How rotation, lubricant viscosity and converging geometry create the pressure that supports a shaft without continuous surface contact.
Hydrodynamic lubrication occurs when relative motion draws lubricant into a narrowing gap between two surfaces. The resulting pressure within that moving film supports the applied load and separates the surfaces during normal operation.
All five conditions interact. Supplying oil alone does not guarantee hydrodynamic separation.
The system provides lubricant of the required viscosity, temperature, flow and cleanliness to the bearing entrance.
Rotation drags lubricant along through viscous shear. At zero speed, this self-generated hydrodynamic action does not exist.
Journal eccentricity, a machined profile or pad tilt creates an inlet that becomes progressively narrower in the direction of motion.
Lubricant resists being forced through the narrowing gap. A pressure distribution develops, rising to a peak and then falling toward the film outlet.
The integrated film pressure balances the applied load. The shaft or collar runs separated from the bearing surface by a very thin lubricant film.
The shaft settles slightly away from the bearing centre under load. Rotation pulls oil into the converging clearance between the journal and bore. Fixed-profile bearings use bore geometry; tilting-pad journal bearings allow each pad to establish its own wedge.
Explore journal bearings
The rotating collar moves oil across the pad face. A taper-land profile or pad tilt creates the converging wedge. In a tilting-pad bearing, each pad finds an operating angle determined by load, speed, pivot geometry and oil conditions.
Explore thrust bearings| Regime | How load is supported | When it occurs | Key distinction |
|---|---|---|---|
| Full-film hydrodynamic | Pressure generated by relative motion in a converging film | Normal running at sufficient speed with suitable oil and geometry | Surfaces are separated by the fluid film |
| Hydrostatic | Pressure supplied by an external high-pressure system | Jacking oil, low-speed turning or a continuously hydrostatic design | Can support load without normal running speed |
| Mixed lubrication | Part fluid film and part local surface contact | Acceleration, coast-down, overload or marginal film conditions | Some asperity contact remains |
| Boundary lubrication | Mostly surface contact and lubricant boundary layers | Start, stop or severe loss of film | Wear and wiping risk rises significantly |
More surface speed generally increases the lubricant-dragging action, but also raises viscous shear and power loss.
Viscosity must be considered at operating temperature. Oil that becomes too thin can reduce film thickness; excessive viscosity increases losses and heat.
The gap must allow oil flow and form the intended wedge while accommodating tolerance, shaft motion and thermal expansion.
Load changes shaft position, pad angle, pressure and film thickness. Dynamic and transient loads can be as important as steady load.
Continuous clean oil replaces leakage and carries away generated heat. Flow at the bearing matters more than header pressure alone.
Misalignment, runout, roughness or contamination can concentrate load and reduce the local film safety margin.
Static load may leave the shaft or collar resting on the bearing.
Film develops as speed rises. Pre-lube and, where fitted, jacking oil reduce contact.
Designed speed and oil conditions establish separation and load capacity.
Post-lube and lift systems may remain required until the rotor stops.
Relative motion pulls viscous lubricant into the converging gap. Resistance to flow creates a pressure distribution that supports the applied load.
Normally no. The pump supplies and cools the oil; the hydrodynamic film generates its load-carrying pressure locally. A hydrostatic or jacking system is different.
Not during healthy full-film operation. Contact can occur while starting or stopping, or if speed, viscosity, flow, geometry or load no longer supports adequate separation.
Clearance provides space for oil flow, thermal expansion and the converging film geometry. Too little and too much clearance can both damage performance.
This explanation is aligned with Waukesha Bearings' fluid-film operating principle, Kingsbury's hydrodynamic bearing guidance and the lubrication terminology covered by ISO 4378-3:2024.
Use the rules-of-thumb reference for preliminary checks, or send Oiltech the operating data for a full bearing performance review.