How do you read a failed bushing?
The debris and the damage pattern tell the story:
| What you see | What it usually means | Where to look |
|---|---|---|
| Fine powder around the bushing | Slow wear as two surfaces grind. Expected at end of life. If it appears early, look at the shaft surface, at grit, or whether the PV limit was exceeded | Q1-05, Q1-02 |
| Large chunks or fragments | Brittle failure, not wear: overload, shock or impact, or the wrong material. Also seen with stick-slip, where material tears off during the "slip" | Q1-01, K02-Q1-13 |
| White stress lines or visible deformation | The plastic was strained past its elastic range. A warning that the safety margin on load was too small | Q1-02 |
| Melted or smeared material | Too much heat from high PV, poor heat escape, or chemical attack that softened the plastic | Q1-02, Q1-07 |
| Damage on one side only | Misalignment or edge loading: the bushing was not carrying load evenly | Q1-13 |
Failures cascade. Corrosion roughens a shaft, that speeds up abrasive wear, which makes heat, which softens the plastic, which increases deformation and friction. The first failure in the chain is the root cause, so trace back from the worst damage to find it.
Why does a plastic bushing wear out early?
Check the four usual causes in this order.
- Load times speed too high. Pressure and sliding speed together make frictional heat. Every bearing plastic has a PV limit above which wear speeds up and the material softens or fails, and a steel housing carries heat away much faster than a plastic one, so the same PV can give different results (Q1-02).
- The shaft. Softer shafts and rough finishes speed up bushing wear. Hard-chromed or hardened stainless shafts usually give the lowest wear (Q1-05).
- Grit. See the next section.
- A harder plastic is not always better. Harder, glass-filled structural plastics can wear faster than a purpose-made bearing compound, because they lack the solid lubricant and rub more abrasively against the shaft.
What does grit do, and why is the seal the first fix?
Dust turns the joint into a grinder. Sand, mineral dust or metal shavings get between the shaft and the bushing and act like sandpaper, wearing both surfaces far faster than the material's strength would suggest. A single particle slipping past a worn seal can also turn lubricant into an abrasive paste.
That is why the seal comes before the bushing. In most dusty places the seal design matters more than the bushing material, and upgrading the material alone is rarely enough. A quarry conveyor pivot reported by the manufacturer cnfesoda failed every three months because a felt washer had become saturated with dust and held it against the shaft; after the housing was changed to a multi-lip polyurethane seal, the same bushing lasted more than two years (a vendor-reported case). Standard lip seals also degrade under mud caking and chemical washdown, and over-greasing can create escape paths that pull grit into the joint, so more grease is not a cure for a poor seal.
Plastic and bronze differ here. Plastics do not embed grit, so a rough shaft abrades them, whereas bronze can embed particles in its softer surface so they do not score the hardened pin. A plastic bushing therefore leans harder on the seal and the shaft. The soft PTFE layer of a composite bushing can also be damaged by sharp, hard particles, so such composites are not suitable for severely abrasive places without good sealing.
Why does a plastic bushing seize or bind?
Because the bore got smaller than the shaft, or the part got hot.
- Swelling. Some plastics absorb water and swell, which changes clearance and fit, and standard nylon in wet service would swell and bind. Nylon takes up about 1.5 to 3.0% water by weight at equilibrium in ambient humidity, and more when soaked (Q1-03, Q1-07).
- Heat closes a pressed-in bushing. Polymers expand substantially more than steel for each degree. A bushing pressed into a rigid housing cannot grow outward, so its wall grows inward and the bore gets smaller. For a nylon bushing in a steel housing with a 25 mm bore and a 50 K rise, the running clearance shrinks by about 0.022 mm with a 2.5 mm wall and 0.044 mm with a 5 mm wall (the bore closes and the steel shaft grows), a simplified estimate worked in Q1-03, where an elastic calculation gives roughly a quarter to a third more. A clearance set at room temperature can close up when hot. A free bushing, not held by a housing, does the opposite and gets looser.
- Measuring the wrong size. A press-fit bushing reaches its final bore size only after it is pressed in. A clearance worked out from the free-state size, before pressing, leads to interference with the shaft (Q1-04).
- Heat from the bushing itself. If PV is too high, the plastic softens and smears (see the table above).
Why does a plastic bushing squeak?
Two causes.
- Stick-slip. Squeaking is the key symptom to listen for. In stick-slip the shaft sticks, then jumps, and material can tear off in the slip phase. Very smooth chromed surfaces can sometimes cause stick-slip at low speed. See K02-Q1-13 for the general cause, and Q1-05 for the right finish.
- A bushing turning in its housing. A bushing that has lost its grip can still rotate in the bore. This leads to squeaking and worse running.
Why does a plastic bushing crack or break?
Chunks and fragments point to overload, shock or impact, or an unsuitable material. White stress lines show the plastic was pushed beyond its elastic range but not yet to a fracture. Check whether the load case includes shock or impact beyond what the material can take (Q1-01, Q1-02).
Why does a plastic bushing walk out of its housing?
A press-fit bushing is held by squeeze: the bore is made slightly smaller than the bushing's outer diameter. If that grip is lower than the push-out force from load and movement at the pivot, the bushing moves. The result is a metal shaft scraping a metal housing and a bushing torn to pieces.
The grip can be lost in three ways:
- Temperature above the material's limit. The plastic relaxes and cannot hold its tension. Even a hot cycle that stays within the limit can leave a bushing looser once it cools, if the plastic has crept or relaxed while it was squeezed.
- A high load for a long time. This is "creep", a slow deformation under load.
- A housing bore that is out of tolerance. It sounds trivial, but it is a common cause. A bore that is too big makes walking out more likely.
Check the temperature and the surface pressure, and keep the bore and shaft tolerances correct, with an H7 housing bore and at least an h9 shaft as a typical starting point. Where the bushing is moved axially or sees axial force, press fit alone is not enough: add axial securing such as flanges, snap-in features, reflanging or screws. Ask your maker for the temperature above which securing is needed, even for low axial forces, for your grade (Q1-04).
What should you do after a failure?
Read the debris and the damage pattern first (table above), then trace back to the first failure in the chain. Recheck the usual suspects before you fit another: PV and temperature, shaft hardness and finish, grit and the seal, bore size and fit, and moisture and chemicals (Q1-10).
If you swapped a greased metal bushing for a dry-running plastic one, removing the grease also removes a barrier that protected the steel. The shaft and housing then need their own corrosion protection. If the bushing is plastic and the machine works in the open, check the grade: nylon and POM need a UV-stabilised grade outdoors (Q1-07).