Journal bearings are a type of sliding bearing designed to support radial loads. They are primarily composed of components such as the journal, bearing shell (bushing), bearing cap, and bearing housing. The bearing shell is the core component and is often made of copper alloys, Babbitt alloys, or non-metallic materials. Some bearing shells are plated with antifriction metal layers to enhance performance. The bearing cap and housing are typically made of cast iron or cast steel and are fastened with bolts. Shims are used to adjust the clearance, while oil holes and axial oil grooves ensure proper lubrication.
Journal bearings can be classified by structure into split type, solid type, flanged type, and self-aligning type:
• The split type allows for easier installation on the journal and facilitates clearance adjustment.
• The solid type has a simpler structure but limited maintainability.
• The flanged type has mounting surfaces perpendicular to the bearing's centerline.
• The self-aligning type can automatically adapt to shaft deflection.
The ratio of bearing width to diameter (W/D) generally ranges from 0.4 to 1.5. In some applications, such as in internal combustion engines, it may be reduced to 0.25 to minimize axial dimensions. Thin-wall bearing shells are widely used in mass-produced equipment due to their strong adaptability.
Composition
The portion of the shaft supported by the bearing is called the journal, and the matching part is the bearing shell. A fully cylindrical shell is known as a bushing. The housing that holds the bearing shell includes the bearing cap (upper part) and the bearing seat (lower part). These are connected by studs, with locating surfaces or pins for alignment. Shims of various thicknesses can be inserted at the joint to adjust clearance. Most bearing joints are horizontal, though inclined joints are sometimes used to better withstand loads perpendicular to the joint surface.
To ensure lubricant access to the friction surface, the bearing cap typically features oil holes, and the bearing shell includes axial oil grooves to distribute lubricant. Cast iron is the most common material for caps and seats, though cast steel or welded steel structures may be used for high-load applications.
Structural Types
Journal bearings come in several forms:
• Split journal bearings feature separable caps for easy installation and clearance adjustment after wear.
• Solid journal bearings are structurally simple but require journal insertion from one end and do not allow for post-wear clearance adjustment.
• Flanged journal bearings have mounting surfaces perpendicular to the shaft axis.
• Self-aligning journal bearings can compensate for shaft deflection and misalignment.
Dimensions
• The bearing diameter refers to the inner diameter of the journal bearing.
• The bearing width refers to its axial dimension.
• The width-to-diameter ratio (W/D) is generally 0.4–1.5. Lower ratios like 0.25 are used in compact designs such as internal combustion engines.
Materials
Bearing shells are made from materials with excellent antifriction properties, including:
• Metallic materials: copper alloys, aluminum alloys, and Babbitt alloys.
• Non-metallic materials: plastics, graphite, rubber, and wood.
Some metals like Babbitt alloys offer good antifriction but have low strength, so a thin layer is cast on a stronger backing, referred to as the bearing lining. The backing is often made of mild steel, cast iron, or bronze. In some cases, the lining is further plated with ultra-low-friction metals like indium. Such multilayer metal shells enhance performance.
• Shells whose inner shape is unaffected by the housing bore shape are known as thick-wall bearings.
• Shells whose inner shape conforms to the housing bore are known as thin-wall bearings, widely used in mass-produced machines like engines and air compressors.
Lubrication
Common lubricants include:
• Fluids: oil, water, air.
• Solids: graphite, molybdenum disulfide.
The type of lubricant depends on application requirements. Liquid-lubricated journal bearings are further divided by film thickness:
• Thin-film lubrication bearings
• Thick-film lubrication bearings, which include:
• Hydrodynamic journal bearings (relying on motion to build pressure)
• Hydrostatic journal bearings (using external pressure)
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