Bearings and Shafts

Part of Gear Making

Designing and making shafts and plain bearings to support rotating gears in low-technology machinery.

Why This Matters

Gears transmit torque through their meshing teeth, but they need shafts to rotate on and bearings to support those shafts. Without adequate shafts and bearings, even perfectly made gears will wobble, wear rapidly, overheat, and fail. The shaft and bearing system is the foundation the gear drive is built on — its quality directly limits the performance and life of everything mounted on it.

In the early stages of rebuilding, ball and roller bearings are not available. All rotating machinery must use plain bearings — a shaft turning inside a close-fitting bore, separated by a film of oil or grease. Understanding how plain bearings work, what makes them succeed or fail, and how to make them from available materials allows machinery to function reliably without modern bearing manufacturing.

Plain bearings are actually excellent in many applications. Large industrial plain bearings in turbines and engines outlast rolling element bearings in high-load, steady-speed applications because they develop a hydrodynamic oil film that keeps the surfaces apart entirely — pure fluid lubrication with no metal-to-metal contact. The key is adequate lubrication, correct clearance, and appropriate materials.

Shaft Design

The shaft transmits the torque from the gear to the driven load (or from the driving source to the gear). It must be strong enough to carry the torques and bending moments imposed on it without yielding or fatigue fracture.

Torque. The torque a shaft must transmit equals the power divided by the rotational speed (T = P/ω). For hand-tool machinery at modest power levels, shaft sizes of 20–50 mm diameter wrought iron or steel are generally adequate.

Bending. Gears impose radial loads on the shaft through the normal tooth force (the force perpendicular to the tooth surface at the contact point). This force has a component along the line of centers (separating force) and a tangential component (transmitted force). Both must be carried as bending moments in the shaft between its supports (bearings). Calculate or estimate the gear tooth load and ensure the shaft diameter provides adequate bending strength.

Stiffness. Beyond strength, shafts must be stiff enough that they do not deflect significantly under load. Shaft deflection at the gear changes the center distance and alignment — the exact problems alignment checking tries to eliminate. Deflection under tooth load should be less than 0.05–0.1 mm at the gear face location.

Materials. Wrought iron has adequate strength for moderate-duty shafts. Cast iron is brittle in torsion and should not be used for shafts. Low-carbon steel (if available) is better — stronger and tougher. For high-duty applications, medium-carbon steel hardened and tempered provides excellent shaft performance.

Surface finish at bearing locations. The shaft journal (the portion running inside the bearing) must be smooth and cylindrical. Surface roughness that would be imperceptible to the touch creates high local stresses in a thin oil film bearing. Grind, file, and polish the journal surfaces. Achievable without a lathe using draw-filing (flat file drawn along the shaft axis) followed by polishing with fine abrasive wrapped around a flat stick.

Plain Bearing Geometry

The plain bearing consists of a bearing housing with a bore that closely fits the shaft journal. The clearance between shaft and bore — the difference in diameter — is critical.

Too little clearance: Insufficient room for oil film, high friction, tendency to gall (cold-weld and seize). Too much clearance: Oil film is weak and easily broken, shaft runs eccentrically, noise and vibration.

The correct radial clearance (half the diameter difference) is approximately 0.001 × shaft diameter for smooth, well-lubricated bearings. For a 30 mm diameter shaft, this is 0.03 mm radial clearance — 0.06 mm diameter clearance. This is tight but achievable with careful fitting.

For low-speed, intermittently loaded bearings (as in many human-powered machines), looser clearances of 0.002–0.003 × diameter are acceptable and easier to achieve.

Length-to-diameter ratio. Bearing length should be 0.5–1.5 times the diameter. Very short bearings allow shaft tilting; very long bearings are difficult to lubricate uniformly and sensitive to shaft misalignment.

Bearing Materials

The bearing material must be:

• Softer than the shaft (so the bearing wears rather than the shaft — the bearing is replaceable, the shaft is not)
• Capable of embedding abrasive particles without letting them scratch the shaft
• Resistant to galling
• Thermally conductive (to remove heat)

Bronze (copper-tin alloy): The classic plain bearing material. Excellent anti-friction properties, embeddability, and hardness compatibility with iron and steel shafts. Bronze was used for plain bearings for thousands of years before ball bearings were invented. Ideal composition for bearings: 88% copper, 10% tin, 2% zinc. Casting bronze bushings is achievable with basic metalworking.

Babbitt metal (tin-base white metal): Extremely good bearing material, used in all large industrial plain bearings. Very soft, embeds abrasives without damage, excellent anti-friction properties. Must be cast onto a steel or bronze backing shell. Tin alloy: approximately 88% tin, 8% antimony, 4% copper.

Wood: Hardwoods (lignum vitae especially, also oak, maple) work as plain bearings in water-lubricated applications (water mills, ships) and for light-duty land machinery. Lignum vitae is so dense and oily it is self-lubricating in many conditions. Suitable for low-speed, moderate-load applications.

Iron-on-iron: Functions as a bearing with continuous lubrication and low surface speed but not recommended — galling risk is high and wear is much faster than proper bearing materials.

Lubrication Systems

Plain bearings require continuous lubrication to maintain the oil film. Options for early rebuilding:

Oil cup: A reservoir directly above the bearing, connected by a hole into the bearing bore. Oil drips by gravity. Simple, reliable, requires periodic refilling. Adequate for light to moderate duty.

Grease packing: Bearing cavity filled with grease (animal fat or mineral grease). Grease is retained by seals or labyrinth clearances. Works at lower speeds where dynamic oil film cannot form. Requires repacking every few weeks to months depending on duty.

Forced lubrication: An oil pump circulates oil from a reservoir through channels to the bearing surface. Necessary for high-speed, high-load bearings. Complex to build but enables much higher performance.

Animal fats (tallow, lard) served as lubricants for millennia and work adequately for low-speed plain bearings. They are degraded by high temperatures, oxidize over time, and can harbor microorganisms, so they must be replaced regularly. Mineral oil is far superior when available.