Bearings and Bushings

Phase 4 — Village Scale

Reducing friction in machines. Every rotating shaft, every wheel, every moving part in every machine needs bearings. The quality of your bearings determines the efficiency, reliability, and lifespan of all your mechanical equipment.

Why This Matters

A machine without proper bearings is a machine that wastes energy, wears out fast, and breaks when you need it most. The difference between a wooden bushing and a properly made bronze bearing is the difference between replacing parts every month and running for years.

The Industrial Revolution was as much about better bearings as it was about steam engines. Every improvement in bearing quality unlocks more efficient machines.

Friction Fundamentals

Sliding vs Rolling

Sliding friction (bushings): The shaft slides against a stationary sleeve. Simple to make, tolerates dirt and shock loads, but higher friction.

Rolling friction (ball/roller bearings): Balls or rollers roll between races. Much lower friction (1/10th to 1/100th of sliding), but harder to manufacture and less tolerant of contamination.

Rule of thumb: Use bushings for slow speeds (<500 RPM) and heavy loads. Use ball bearings for high speeds (>500 RPM) and where low friction matters (generators, spindles, vehicles).

Lubrication

Three regimes:

1. Boundary lubrication: Metal-to-metal contact with thin oil film. High friction, high wear. Acceptable for slow, intermittent motion.
2. Mixed lubrication: Partial oil film. Most bushing bearings operate here.
3. Hydrodynamic lubrication: Full oil film separates surfaces completely. Shaft “floats” on oil. Minimum friction and wear. Achieved at higher speeds with adequate oil supply.

Plain Bearings (Bushings)

Bronze Bushings

The standard bearing material for all pre-industrial and early industrial machinery.

Alloy: Cast bronze — 85% copper, 10% tin, 5% lead. The lead provides a sacrificial soft phase that traps contaminants and aids break-in.

Sand casting method:

1. Turn a pattern on the lathe (slightly oversize to account for shrinkage — bronze shrinks ~1.5%)
2. Ram green sand around the pattern in a two-part flask
3. Remove pattern, add a core for the bore (made from oil-bonded sand)
4. Pour bronze at 1,000–1,050°C
5. Cool slowly, break out, clean

Centrifugal casting (better quality):

1. Spin a steel or cast-iron mold at 500–1,000 RPM
2. Pour molten bronze into the spinning mold
3. Centrifugal force pushes denser metal outward, concentrating impurities and porosity at the inner surface
4. Bore out the inner surface to remove impurities
5. Result: dense, defect-free bushing

Finishing:

1. Chuck the bushing on a lathe
2. Bore to final dimensions: bore diameter = shaft diameter + 0.05 to 0.10 mm clearance
3. Surface finish: as smooth as possible. Target <1.6 μm Ra (achievable with careful boring and burnishing)
4. Cut an oil groove: a helical groove 1–2 mm wide × 0.5 mm deep, spiraling the length of the bore

Oil grooves

A single helical groove distributes oil across the entire bearing surface. For heavy loads, add a circumferential groove at mid-length to act as an oil reservoir. Never put grooves in the load zone (bottom of the bearing).

Wood Bearings

When bronze is unavailable:

• Lignum vitae: Self-lubricating, extremely durable. Used in ship stern tubes into the 20th century.
• Elm: Good shock resistance, absorbs grease well
• Maple/beech: Dense, fine-grained, adequate for light duty

Soak wood bearings in hot tallow or lard oil for 24 hours before installation. Relubricate frequently.

Split Bearings

For large, heavy shafts that can’t be slid through a one-piece bushing:

1. Cast two half-shells of bronze
2. Machine mating surfaces flat
3. Bolt together around the shaft
4. Shim between halves to adjust clearance as bearing wears
5. Cap bolts allow tightening to take up wear

Most water wheel, mill, and heavy machinery bearings use this design.

Ball Bearings

Difficulty level

Ball bearings are the most precision-demanding items in this entire project. They require hardened steel, grinding to within 0.01 mm, and polished surfaces. Don’t attempt these until your machine shop can reliably hold ±0.02 mm tolerances.

