What does each material do best?
Each material is the right answer to a different question, and the question is set by the load, the speed, the environment and the lubrication plan.
Bronze is the strong, heat-tolerant option. Its strength, heat conduction and wear resistance suit high loads and high temperatures, and it gives consistent load support. The price is weight and a lubrication job that has to sit in the maintenance plan.
Plastic is the low-maintenance option. It suits light to medium loads, joints that are hard to lubricate, humid or chemically corrosive places, and jobs where some wear and a little elasticity are acceptable. Self-lubricating grades need no oil or grease.
Hardened steel is the third option for the roughest joints. Bronze is soft and forgiving (a common bearing bronze is around 65 Brinell), whereas hardened steel sits at about HRC 50 to 62 (ebushing). Bronze suits continuous rotation at moderate, steady load. Hardened steel suits shock, slow movement through a small arc, and high pressure on a small area.
When does plastic win?
Plastic is the better first choice when one or more of these is true:
- You cannot grease the joint. A bushing behind a guard, or on a machine that is rarely opened, will not be greased on schedule. A plastic bushing with built-in lubricant removes the job (Q1-06).
- The place is wet or corrosive. Plastic does not rust. Check the exact grade against the chemicals and the temperature (Q1-07).
- Grease would be a problem. Where grease attracts dirt or must stay out of a process, running dry is an advantage.
- Noise or weight matters. Plastic is light and runs quietly.
- Shaft cost matters. Bronze needs a shaft harder than the bronze, while a plastic bushing can run on many shaft materials, which gives you more options on shaft cost. Check the shaft hardness and finish the grade needs (Q1-05).
When do bronze or steel win?
Keep the metal bushing where:
- The load is heavy or comes as shocks. A steady running load says very little about the impact when a machine jams. Under sustained shock loading, metal is often the safer choice even where plastic looks adequate on a steady-load calculation.
- The joint is hot. Plastics have a more limited temperature range than bronze, so check the grade's limit first (Q1-16).
- You will really grease it. A greased bronze or steel joint works well, but only if the grease goes in.
- The pin is the expensive part and you want a sacrificial liner. A bronze bushing is designed to wear so that the pin does not. A plastic bushing can do the same job, but only inside its pressure and speed limits (Q1-02).
Between bronze and steel, bronze suits steady loads and continuous turning, while hardened steel suits shock, a slow sweep through a few degrees, and high unit pressure. Many pin joints on construction, mining, logging and farm equipment fall into the second group. A plastic bushing is not a substitute in that second group unless the maker confirms the load.
What changes if you swap?
Four things change, and each one can undo the swap if it is missed.
The shaft. A bronze bushing is made to run against a harder shaft. A plastic bushing also needs the shaft to be hard enough and finished to a suitable roughness, or it wears fast. Do not assume the old shaft will do. Check it against the grade (Q1-05).
The clearance and the heat. Plastic expands more with temperature than steel does, and some plastics take up water, so the running clearance behaves differently (Q1-03). Heat also leaves the joint differently: a steel housing carries heat away much faster than a plastic one, and a plastic bushing itself conducts almost nothing, so the same load and speed can give different results. Bronze itself expands about half again as much as steel, and most bearing plastics expand several times as much as either, so the clearance set for bronze will not suit plastic.
The lubrication and the corrosion protection. Dropping grease also drops the barrier that protected the steel. The shaft and the housing may now need their own corrosion protection (Q1-09).
The fit in the housing. A press-fit plastic bushing reaches its final bore only after it is pressed in, so a clearance worked out from the free size will be wrong (Q1-04).
What about a dusty place?
In dust, abrasive wear matters more than material strength, so the seal comes before the material. Plastic and bronze also react differently to grit. Plastics do not embed grit, so a rough shaft abrades them, while bronze can embed particles in its softer surface so they do not score the hardened pin, although porous sintered bronze can be clogged by dust. A swap from bronze to plastic therefore hands the protection to the seal and the shaft: check that the seal is sound first, then the shaft hardness and finish (Q1-09, Q1-05).
What should you check before you swap?
Work through this in order. Start with bearing pressure: divide the load by the projected area, which is bore times length. A 2 kN load on a bushing with a 25 mm bore and 25 mm length gives 2 000 ÷ (25 × 25) = 3.2 MPa, which you compare with the maker's limits for the grade, together with the speed (Q1-02). Then find the peak load, the worst case such as a jam or an impact, not just the steady load. Compare the highest temperature, including heat from friction, with the grade's limit. Check the shaft hardness and finish and the housing bore tolerance. Decide whether the joint will still be greased, and protect the steel if it will not. Finally, ask the maker for the grade's data for your load, speed and temperature, and say what you are replacing.