What is a Journal Bearing? Hydrodynamic Support • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors What is a Journal Bearing? Hydrodynamic Support • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors

What is a Journal Bearing? Hydrodynamic Support

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Understanding Journal Bearings

Definition: What is a Journal Bearing?

A journal bearing (also called a plain bearing, sleeve bearing, or fluid-film bearing) is a type of bearing that supports a rotating shaft through a thin film of pressurized lubricant rather than through rolling elements. The rotating shaft (the “journal”) is separated from the stationary bearing surface by a hydrodynamic oil film that is generated by the shaft’s rotation dragging oil into a converging wedge-shaped gap. This pressurized oil wedge supports the shaft load without metal-to-metal contact.

Journal bearings are fundamental to high-speed, high-load rotating machinery such as turbines, generators, and large compressors because they provide excellent load capacity, low friction at high speeds, and significant damping that helps control vibration and stabilize rotor systems.

Operating Principle: Hydrodynamic Lubrication

How the Oil Film Forms

The journal bearing operates on the principle of hydrodynamic lubrication:

  1. Initial Contact: When stationary, shaft rests on bearing surface under gravity
  2. Rotation Begins: As shaft starts turning, it drags oil into the gap by adhesion
  3. Wedge Formation: Convergent geometry between shaft and bearing creates a wedge-shaped space
  4. Pressure Generation: Oil dragged into converging wedge generates hydrodynamic pressure
  5. Lift-Off: Pressure force exceeds shaft weight, lifting it onto full oil film
  6. Steady State: Shaft floats on pressurized oil film with no metal contact

Oil Film Thickness

  • Typical thickness: 10-100 micrometers (0.0004-0.004 inches)
  • Extremely thin but sufficient to prevent contact
  • Thickness varies around circumference (minimum at point of closest approach)
  • Dependent on speed, load, oil viscosity, and bearing clearance

Types of Journal Bearings

1. Plain Cylindrical (Full Journal)

  • Simplest design: cylindrical bore with oil supply groove
  • 360° wrap angle
  • Good load capacity but can be prone to instability at high speeds
  • Common in motors, pumps, general industrial equipment

2. Partial Arc Bearings

  • Bearing surface covers only portion of circumference (120-180°)
  • Lighter weight, less oil flow required
  • Lower stiffness than full journal
  • Used in lightly loaded applications

3. Tilting Pad Bearings

  • Bearing surface divided into several independent pads that pivot
  • Each pad develops own hydrodynamic wedge
  • Inherently stable against oil whirl/whip
  • Industry standard for high-speed turbomachinery
  • More expensive but superior dynamic characteristics

4. Pressure Dam and Offset Bearings

  • Modified cylindrical bearings with geometric features to enhance stability
  • Grooves, dams, or offset bores increase effective damping
  • Compromise between simple cylindrical and tilting pad

Dynamic Characteristics

Stiffness

Journal bearing stiffness is complex and speed-dependent:

  • Low Speed: Low stiffness, shaft position changes significantly with load
  • High Speed: Higher stiffness from more developed hydrodynamic pressure
  • Directional Variation: Stiffness different in horizontal vs. vertical directions
  • Cross-Coupled Stiffness: Deflection in one direction creates force in perpendicular direction

Damping

Journal bearings provide significant damping:

  • Energy dissipated through viscous shearing of oil film
  • Damping increases with speed and oil viscosity
  • Critical for limiting vibration at critical speeds
  • Essential for preventing rotor instability

Speed Dependence

All journal bearing properties change with rotational speed:

Advantages of Journal Bearings

  • High Load Capacity: Can support very heavy rotors
  • High Speed Capability: Suitable for speeds up to 50,000+ RPM
  • Low Friction at Speed: Once hydrodynamic film established, friction coefficient very low (0.001-0.003)
  • Excellent Damping: Controls vibration at critical speeds
  • Quiet Operation: No rolling element noise
  • Shock Resistance: Oil film absorbs transient loads
  • Long Life: No metal contact means minimal wear (decades of operation possible)
  • Simple Design: Basic types are simple and economical

Disadvantages and Challenges

  • High Starting Friction: No oil film at rest, requires break-away torque
  • Lubrication System Required: Must supply clean, cooled oil continuously
  • Oil Whirl/Whip Risk: Plain cylindrical bearings susceptible to instability
  • Slower Response: Oil film adds compliance, less stiff than rolling element bearings at low speeds
  • Temperature Sensitivity: Performance changes with oil temperature (affects viscosity)
  • Contamination Sensitivity: Particles can damage bearing surface or block oil passages
  • Axial Positioning: Provides no inherent axial restraint (requires separate thrust bearing)

Applications

Journal bearings are standard in:

  • Steam and Gas Turbines: Multi-megawatt power generation units
  • Large Generators: Synchronous generators in power plants
  • Centrifugal Compressors: High-speed, high-load industrial compressors
  • Large Electric Motors: Motors > 500 HP often use journal bearings
  • Marine Propulsion: Ship propeller shaft bearings
  • Paper Machines: Large roll bearings
  • Internal Combustion Engines: Crankshaft main and rod bearings

Relationship to Rotor Dynamics

Journal bearings critically influence rotor dynamic behavior:

  • Critical Speed Determination: Bearing stiffness and damping directly affect critical speed locations and amplitudes
  • Stability: Bearing type and design determine susceptibility to oil whirl and shaft whip
  • Campbell Diagrams: Show how natural frequencies change with speed due to bearing stiffness variations
  • Balancing: Bearing characteristics affect influence coefficients and balance response

Journal bearings represent a sophisticated, mature technology essential for high-performance rotating machinery. Their unique combination of load capacity, speed capability, and damping makes them irreplaceable in critical applications despite the complexity of their lubrication requirements and dynamic behavior.

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