Plain Bearings
Part of Gear Making
How to design and build simple sliding-contact bearings that support rotating shafts with minimal friction.
Why This Matters
Every machine that uses rotating parts — mills, generators, lathes, pumps — depends on bearings to support the shaft while allowing it to spin freely. Without adequate bearings, friction destroys shafts, wastes power, and causes mechanical failure within hours. A collapsed bearing in a water mill can halt grain grinding at the worst possible time.
Plain bearings, also called sleeve bearings or bushings, are the simplest bearing type: a shaft rotates inside a closely fitted housing lined with a low-friction material. Unlike roller bearings, they have no rolling elements — just a thin film of lubricant separating metal from metal. This simplicity makes them ideal for post-collapse manufacturing because they require no precision ball-making equipment and can be fabricated with basic metalworking tools.
Understanding plain bearings lets you build machinery that runs quietly, lasts years, and can be repaired with materials available in any workshop. A well-made plain bearing, properly lubricated, will outlast roller bearings under moderate loads because there are no small parts to crack or corrode.
Load Types and Basic Principles
Plain bearings carry two types of load: radial load acts perpendicular to the shaft axis (the weight of a millstone pressing down on a horizontal shaft), and axial load or thrust acts along the shaft axis (a propeller pushing the shaft forward or backward). Most shafts experience radial loads; dedicated thrust bearings handle axial loads.
The key physics: when a shaft rotates inside a bearing, it drags lubricant into the narrow wedge-shaped gap between shaft and housing. This builds a hydrodynamic pressure film that actually lifts the shaft off the bearing surface. The shaft floats on oil rather than riding metal-on-metal. This film only forms above a critical speed — at startup, there is brief metal contact, which is why bearing materials must also tolerate some dry rubbing.
Clearance is critical. Too tight: no room for the oil film, the shaft seizes. Too loose: shaft wobbles, causes vibration and uneven wear. The standard clearance for a plain bearing is roughly 0.001 inches per inch of shaft diameter — so a 1-inch shaft needs about 0.001 inches of clearance all around. In workshop practice, this means the shaft should slide in with light thumb pressure but have no perceptible wobble.
Bearing Materials
Babbitt metal is the traditional bearing lining material: a soft alloy of tin, antimony, and copper (or lead-based variants). Its softness allows it to conform to minor shaft irregularities and embed small dirt particles harmlessly. Babbitt can be cast directly into a housing, making it ideal for fabrication. A basic formula: 83% tin, 11% antimony, 6% copper by weight. Melt in a steel ladle, preheat the housing to prevent cold shuts, then pour around a mandrel slightly smaller than the final shaft diameter.
Bronze (copper-tin alloy) makes excellent plain bearings for higher loads. Cast bronze bushings can be pressed or screwed into housings. Phosphor bronze (with 0.1-0.35% phosphorus) has particularly good wear resistance. Bronze bearings for moderate applications can be turned on a lathe to fit, then pressed in.
Hardwood works surprisingly well for low-speed, heavily loaded applications — particularly lignum vitae, a naturally oily tropical hardwood. Water-lubricated wooden bearings were used in ship propeller shafts for centuries. Domestic hardwoods (oak, maple, ash) work for slow-moving shafts if kept lubricated with grease or tallow.
Cast iron on cast iron: for very low speeds (hand-cranked machinery), a cast iron shaft journal running directly in a cast iron housing with regular grease lubrication is acceptable. Both surfaces work-harden slightly and develop a compatible surface.
Constructing a Split Plain Bearing
The most practical design for rebuilding is the split bearing: two halves that bolt together around the shaft, allowing installation and removal without dismantling the entire machine.
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Machine or cast the housing: Two matching halves, typically in cast iron or bronze. The bore should be slightly undersized — leave 0.010-0.020 inches for finishing.
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Line with babbitt: Heat the housing halves to 300°F to drive out moisture. Apply a thin flux (rosin or zinc chloride solution) to the bore. Assemble the halves around a shaft mandrel (wrapped in thin shim stock to set clearance). Pour molten babbitt at about 700°F into the gap. Allow to cool slowly.
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Bore to size: Remove the mandrel and run a boring bar or carefully fitted reamer through the bearing to achieve the correct clearance. The bore must be round and concentric with the housing OD.
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Cut oil grooves: Machine shallow grooves (about 1/16 inch deep, 1/8 inch wide) running along the bearing length, offset from the load zone. These grooves distribute oil around the bearing but must not run through the loaded area where the hydrodynamic film forms.
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Drill oil holes: A hole from the outside of the housing into each oil groove, capped with an oil cup or pressure fitting.
Lubrication Systems
The simplest system is a cup oiler: a small cup screwed into the oil hole, filled with oil or grease. The operator fills it regularly. For low-speed, intermittently used machinery, weekly lubrication may suffice.
Wick oilers (Stauffer cups) use a felt wick to feed oil continuously by capillary action. A reservoir holds a day’s worth of oil; gravity and capillary action distribute it. These are ideal for unattended operation.
Grease packing: For dusty or wet environments, pack the bearing housing with thick grease (tallow mixed with lime, or animal fat) that resists washing out. Grease lubrication tolerates lower surface speeds than oil but is much more forgiving of infrequent maintenance.
Ring oilers: A loose ring hangs on the shaft, dipping into an oil reservoir in the bearing housing as the shaft rotates, carrying oil up to the shaft surface continuously. Self-feeding and reliable for horizontal shafts.
Troubleshooting and Maintenance
Overheating: The bearing runs too hot (cannot hold your hand on the housing for more than 3 seconds). Causes: insufficient lubrication, clearance too tight, shaft misalignment, overload. Check oil supply first; then check alignment with a straight edge; finally measure bearing clearance.
Knocking or rumbling: Clearance too large, or bearing material has worn away leaving the shaft running on the housing. Rebabbitt or replace bronze bushing.
Shaft wear: If the shaft journal (the section running inside the bearing) has worn grooves or ridges, it must be ground or turned smooth before installing a new bearing — a rough shaft destroys new babbitt in hours.
Bearing seizure: Shaft spins so fast and hot that it welds to the bearing metal. This is usually a lubrication failure. Prevent by ensuring oil supply before startup. A seized bearing must be bored out completely; if the shaft is damaged, it needs grinding or replacement.
Routine inspection involves checking oil level daily on running machines, feeling bearing housings for unusual heat, and listening for changes in sound. A well-maintained plain bearing in a workshop machine should last 5-10 years before requiring rebabbitting.