A bronze bushing is a sleeve of cast bronze, typically 85% copper, 10% tin and 5% lead, bored 0.05-0.10 mm larger than the shaft; it outlasts iron or wood because it is hard enough to resist wear, soft enough to conform to the shaft, and does not gall against steel. How much load and speed it carries depends on its oil supply and how fast its housing sheds heat, so size it from the pressure and heat sums rather than from a quoted limit.
Part of Bearings and Bushings
The standard sliding bearing of pre-industrial machinery: a cast and bored bronze sleeve pressed into a housing, with an oil path cut into the bore.
Bronze Resists Wear Without Welding Itself To The Shaft
Two bare iron surfaces under load tear and weld together (galling); iron on iron seizes almost at once without oil. Bronze avoids this, which is why Bearing Materials calls it the best bearing material of the period. It is hard enough to resist abrasion, soft enough to deform slightly and spread the load across the shaft, holds lubricant on its surface, and has a lower friction coefficient than iron or steel: 0.02-0.06 lubricated against steel, compared with 0.05-0.15 for steel on steel.
The lead in the usual mix is a soft phase scattered through the harder bronze. It wears away first, which helps the bearing run in to the shape of the shaft, and it traps small contaminants. It is a sacrificial phase, there to run in and trap dirt, so more lead does not make a better bearing.
| Alloy | Cu | Sn | Pb | Other | Use |
|---|---|---|---|---|---|
| Cast bronze (basic) | 85% | 10% | 5% | - | General bearing use |
| Leaded bronze | 83% | 7% | 7% | 3% Zn | Heavier loads |
| Phosphor bronze | 95% | 4-8% | - | 0.1% P | Hard; precision work |
| Gunmetal | 88% | 10% | - | 2% Zn | Cannon and bearings |
The alloy table is taken from Bearing Materials. For how to melt and alloy bronze, see Bronze Making.
What You Need
- A cast bronze blank (see Casting Bronze Bushings)
- A lathe able to bore to a diametral clearance of 0.05-0.10 mm (see Boring and Finishing)
- A smooth shaft journal: a rough shaft destroys a new bearing in hours (Plain Bearings)
- An oil path: a hole to a groove, a cup, or a wick (see Lubrication Systems)
Method: Size The Bushing From Pressure And Heat
Step 1: Pick the alloy. Basic cast bronze for general work; leaded for heavier loads; phosphor bronze where hardness matters.
Step 2: Set the diameter from the shaft, not the other way round. The shaft is sized for torque and bending first (Bearings and Shafts). The bushing bore is that diameter plus the clearance.
Step 3: Choose the length and compute the bearing pressure. Projected pressure is load divided by bore diameter times length.
Step 4: Compute the heat. Use the sum in Friction Fundamentals.
Step 5: Compare against a proven design. No allowable pressure for bronze is given in this wiki, so do not borrow the babbitt thrust-face figure from Thrust Bearings, which is for a different geometry and metal. Copy the length-to-diameter proportions of a bushing known to work at a similar load and speed, and run yours in under reduced load.
Step 6: Fit and align. Press the bushing in (see Interference Fits) and check alignment to the carrier’s 0.1 mm per metre of span.
Worked Example
Load 2,000 N on a 50 mm shaft, bushing length 75 mm (a length chosen for the example, not a rule). Projected area = 0.05 x 0.075 = 0.00375 square metres. Pressure = 2,000 / 0.00375 = 533,000 Pa, about 0.53 MPa.
At 100 rpm the surface speed is 0.262 m/s and the lubricated heat is 10.5-31.4 W (worked in the friction page). Halving the length to 37.5 mm doubles the pressure to about 1.07 MPa but does not change the heat, because heat depends on load and speed, not on length. Length buys pressure margin, not cooling.
Common Mistakes
- Mixing alloys from scrap into a bearing pour. Zinc, lead and tin proportions change hardness and behaviour; weigh the charge.
- Reaching for a thick bushing for a bent shaft. A bushing cannot straighten a shaft; fix alignment instead.
- Forgetting the oil path. A bronze bushing with no way for oil to reach the bore runs dry however good the alloy.
- Putting a groove in the load zone. Grooves go away from the bottom of the bearing.
Safety
Molten bronze and a spinning chuck
Bronze is poured around 1,000-1,050°C into a mould that must be dry; any trapped moisture flashes to steam and throws metal. Wear eye and face protection and leather. When boring, the work spins in the chuck: clamp it as described in Workholding, keep long hair and sleeves clear, and know where the lathe stop is.
What’s Next
- Learn next: Casting Bronze Bushings - turning a pattern into a bronze blank
- Combine with: Split Bearings - when the shaft cannot be slid through a one-piece sleeve