Plain Bearings

§1The oldest bearing

Before rolling elements there was the sleeve — and long after them there still is, because for half the jobs in machinery the simplest bearing remains the best one.

A plain (journal, sleeve) bearing is nothing but a bush of suitable material around a rotating shaft, and its case never went away. It is cheap and compact — a pressed-in bush against a precision assembly of rings and balls; it is quiet, with no rolling elements to drum; it shrugs at shock, spreading impact over a broad film and surface where a ball concentrates it on a point; it can be split, dropping over a crankshaft journal that no one-piece bearing could ever reach — the reason every engine big-end in the world is plain; and it scales both ways, from watch pivots to turbine and ship bearings a metre across, sizes where rolling elements simply do not exist. Its terms of trade are equally plain: it slides where the next page’s bearings roll, so friction and its consequences dominate the design — and the whole page is really one question asked three ways: is there metal-to-metal contact, or a film? Section §2 gives the catalogue’s answer for the contact regime; §3 and §4 give physics’ answer for the film; and the lubrication page next door owns the territory in between.

§2Pressure, speed and PV

For the everyday bush — boundary-lubricated, greased or self-lubricating — the whole selection problem compresses into two numbers and their product.

§3The hydrodynamic wedge

Give the same bearing enough speed, enough oil and a little clearance, and it changes species: the shaft drags oil into a converging gap, the gap becomes a pump, and the pump lifts the shaft clear of the metal entirely.

The hero shows the mechanism, and it needs nothing but geometry. Under load the journal cannot sit concentric — it settles slightly off-centre, so the clearance space becomes a crescent: wide on one side, narrowing to a minimum film h_min near the load line. The rotating shaft surface drags viscous oil along with it (oil sticks to metal — the no-slip fact underlying all of this), and oil dragged into a converging passage has nowhere to go but into pressure: the film in the narrowing zone pressurises exactly as the hero’s arrows show, and the integrated pressure carries the entire load with no solid contact at all. Everything about full-film behaviour follows from that picture. More load pushes the shaft further off-centre, narrowing h_min and strengthening the wedge — the bearing is self-adjusting. More speed or thicker oil builds pressure sooner, thickening the film. And the three ingredients of the wedge — speed, viscosity, converging clearance — are precisely the things a designer controls, which §4 turns into numbers. The one thing the wedge cannot do is exist at zero speed: every start begins with metal on metal, a fact §6 will hold onto, and the lubrication page will draw as the left-hand end of its famous curve.

§4Numbers of the film

Two working numbers frame full-film design: the clearance the wedge needs, and the friction the film still charges — Petroff's beautifully simple estimate of a shaft shearing its own oil.

§5Materials and embeddability

A plain bearing’s material is chosen to lose — softer than the shaft, tolerant of dirt, incapable of welding to steel — because the bush is the cheap, replaceable half of the pair.

The logic is the Acme page’s bronze nut, generalised. The bush must be dissimilar and non-seizing against a steel shaft — like metals under pressure micro-weld, and a bearing that grabs its shaft destroys both — and it should be soft enough to conform and to embed: to bed in over slight misalignment, and to swallow the inevitable grit particle harmlessly into its own surface rather than lap it endlessly against the journal. Hence the classic ladder. Whitemetal (babbitt), the tin- or lead-based lining of engine shells: supremely conformable and embeddable, run as a thin layer on a steel backing that supplies the strength it lacks. Bronzes, the general-purpose middle: stronger, hotter-running, the default bush of ordinary machinery — including sintered porous bronze (Oilite-type), vacuum-charged with oil that the pores feed to the surface for the life of the part: the fit-and-forget bush of small motors and hinges everywhere. And the polymer and composite liners — PTFE-faced steel-backed bushes and filled plastics — that run dry or nearly so, owning the low-speed, maintenance-free and food-safe territory where oil is unwelcome. One family, one philosophy: the shaft is precious, the bush is sacrificial, and a worn bush pressed out and replaced is the system working as designed.

§6Failure and practice

Plain bearings die three ways — at the starts, at the edges, and in the seizure spiral — and the field rules exist to block each door.

At the starts: §3’s wedge needs speed, so every start and stop is a boundary-contact event, and a bearing that starts and stops all day wears at a rate its full-film hours would never predict — the case for high-load start-up oil (or hydrostatic jacking on big machines), for materials chosen on §2’s contact limits even in “hydrodynamic” duty, and for judging bearing life in start cycles as much as running hours. At the edges: a misaligned shaft loads a bush at its ends, collapsing the local film and wiping the lining — the concentrated preview of the shaft-alignment page that closes this section, and the reason long L/D bushes demand line-boring and careful fitting. The seizure spiral: lost oil or overload thins the film, contact makes heat, heat thins the oil and expands the journal into its clearance, contact grows — a feedback loop that runs to a welded shaft in seconds once started, which is why lubrication failure is treated as an emergency, not a maintenance item. And one drafting rule guards the film itself: oil is fed, and any distribution grooves are cut, only in the unloaded region — the hero’s top — because a groove in the load zone is a slot cut through the pressure hill that carries the shaft. Feed the wedge from the low-pressure side and let the physics do the lifting.

§7Quick reference

The working core of the page on one card rack.