and moisture tolerance |
Acetal is the default for a general bushing because it is the best compromise between stiffness, friction and dimensional stability. Filling it with PTFE lifts the PV limit to roughly 15,000 psi·ft/min and lowers friction further; Ready Plastics' bushing guide is the source for these PV figures. The rule of thumb underneath the table: choose acetal when the part must hold its size and resist creep, and UHMW when the surface will be hit, rubbed or scraped.
What does the load and speed tell you?
PV, the pressure on the bushing times its sliding speed, is the first filter (Q1-02). A material that passes on PV still has to survive the temperature, the moisture and the chemicals around it, so treat PV as the first screen and not the final answer.
What does water do?
- Acetal and UHMW absorb little or no water, so their size stays predictable (Q1-03).
- Nylon swells. Size the bore for the worst-case swollen size, not the dry one. As an illustration, a nylon bushing machined to 1.000 inch dry can close to about 0.994 inch after soaking, which is enough to turn a free-running fit into a tight one.
- Polyurethane does not tolerate heat plus moisture. Steam and long exposure to humid heat above about 70 °C break down the polymer chains permanently, so a bushing that has been through it does not recover when it dries.
What does temperature do?
Every polymer has a ceiling that steel does not. Polyurethane's usual range runs from about −62 °C to 93 °C, with special formulations reaching 150 °C (Gallagher), and it expands roughly ten times as much as steel, so the fit must allow for it (Q1-03). UHMW has a lower continuous-service temperature than acetal. The ceiling for a given grade is a datasheet value (Q1-16).
How do you decide?
Start from the demand that would hurt most if missed. A precise, stiff pivot points to acetal. A wear surface with impact, abrasion or washdown points to UHMW. Shock and tough duty in a dry place points to nylon, with the bore sized for swell if it will ever be wet. A need for damping and some give points to polyurethane, within its temperature limit. If lubrication is impossible, look at filled grades such as PTFE-filled acetal, and see Q1-06. If the part replaces bronze or steel, the material is only part of the change; see Q1-12.
Which article answers which question?
| If you need to… | Read |
|---|---|
| Check the load and speed against a PV limit | Q1-02 |
| Size the running clearance for heat and moisture | Q1-03 |
| Press the bushing in and keep it there | Q1-04 |
| Choose the shaft material and finish | Q1-05 |
| Run it dry or lubricate it | Q1-06 |
| Use it in washdown or chemicals | Q1-07 |
| Diagnose wear, seizing, squeaking or walk-out, or deal with grit | Q1-09 |
| Inspect and replace it | Q1-10 |
| Swap a bronze or steel bushing for plastic | Q1-12 |
| Choose sleeve, flanged, split or thrust form | Q1-13 |
| Find the maximum operating temperature | Q1-16 |
Does any of this change on an ATV, UTV or off-road vehicle?
Mostly the choice is the same, but one part of the vehicle is covered elsewhere. Rubber and polyurethane suspension bushings, in control arms, radius rods and sway-bar links, are a different job with different loads: see the Bushings Resource Center on our sister site, syzmachine. Our forged poly bushings page covers polyurethane bushings for link-type flexible joints (flex joints).
This article applies to the other pivots on the vehicle: pedal and shifter pivots, throttle and brake-pedal linkage, latch and hinge pins, and the pin sleeves on racks, doors and tailgates. These pivots are lightly loaded and slow, so PV rarely decides the material (Q1-02). The environment does. Mud and sand are abrasive, so keep grit out with a seal or a flange before you change the material (Q1-09). A vehicle that is pressure-washed often, or stored wet, has a wet duty, which is where nylon's moisture swelling matters (Q1-07). Temperatures swing wider than in a plant, from a sun-baked panel to a winter start, so check the grade's limits (Q1-16). Pivots in steering, brake or suspension linkages are safety parts: take the material and the service limit from the vehicle maker.