EngineeringMechanical EngineeringPart 03 of 15
Journal and Porous Bronze Bearing Selection
A journal bearing is a bush, a shaft and a film of oil. Everything that matters in its selection follows from whether that film ever becomes thick enough to separate the two metal surfaces.
- Pressure and pv limits
- Bearing modulus
- Lubrication regimes
- Worked example
Executive summary
Journal bearings — also called plain bearings or bushes — divide into pressure-lubricated types, where oil is pumped into the bearing under pressure, and non-pressure-lubricated types selected from a supplier range. Pressure-lubricated designs require specialist hydrodynamic analysis and cannot be bought off the shelf; this page addresses the second group, in particular sintered porous bronze bushes.
Selection is governed by three quantities: the bearing pressure on the projected area, the pv factor that determines whether auxiliary lubrication is needed, and the bearing modulus that indicates whether the bearing will reach thick-film operation.
How a plain bearing is constructed
The journal is the portion of the shaft running inside the bearing; it is often the same diameter as the shaft rather than a raised or reduced section. The bearing itself is a sleeve in a housing, sometimes with a flange on one side to take light thrust.
The material pairing is deliberate. The journal is hard, finely finished and dimensionally accurate. The bearing is a dissimilar, softer material with a relatively open, porous surface. Each of those three properties does specific work:
Prevents welding
Like-on-like metal contact under load and heat produces localised welding and seizure. A dissimilar pairing avoids it.
Allows embeddability
Hard foreign particles bed into the softer bearing surface rather than ploughing the journal.
Retains lubricant
An open surface holds oil at the interface. Sintered bushes carry this further, being impregnated with oil throughout their volume.
Common bearing materials include bronze (copper–tin), white metals based on lead, tin, aluminium, antimony and copper, and non-metals such as nylon, phenolics and PTFE. Cast iron was widely used historically and is now uncommon. Oils and greases dominate as lubricants, but special bearings run on water or even air.
Produced by powder metallurgy from copper and tin powders, then vacuum impregnated with oil to roughly thirty per cent by volume. In many duties they need no further lubrication; in heavier duties auxiliary lubrication extends life substantially.
Advantages, limits and where they belong
Advantages over rolling element bearings
- Low cost, particularly in volume.
- Quiet operation with little running noise.
- Very small radial envelope.
- High speed capability.
- Can be designed for lubricants other than oil or grease, including water or dry running.
Limitations
- Relatively low radial load capacity.
- Effectively no thrust capacity unless a flange bearing and a stepped shaft are used.
- Low misalignment tolerance; self-aligning types exist only in small sizes.
- Shaft material, hardness and surface finish are critical, not incidental.
- Large sizes are generally not available off the shelf.
The natural home for a plain bearing is therefore a relatively high-speed shaft with a moderate radial load and little or no thrust, where cost, noise and radial space all matter.
Factors that decide whether it lasts
The three lubrication regimes
Consider a loaded shaft starting from rest and accelerating to running speed. The bearing passes through three distinct regimes.
- Regime 1Boundary lubrication. At rest and low speed the journal sits against the lower bore. Metal contact occurs and wear is significant.
- Regime 2Thin-film lubrication. Oil dragged around by the shaft begins to wedge into the gap. The journal starts to float but may still touch under shock load; moderate wear occurs.
- Regime 3Thick-film lubrication. Film thickness grows until the journal no longer contacts the bore at all. In principle no wear occurs, because nothing rubs.
Plotting coefficient of friction (or frictional torque) against shaft speed at constant load and viscosity gives a characteristic curve. Friction falls steeply as metal contact diminishes, reaches a minimum around the onset of thick-film operation, then rises again — once the shaft is floating, fluid friction increases with velocity exactly as it does in any fluid flow.
The best place to run is just past the onset of thick-film lubrication. Below it the bearing wears; well above it the bearing survives but wastes power in fluid shear.
- M
- bearing modulus (dimensionless in the unit set below)
- η
- dynamic viscosity of the lubricant at bearing operating temperature, centipoise
- v
- surface (linear) velocity of the journal, m/s
- p
- bearing pressure on the projected area, MPa
A widely used design rule of thumb is that the onset of thick-film lubrication corresponds to a bearing modulus of about 75 in these units. Well above 75, thick-film operation is assured but frictional torque is high — consider a lower viscosity. Below 75, consider a higher viscosity or another change that raises M.
Selection procedure
- Assemble the duty dataJournal diameter, running speed and radial load per bearing.
- Choose a trial lengthFor the given journal size, start at L/d = 1 and work from the supplier's standard length range.
- Calculate bearing pressurep = F / (d L), using the projected area rather than the wrapped area.
- Calculate surface velocityv = π d N / 60 000 with d in mm and N in rev/min, giving v in m/s.
- Check pressure against the allowable limitAllowable pressure falls sharply as velocity rises. If the limit is exceeded, try a longer bush; if the longest available bush still fails, the duty is wrong for a porous bronze bearing.
- Calculate the pv factorMultiply pressure in MPa by velocity in m/s. Beyond the supplier's pv limit, auxiliary lubrication is required.
- Calculate the bearing modulusAssess whether thick-film operation is likely and adjust viscosity if not.
- Record the specificationBush designation, housing fit, running clearance, lubricant grade and any auxiliary lubrication arrangement.
The allowable pressure at a given surface velocity, and the pv threshold above which auxiliary lubrication becomes necessary, are product-specific. They are published by the bush manufacturer and must be read from the current catalogue — they are not general constants and vary between material grades and manufacturing processes.
Worked example
A 30 mm shaft rotates at 1450 rev/min and carries a 500 N radial load midway between two supporting bearings, running continuously.
The pv factor is 0.278 × 2.28 ≈ 0.63. That sits above the threshold at which many porous bronze bushes call for auxiliary lubrication, so either a lubrication arrangement is provided or the bush is lengthened to bring the pressure — and therefore pv — down. Lengthening to 40 mm gives p = 0.208 MPa and pv ≈ 0.47, which changes the answer.
For the bearing modulus, assuming the impregnating oil sits near 20 cp at operating temperature: M = 20 × 2.28 / 0.278 ≈ 164. Comfortably above the rule-of-thumb threshold of 75, so thick-film operation is likely — at some cost in fluid friction.
A single geometry change — ten millimetres of extra bush length — moved the design from needing an auxiliary lubrication system to not needing one. Iterating the L/d ratio before adding hardware is almost always the cheaper engineering.
Specification checklist
- Application confirmed as suitable for a non-pressure-lubricated bearing.
- Journal material, hardness and surface finish specified, not assumed.
- L/d ratio within roughly 0.5 to 1.5.
- Bearing pressure below the allowable limit at the actual surface velocity.
- pv factor evaluated and auxiliary lubrication provided if required.
- Bearing modulus assessed and lubricant viscosity chosen accordingly.
- Running clearance specified, typically about one thousandth of journal diameter.
- Housing fit and installation method defined, including the pressing tool.
- Housing material considered as part of the heat dissipation path.
- Thrust path resolved — flange bearing and shaft step, or a separate thrust element.