Making Steel Balls

Method 1 — Forging and grinding:

1. Cut short cylinders from high-carbon steel wire (1% carbon)
2. Heat to cherry red, forge into rough spheres between hemispherical dies
3. Harden: heat to 800°C, quench in oil
4. Temper: reheat to 150°C, air cool (extremely hard, ~60 HRC)
5. Grind between two grooved cast-iron plates, one rotating, one stationary
6. Add progressively finer abrasive (silicon carbide → aluminum oxide → polishing compound)
7. Final tolerance: all balls in a set must be within ±0.01 mm diameter

Method 2 — Casting: Cast small steel spheres in gang molds. Grind and polish as above. Casting gives less uniform internal structure.

Race Machining

Inner and outer races need hardened grooves:

1. Machine races from high-carbon steel on a lathe
2. Cut the ball groove with a form tool radiused to match the ball diameter
3. Groove radius = ball radius + 0.5–1 mm (allows angular contact)
4. Harden races: heat to 830°C, oil quench, temper to 200°C
5. Grind the groove surface smooth using a grinding wheel

Assembly

1. Insert balls into the space between inner and outer races (offset the races to create a gap for loading)
2. Redistribute balls evenly
3. Insert cage (retainer) — machined brass or stamped steel with pockets for each ball
4. Pack with grease
5. Fit shields or seals if needed

A well-made ball bearing should spin freely with a smooth, quiet feel. Any roughness indicates dimensional errors or surface defects.

Thrust Bearings

For loads along the shaft axis (water wheel shafts, screw presses, vertical shafts):

Flat Thrust Bearing

Two hardened, polished steel washers with a bronze or hardened ball-bearing set between them. The simplest axial bearing. Adequate for moderate loads and low speeds.

Step Bearing

For vertical shafts (windmills, vertical water wheels):

1. A hardened steel pivot on the shaft end
2. A hardened steel cup in a fixed mount
3. The entire weight of the shaft assembly rests on this point
4. Lubricate with heavy grease or oil bath
5. For heavy loads, use a bronze pad with oil groove

Lubrication Systems

Manual Lubrication

• Grease cups: A threaded cap over a reservoir above the bearing. Screw down periodically to push grease into the bearing.
• Oil holes: A simple drilled hole with a hinged cover. Add oil daily.

Continuous Lubrication

• Drip oiler: A small reservoir with an adjustable needle valve. Drips oil onto the bearing at a controlled rate.
• Wick oiler: A felt or cotton wick draws oil from a reservoir by capillary action. Self-regulating — draws more oil when bearing runs hot.
• Ring oiler: A loose ring sits on the shaft, dipping into an oil bath below. The rotating shaft carries the ring, which lifts oil to bearing level.

Lubricant Production

Lubricant	Source	Best for	Temperature limit
Tallow	Rendered beef fat	Slow, heavy bearings	40°C
Lard oil	Rendered pork fat	General purpose	50°C
Castor oil	Castor beans	High-speed bearings	80°C
Mineral oil	Petroleum distillation	All applications	120°C+
Grease	Oil + soap thickener	Sealed bearings	Varies

Making grease: Heat oil to 80°C, add 10–15% by weight of calcium soap (lime + tallow), stir until smooth and uniform. Cool slowly while stirring. Adjust consistency by varying soap content.

Fitting and Alignment

Press Fits

Bearings must be installed with the correct fit:

• Housing bore: Bearing OD + 0.00 to +0.02 mm (light press fit)
• Shaft: Bearing ID − 0.00 to −0.02 mm (bearing turns freely on shaft) for bushings
• For ball bearings: inner race is press-fit on shaft, outer race slides into housing

Never force a bearing

If a bearing won’t slide in, the fit is too tight. Forcing damages the bearing. Heat the housing (to expand it) or cool the bearing (to shrink it) for installation.

Shaft Alignment

Misaligned bearings wear rapidly and generate heat. Check alignment by:

1. Straight edge across bearing housings
2. Dial indicator on shaft — rotate and check runout
3. String line for long spans

Target: parallel to within 0.1 mm per meter of span. Angular misalignment under 0.5° for ball bearings, up to 2° for self-aligning bushings.

What’s Next

With reliable bearing production, you can build:

• Power looms and spinning frames (textile mechanization)
• Efficient water wheels and windmills
• Lathes, drill presses, and other machine tools
• Vehicles with rolling-element wheel bearings
• Generators and motors with low-friction supports