Understanding Journal Bearings
A journal bearing — also called a plain bearing, sleeve bearing, or fluid-film bearing — supports a rotating shaft on a thin, pressurised film of lubricant instead of on rolling elements. The rotating portion of the shaft inside the bearing is the journal; it is held clear of the stationary bearing surface by a hydrodynamic oil film that the shaft itself generates as it drags lubricant into a converging, wedge-shaped gap. That pressurised wedge carries the full rotor load with no metal-to-metal contact. Because the oil film also provides generous damping, journal bearings are the natural choice for the high-speed, high-load machinery — turbines, generators, large compressors — where controlling vibration and stabilising the rotor matter most.
1. Definition: What Is a Journal Bearing?
In a journal bearing the shaft does not touch the bearing at running speed. Instead it floats, slightly off-centre, on a wedge of lubricant only tens of micrometres thick. This single fact distinguishes it from a rolling-element bearing, which carries load through balls or rollers in Hertzian contact. The journal bearing’s strengths flow directly from the oil film: very high load capacity, extremely low friction once the film is established, quiet running, and the damping that makes it possible to run large rotors smoothly through and above their critical speeds. The behaviour of the shaft and its bearings together is studied as a rotor-bearing system, because neither can be understood in isolation.
2. Operating Principle: Hydrodynamic Lubrication
How the Oil Film Forms
The journal bearing relies on hydrodynamic lubrication, which develops in a predictable sequence as the shaft comes up to speed:
- Initial contact: at rest the shaft sits on the bottom of the bore under its own weight, with metal touching metal.
- Rotation begins: as the shaft starts to turn, adhesion drags lubricant into the clearance gap.
- Wedge formation: the convergent geometry between shaft and bore squeezes that oil into a wedge-shaped space.
- Pressure generation: oil forced into the narrowing wedge develops hydrodynamic pressure.
- Lift-off: once that pressure force exceeds the shaft weight, the journal lifts clear and rides on a full film.
- Steady state: the shaft floats on the pressurised film, finding an equilibrium position offset from the bore centre, with no metal contact.
The position the journal settles into — its eccentricity within the clearance — is not fixed. It shifts with load and speed, and that shifting equilibrium is the root of the bearing’s complex dynamic behaviour described below.
Oil Film Thickness
- Typical minimum film thickness is 10–100 micrometres (0.0004–0.004 in) — extremely thin, yet enough to keep the surfaces apart.
- The film is not uniform: it varies around the circumference, reaching its minimum at the point of closest approach between journal and bore.
- Thickness depends on speed, load, lubricant viscosity and bearing clearance — raise the speed or viscosity and the film thickens; raise the load and it thins.
- Because viscosity falls as oil warms, film thickness is also sensitive to operating temperature, which is why oil supply temperature is a monitored parameter on large machines.
3. Types of Journal Bearing
Plain Cylindrical (Full Journal)
- The simplest design: a plain cylindrical bore with an oil-supply groove and a full 360° wrap angle.
- Good load capacity, but the symmetric film makes it prone to instability — oil whirl — at high speed and light load.
- Common in motors, pumps and general industrial equipment where speeds are moderate.
Partial-Arc Bearings
- The bearing surface covers only part of the circumference, typically 120–180°.
- Lighter and demanding less oil flow, but offering lower stiffness than a full journal.
- Suited to lightly loaded applications where the load direction is well defined.
Tilting-Pad Bearings
- The surface is divided into several independent pads, each free to pivot.
- Every pad develops its own hydrodynamic wedge, which suppresses the cross-coupling that drives oil whirl.
- Inherently stable against whirl and whip, they are the industry standard for high-speed turbomachinery.
- More expensive and complex, but with markedly superior dynamic characteristics.
Pressure-Dam and Offset Bearings
- Modified cylindrical bearings with geometric features — grooves, a step “dam”, or an offset (lemon-bore) split — added to improve stability.
- These features deliberately load the film to raise effective damping.
- They are a practical compromise between the simple cylindrical bearing and the costly tilting-pad design.
Where even a tilting-pad bearing cannot supply enough damping for a flexible rotor, designers may add a squeeze-film damper in series with the bearing to dissipate additional energy.
4. Dynamic Characteristics
Stiffness
Journal-bearing stiffness is not a single number; it is a set of speed- and load-dependent coefficients:
- Low speed: low stiffness — the journal position changes a great deal as load varies.
- High speed: higher stiffness as the hydrodynamic pressure field becomes fully developed.
