Bearing Materials

Part of Roads and Transport

Selecting and preparing materials for plain bearings in axles, wheels, and machinery.

Why This Matters

A bearing is the interface where two surfaces move relative to each other. Its job is to support load while minimizing friction. The materials chosen for the bearing surfaces determine how much friction is generated, how fast the bearing wears, how often it needs lubrication, and how long the vehicle or machine lasts.

Choosing the wrong bearing material is costly. Iron-on-iron bearings without lubrication seize almost immediately. Wood-on-wood bearings wear rapidly, require frequent replacement, and need constant lubrication. Bronze-on-iron bearings, properly made and occasionally greased, last years of heavy service.

In a rebuilding environment where bearing replacement is difficult and downtime is expensive, choosing the best available bearing materials and manufacturing them correctly is a technical decision with major economic consequences.

Tribology: The Science of Friction

Friction coefficient (μ): The ratio of friction force to normal load. Lower is better.

Material pair	Dry μ	Lubricated μ
Steel on steel	0.50–0.80	0.05–0.15
Cast iron on cast iron	0.40–0.70	0.05–0.10
Bronze on steel	0.10–0.20	0.02–0.06
Bronze on iron	0.10–0.18	0.02–0.05
Wood on wood	0.25–0.50	0.10–0.25
Wood on iron	0.20–0.40	0.05–0.15
PTFE on steel	0.04–0.10	0.02–0.04

Key insight: Bronze against iron, properly lubricated, generates less friction than steel against steel even when poorly lubricated. Bronze is the pre-industrial era’s best bearing material and remains excellent today.

Wood Bearings

The earliest bearing material was wood:

Best woods for bearings:

• Lignum vitae (self-lubricating due to natural oils; exceptionally hard and dense)
• Elm (used for wooden axle bearings)
• Oak (general purpose; harder than elm)
• Any dense, fine-grained hardwood

Construction: The bearing block is shaped to cradle the axle. A slot in the top allows periodic greasing. The wood compresses slightly under load, conforming to the axle surface — this helps distribute pressure.

Lubrication for wood bearings: Tallow (animal fat) is the traditional lubricant. The wood absorbs some fat, providing reserve lubrication. Relubricate every 20–50 km depending on load.

Failure mode: Wood wears into the shape of the shaft, eventually allowing so much play that the vehicle wobbles or the hub shifts. Worn bearing blocks are easily replaced.

When to use: Light loads; slow speeds; when metal is not available. An adequate stopgap; not a permanent solution for heavy service.

Bronze Bearings

Bronze (copper-tin alloy) is the gold standard of plain bearing materials:

Why bronze is so good:

• Hard enough to resist wear
• Soft enough to deform slightly under load (conforming to the shaft)
• Natural affinity for lubrication (lubricant sticks to bronze surface)
• Low friction coefficient against iron and steel
• Does not gall (weld under pressure) against iron — unlike two iron surfaces

Standard bearing bronze compositions:

Alloy	Cu	Sn	Pb	Other	Notes
Cast bronze (basic)	85%	10%	5%	—	General bearing use
Leaded bronze	83%	7%	7%	3% Zn	Better for high load
Phosphor bronze	95%	4–8%	—	0.1% P	Hard; good for precision work
Gunmetal	88%	10%	—	2% Zn	Traditional cannon & bearing alloy

Lead additions reduce friction slightly and help the bearing “run in” by sacrificially wearing to conform to the shaft. Phosphorus in small amounts hardens the alloy significantly.

Making bronze: Melt copper, add tin (and optionally lead). Bronze melts at approximately 950–1000°C depending on composition — achievable in a charcoal crucible furnace. Pour into molds. Allow to cool slowly for best structure.

Machining bronze: Machines very well. Turn on a lathe with HSS tooling. The finished bearing should be press-fit into the hub (typically 0.01–0.03 mm interference) and have a running clearance on the axle (0.05–0.10 mm for heavy loads; 0.02–0.05 mm for light precision bearings).

Babbit Metal (White Metal)

Invented in 1839 by Isaac Babbitt, this soft alloy is used for lining harder housings:

Basic composition: Tin (~88%), antimony (~7%), copper (~5%)

Why it works: Babbit is soft enough to embed particles (preventing scratching of the shaft). Hard crystals in a soft matrix — the hard antimony-tin crystals carry the load while the soft tin matrix lubricates.

How to use: The bearing housing (usually iron or bronze) is tinned, then molten Babbit is poured in. When the Babbit solidifies, it is machined to final size. The soft metal is sacrificial — when the bearing eventually fails, the soft Babbit wears before the hard shaft.

When to use: High-speed shaft bearings (mills, engines); when shaft runout must be minimized; when shafts are valuable and must be protected.

Iron-on-Iron (Last Resort)

Cast iron against cast iron or steel runs acceptably if:

• Both surfaces are machined smooth
• Lubricated before and during every run
• Load is moderate
• Speed is slow

Failure mode: Galling — the surfaces weld microscopically and tear apart. This creates a surface rougher than what you started with, which wears faster, which warms up more, which causes more galling. A runaway failure process.

When acceptable: Temporary repairs; very light loads; when bronze is not available; with constant monitoring and lubrication.

Oil Groove Design

Any plain bearing needs a method to distribute lubricant:

Straight groove: A straight channel along the length of the bearing bore. Simple; adequate for very slow bearings.

Helical groove: A spiral groove wound around the bore. As the shaft rotates, it pumps oil along the groove and distributes it circumferentially. Much better than straight groove for rotating shafts.

Circumferential groove at mid-length: A ring groove at the middle of the bearing where oil is introduced through a hole from outside. Oil distributes sideways from the ring. Good for long bearings.

Best practice: Use a helical groove cut to 30–50% of the bearing width. The groove provides oil storage and distribution. Fill with grease before assembly; replenish through an external oiling hole.

Choosing the right bearing material — bronze whenever possible, with oil grooves and regular lubrication — is one of the most cost-effective engineering decisions in vehicle design. The difference in lifespan between well-designed bronze bearings and improvised iron-on-iron bearings can be a factor of 10 or more.