Lubricant Types

Part of Gear Making

Choosing and preparing lubricants for gears and bearings from available materials.

Why This Matters

Lubrication is not optional in gear systems. Metal sliding on metal without lubrication generates heat, causes adhesive wear (galling), and leads to rapid failure. A thin film of oil between gear tooth flanks reduces friction by orders of magnitude and dramatically extends the life of both gears and bearings.

In the early rebuilding period, petroleum-based lubricants may not be available. This is not a crisis — industrial machinery ran for millennia on plant and animal-derived oils and greases before petroleum lubrication was understood. Many of these traditional lubricants are excellent for slow-to-moderate speed applications. Understanding what makes a good lubricant, and which available materials serve that function, allows you to lubricate machinery effectively from local resources.

Choosing the wrong lubricant — too thin, too contaminated, or the wrong type for the temperature or load — is as bad as no lubricant at all. Knowing the properties that matter and how available materials compare on those properties enables informed choices.

What a Lubricant Must Do

Separate surfaces: The primary function. A lubricant must maintain a film between moving surfaces under the operating loads and speeds. If the film collapses, metal-to-metal contact occurs.

Reduce friction: A fluid lubricant in a bearing or gear contact allows surfaces to slide on the fluid rather than on each other. The viscosity of the fluid determines the friction force. Too thick (high viscosity): high churning losses. Too thin (low viscosity): film breaks down under load.

Carry away heat: Friction generates heat. The lubricant must conduct heat away from the contact zone to the surrounding structure. Lubricant flow (in a pressurized or splash system) carries heat away; stagnant grease only conducts it away slowly.

Resist degradation: Lubricants are chemically attacked by heat, water, air, and the acids produced by combustion or oxidation. A lubricant that breaks down quickly must be replaced frequently; a more stable one lasts longer.

Animal Fats and Oils

Animal-derived lubricants served machinery for millennia and remain entirely adequate for low-to-moderate speed applications.

Tallow (rendered beef or mutton fat): A semi-solid grease at room temperature. Melts at about 40–45°C. Used as a bearing grease since ancient times. Works well for slow-moving bearings (cart axles, millstone spindles, door hinges). Limitations: attracts insects and rodents (it is food), oxidizes and goes rancid within months, unsuitable for temperatures above 40°C.

Lard (rendered pork fat): Similar to tallow but slightly softer. More likely to remain workable at moderate temperatures. Same limitations as tallow.

Whale oil (historically important, now unavailable): Excellent lubricant, highly refined, with good low-temperature properties. Mentioned for historical context; not a viable source.

Neatsfoot oil (rendered from cattle hooves and leg bones): A liquid oil with good lubricating properties. Traditionally used for leather conditioning but also as a machinery lubricant. Slower to oxidize than tallow. Works for gears and bearings at low speeds.

Goose or duck fat: More viscous than lard, better temperature stability among animal fats. Traditional lubricant for weapons, hinges, and machinery.

Fish oil (particularly cod liver oil): High in unsaturated fatty acids. Tends to polymerize (thicken and harden) on exposure to air — this is useful for some applications (oil finishes on wood) but undesirable in lubricants. Not preferred for gear lubrication.

Plant Oils

Castor oil: Excellent lubricant. High viscosity, stable at high temperatures compared to most plant oils, non-drying (does not polymerize in air). Used as a high-performance engine lubricant in aircraft racing engines as recently as the early 20th century. Castor plants grow in warm climates and are the preferred plant-derived lubricant where available.

Linseed oil (raw): Readily available from flax. Moderate viscosity. Unfortunately it is a drying oil — it polymerizes in air, forming a tough varnish-like film. This means linseed oil will eventually solidify in gear housings and clog lubrication passages. Not suitable for gear or bearing lubrication. (Boiled linseed oil dries faster and is even worse.)

Olive oil: Moderate viscosity, non-drying, stable. Good lubricant for lightly loaded gears and bearings where it is available. More valuable as food and not the best use of the resource.

Rapeseed oil (canola): High-erucic-acid rapeseed oil was an important industrial lubricant in the 18th–19th centuries, particularly for steam engine cylinder lubrication. Non-drying, good film strength. Modern canola has low erucic acid and somewhat inferior lubrication properties. Adequate for light-to-moderate gear lubrication.

Sunflower oil: Non-drying, moderate viscosity. Adequate for light machinery lubrication.

Mineral Oils

If petroleum or coal tar products become available through distillation (see petroleum-and-tar article), mineral oils provide the best lubricant performance:

Light mineral oil (similar to machine oil or spindle oil): Low viscosity, suitable for high-speed bearings and light-duty gears.

Medium mineral oil (gear oil equivalent): Moderate viscosity, suitable for moderate-speed, moderate-load gears.

Heavy mineral oil (gear oil, similar to SAE 90–140): High viscosity, suitable for slow-speed, high-load gears (particularly worm gears).

Mineral oils do not go rancid, do not polymerize, and are not food — they resist contamination and last much longer between changes than animal or plant oils.

Selecting Lubricant Viscosity

Higher viscosity lubricants maintain thicker films but generate more churning drag. Lower viscosity lubricants have less drag but thinner films that break down more easily.

Rules of thumb:

• Slow-speed, heavy-load gears (water mill drives, windmill drives): use a thick oil or grease (high viscosity)
• Moderate-speed, moderate-load gears: medium oil
• High-speed, light-load gears (clocks, fine mechanisms): thin oil
• Worm gears: always use heavy-duty oil — the sliding contact generates high temperatures and requires thicker film
• Open gears (not enclosed in a housing): use grease rather than oil to prevent throwing-off by centrifugal force

Replenish animal and plant oil lubricants every 1–3 months in moderate-duty applications. More frequent changes extend gear life significantly by removing wear particles and degraded oil.