Lubrication

§1What oil is for

Separation first — everything else second: a lubricant’s defining job is to keep two loaded surfaces from ever meeting, and its other four jobs come along in the same can.

The plain-bearing page built a pressurised film; the rolling-bearing page trusted a pressure-thickened one; the threads section charged friction against every screw’s efficiency. This page is the common ground: the lubricant is a load-bearing machine element — a component with properties, a selection procedure and failure modes, not a fluid afterthought — whose first duty is separation and whose supporting duties matter almost as much. It cools, carrying away the frictional watts the Petroff arithmetic priced (in many gearboxes and engines the oil moves more heat than it saves in friction); it protects, filming steel against the corrosion pages’ oxygen and moisture between runs; it cleans, suspending wear debris and combustion soot for the filter to catch; and it seals, packing the micro-clearances of rings and glands. Selection is the art of serving all five with one fluid — and the good news of §2 is that, to a first and very useful approximation, one property of the fluid governs the whole business.

§2Viscosity, the one property

Viscosity is the oil’s resistance to being sheared — the property that builds every film and charges for every one — and the grade systems are simply viscosity written on the tin.

§3Reading the Stribeck curve

The hero’s curve organises the whole section: friction against viscosity-times-speed-over-load, falling a hundredfold from the boundary plateau to the full-film dip, then climbing the viscous slope.

Read it right to left. On the full-film side the surfaces never meet: friction is pure oil shear, low (order 0.001) and rising gently with the very quantities — viscosity and speed — that guarantee the film; the plain bearing’s hydrodynamic regime, and the region every continuously running machine is designed to occupy, just right of the dip with margin. Slide left — slower, hotter-and-thinner, or harder loaded — and the film thins into mixed lubrication: the tallest asperities begin to carry share of the load, friction climbs steeply, and wear restarts. At the far left, boundary: the film proper is gone, load rides on asperity contacts dressed only in adsorbed and reacted molecular layers, and friction sits near 0.1 — a hundred times the dip. Now the operational sentence of the section: every machine traverses this entire curve at every start and stop, because speed passes through zero — the plain-bearing page’s start-up wear, the reason bearings are judged in start cycles, and the hero’s teal arrow. Design puts the running point on the right; chemistry (§5) stands guard on the left; and §4 gives the regime boundaries an actual number.

§4Lambda and the regimes

Whether a film “exists” is a comparison, not an absolute: film thickness against surface roughness — and the ratio has a name, a symbol and accepted bands.

The insight is that a two-micrometre film is luxurious over ground surfaces and useless over rough-machined ones: what matters is the minimum film measured in units of the peaks trying to poke through it, the combined roughness of the two surfaces. Above λ of about 3 the peaks essentially never touch — full film, indefinite life; between 1 and 3, occasional asperity traffic — the mixed regime most greased and many oiled contacts actually inhabit, workable but wearing; below 1, boundary, and §5’s chemistry is the only thing left working. Two corollaries earn their keep daily. Finish buys film: polishing a journal or raceway raises λ at zero cost in oil or heat — often the cheapest lubrication upgrade there is. And in rolling contacts the film that λ measures is the elastohydrodynamic one: at gigapascal contact pressures the trapped oil’s viscosity rises enormously and the steels flatten locally, so a film tenths of a micrometre thick survives where naive theory says none could — the quiet miracle that lets the ball-bearing page’s L₁₀ arithmetic assume separation at all, and the reason a starved rolling bearing dies so far short of its computed life.

§5Boundary chemistry

Left of the curve, lubrication stops being fluid mechanics and becomes surface chemistry: molecules that stick, and molecules that react, standing in for the film that is not there.

Two lines of defence hold the boundary regime. The first is adsorption: polar molecules — fatty-oil components and friction modifiers — that anchor to metal and stand up in ordered layers, so contacting asperities shear a molecular carpet instead of welding; the ancient virtue of tallow and rapeseed, bottled. The second is reaction: anti-wear and extreme-pressure additives that use the flash temperature of an asperity collision as their trigger, reacting there — and only there — to form a soft, sacrificial solid film that shears in preference to the steel. ZDDP-type anti-wear packages do this for everyday sliding; sulphur–phosphorus EP chemistries do it for the brutal contacts of hypoid and heavily loaded gears, which is why gear oils smell as they do and why an EP oil in the wrong place (against yellow-metal cages and bushes it can attack) is a mis-prescription, not an upgrade. The design reading: additives are the guard on the left of the curve, spent gradually in service — one honest reason oil is changed while still looking clean — and no additive package is a licence to run there continuously. Chemistry buys the passage through boundary; only geometry and viscosity buy residence on the right.

§6Grease and delivery

Grease is not thick oil — it is oil held in a sponge of soap, released where the contact wants it; and delivery, oil or grease, is its own small discipline.

A grease is a base oil (whose viscosity does all the §2 work) immobilised in a thickener matrix — most commonly lithium soaps — that holds it at the contact, bleeding oil into the working zone and reabsorbing it at rest. That structure is the whole value: grease stays put without pumps, seals the bearing against the ingress §4 warned about, and turns lubrication into a fit-and-forget or occasional-gun affair — which is why the great majority of rolling bearings run greased for life. Its grades measure the sponge, not the oil: the NLGI number ranks consistency from fluid to block, with the buttery middle grades serving almost everything. Two field rules do most of grease’s good. Part-fill: a bearing packed solid churns its grease into heat — cavities are filled only partway, leaving room for the excess to shed. And never assume greases mix: incompatible thickeners can collapse each other’s structure into slop. Oil-side delivery completes the toolkit — bath and splash for gearboxes, drip and wick for the modest, mist and circulating systems (which also carry §1’s cooling and cleaning duties) for the serious, and the plain-bearing page’s standing rule wherever oil is piped in: feed the unloaded zone, and let the wedge do the lifting.

§7Quick reference

The working core of the page on one card rack.