- Directional variation: stiffness differs in the horizontal and vertical directions, so the bearing responds anisotropically.
- Cross-coupled stiffness: a deflection in one direction produces a force at right angles to it. This cross-coupling is precisely the mechanism that can pump energy into a whirling orbit and trigger rotor instability.
Damping
The film’s great virtue is the damping it supplies:
- Energy is dissipated by viscous shearing of the oil as the journal moves within the clearance.
- Damping rises with speed and with oil viscosity.
- It is what limits the vibration amplitude when the rotor passes through a critical speed.
- Adequate damping is essential to keep self-excited instabilities from growing without bound.
Speed Dependence
Because both stiffness and damping change with speed, so does everything that depends on them:
- Stiffness increases with speed.
- Damping increases with speed.
- The system’s natural frequencies rise with speed.
- Critical speeds therefore shift upward as the machine accelerates — an effect made visible on a Campbell diagram.
5. Advantages and Limitations
The oil film is responsible for both the journal bearing’s outstanding strengths and its particular demands.
- High load capacity: can support very heavy rotors that would crush a rolling-element bearing.
- High-speed capability: suitable for speeds up to 50,000 rpm and beyond.
- Low friction at speed: once the hydrodynamic film is established, the friction coefficient is very low (around 0.001–0.003).
- Excellent damping: controls vibration through critical speeds and helps stabilise the rotor.
- Quiet operation: no rolling-element passage means no rolling-element noise.
- Shock resistance: the oil film cushions transient and impact loads.
- Long life: with no metal contact in service, wear is minimal and decades of operation are possible.
- Simple basic design: the plain cylindrical type is mechanically simple and economical.
Against these sit the practical challenges:
- High starting friction: there is no film at rest, so the machine must overcome break-away torque and brief boundary-lubrication wear at each start.
- Lubrication system required: a continuous supply of clean, cool, correctly pressurised oil is mandatory; bearing lubrication is not optional but central to the design.
- Whirl and whip risk: plain cylindrical bearings are susceptible to oil whirl and, near twice a critical speed, to shaft whip.
- Lower low-speed stiffness: the compliant film makes the bearing softer than a rolling-element bearing at low speed, slowing the response.
- Temperature sensitivity: performance tracks oil temperature through its effect on viscosity.
- Contamination sensitivity: hard particles can score the soft babbitt surface or block oil passages.
- No axial restraint: a journal bearing locates the shaft only radially; axial loads need a separate thrust bearing.
6. Where Journal Bearings Are Used
Journal bearings are standard wherever rotors are large, fast, or both:
- Steam and gas turbines: multi-megawatt power-generation units.
- Large generators: synchronous generators in power plants.
- Centrifugal compressors: high-speed, high-load industrial machines.
- Large electric motors: motors above roughly 500 hp frequently use them.
- Marine propulsion: propeller-shaft and stern-tube bearings.
- Paper machines: the large rolls that carry the web.
- Internal-combustion engines: crankshaft main and connecting-rod bearings.
7. Relationship to Rotor Dynamics and Field Balancing
Because their stiffness and damping define so much of a rotor’s behaviour, journal bearings sit at the heart of rotor dynamics:
- Critical-speed placement: bearing stiffness and damping set where critical speeds fall and how high the vibration peaks there.
- Stability: the bearing type largely decides susceptibility to oil whirl and shaft whip; the characteristic sub-synchronous frequencies these produce can be estimated with a dedicated journal-bearing defect-frequency calculator.
- Frequency mapping: a Campbell diagram shows how the natural frequencies migrate with speed as bearing stiffness changes.
- Balancing response: bearing characteristics shape the influence coefficients that govern how the rotor responds to a correction weight.
That last point is where the bearing meets day-to-day maintenance. When a turbine or compressor running in journal bearings shows an elevated 1× unbalance response, it is balanced in place, in its own bearings, at operating speed. A portable two-channel analyser such as the Balanset-1A measures the synchronous amplitude and phase at each bearing, computes the rotor’s influence coefficients from a trial run, and calculates the correction weights needed — capturing the true response of the assembled rotor-bearing system, including the very film stiffness and damping a balancing machine could never reproduce. Verified against the appropriate ISO 21940-11 balance grade, the result reflects how the machine actually behaves in service.
Journal bearings are a mature, sophisticated technology that remains irreplaceable in critical high-performance machinery. Their unique combination of load capacity, speed capability and damping justifies the complexity of their lubrication and dynamic behaviour, and a working grasp of that behaviour is essential to anyone diagnosing or balancing large rotating equipment.
