9.1 Plain Bearings, Bushings & Fits
Key Takeaways
- Plain bearing materials are selected based on conformability, embeddability, anti-seizure resistance, and fatigue strength, ranging from soft babbitt alloys to hard phosphor bronzes and self-lubricating polymers.
- Hydrodynamic lubrication forms a pressurized, converging oil wedge created by journal eccentricity and rotation, lifting the shaft into full fluid film separation with zero metal-to-metal contact.
- The standard industrial rule of thumb for plain bearing clearance is 0.001 inches per inch of journal shaft diameter (0.001 mm/mm), plus base thermal expansion allowances.
- Split sleeve bearing assembly requires measuring bearing crush (0.001" to 0.003" interference height oversize) to prevent shell rotation and ensure thermal heat conduction into the housing.
- Oil distribution grooves must be machined strictly in non-loaded bearing zones and chamfered split-line entry reliefs; grooves cut into the hydrodynamic pressure wedge collapse oil film pressure.
Plain bearings (also termed sleeve bearings, journal bearings, or bushings) rely on sliding contact between a rotating or oscillating journal shaft and a stationary bearing surface. Unlike rolling element bearings, plain bearings transmit heavy radial or thrust loads across a large contact surface area. When properly designed and lubricated, plain bearings operate in the hydrodynamic regime, yielding indefinite service life with virtually no metal-to-metal contact or mechanical wear.
Plain Bearing Metallurgy and Material Selection
Selecting plain bearing materials requires balancing opposing mechanical properties: compressive strength and fatigue resistance (to support heavy dynamic loads) against soft surface properties including conformability (ability to yield to minor shaft misalignment), embeddability (ability to absorb abrasive dirt particles without scratching the journal), and anti-seizure / anti-weld properties (resistance to galling during dry start-up and shutdown).
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| PLAIN BEARING MATERIAL MATRIX |
+-----------------------+-----------------------+-----------------------+---------------------------+
| Material Class | Alloy / Composition | Load / Speed Rating | Key Tribological Features |
+-----------------------+-----------------------+-----------------------+---------------------------+
| Tin-Base Babbitt | 89% Sn, 7% Sb, 4% Cu | Med Load, High Speed | Excellent embeddability, |
| | | (Up to 20 MPa / 3 ksi)| superior anti-seizure |
+-----------------------+-----------------------+-----------------------+---------------------------+
| Lead-Base Babbitt | 80% Pb, 15% Sb, 5% Sn | Low-Med Load & Speed | Economical, soft, lower |
| | | (Up to 15 MPa) | fatigue limit at >100°C |
+-----------------------+-----------------------+-----------------------+---------------------------+
| Phosphor Bronze | Cu-Sn-P Alloy | Heavy Load, Low Speed | High fatigue strength; |
| (SAE 660 / C93200) | (83% Cu, 7% Sn, 7% Pb)| (Up to 70 MPa / 10ksi)| requires hardened shaft |
+-----------------------+-----------------------+-----------------------+---------------------------+
| Aluminum Bronze | Cu-Al-Fe-Ni Alloy | Extreme Load & Shock | Exceptional toughness; |
| | | (Up to 100 MPa) | poor embeddability |
+-----------------------+-----------------------+-----------------------+---------------------------+
| Grey Cast Iron | Fine Graphite Flakes | Low Speed (<1 m/s) | Self-lubricating graphite;|
| | Class 30 / 40 Cast | Light Radial Load | brittle, low conformability|
+-----------------------+-----------------------+-----------------------+---------------------------+
| Engineered Polymers | Nylon 6/6, PTFE, | Low-Med Load, Dry | Corrosion immune, zero |
| (PTFE / Nylon) | PEEK + MoS2 filler | (Up to 35 MPa dry) | stick-slip, low temp limit|
+-----------------------+-----------------------+-----------------------+---------------------------+
Babbitt Alloys (White Metal)
Invented by Isaac Babbitt, white metal alloys consist of a soft matrix embedding harder intermetallic crystals (Cu6Sn5 or SbSn). Tin-base babbitt offers superior corrosion resistance, high fatigue strength, and high speed capability. Lead-base babbitt is more economical but softens rapidly above 100°C (212°F). Babbitt is applied as a thin overlay (0.002" to 0.005" for high-fatigue automotive/turbine inserts, or up to 0.125" for heavy industrial split shells) bonded to steel or bronze backing shells.
Hydrodynamic Lubrication & Converging Wedge Formation
Full fluid film hydrodynamic lubrication separates the journal and bearing shell completely with a continuous oil film under hydraulic pressure. This pressure is generated internally by the pumping action of the rotating shaft itself.
The Three Regimes of Lubrication
- Boundary Lubrication (Static / Start-Up): At rest, the journal rests directly at the bottom of the bearing bore (6 o'clock position). As shaft rotation begins, metal-to-metal contact occurs. Asperity contact creates high friction (coefficient of friction µ ≈ 0.08 to 0.15). Anti-wear (AW) and Extreme Pressure (EP) chemical additives prevent catastrophic welding.
- Mixed Lubrication (Acceleration): As rotational speed increases, oil adheres to the journal surface due to viscosity and surface tension. The journal physically "climbs" up the bearing wall in the direction opposite to rotation (moving to the 7-8 o'clock position for clockwise rotation). Microscopic oil films begin carrying part of the load (µ ≈ 0.02 to 0.08).
- Hydrodynamic Full Film Lubrication (Operating Speed): Viscous drag forces pump oil into the restricted, wedge-shaped converging clearance space between the eccentric journal and bearing bore. The wedge restricts oil flow, generating intense hydrodynamic pressure (reaching peak pressures of 3.5 to 14 MPa / 500 to 2000 PSI). This pressure forces the journal upward and sideways into a stable equilibrium position (4-5 o'clock position), completely floating the shaft on an oil film (0.0005" to 0.003" minimum film thickness h0, yielding friction coefficients µ ≈ 0.001 to 0.005).
AT REST (Boundary) START-UP (Mixed) OPERATING SPEED (Hydrodynamic)
Metal Contact at 6 o'clock Climbs Wall to 7 o'clock Floats on Oil Wedge at 4 o'clock
+-------+ +-------+ +-------+
/ .+--+. \ / .+--+. \ / .+--+. \
| / Journal\| | / Journal\| | / Journal\|
| \ (o) /| | \ (o) /| | \ (o) /|
| `+--+' | | `+--+' | | `+--+' |
\ || / \ // / \ /// ^ /
+---+---+ +-------+ +--|---|--+
Metal-Metal Climbing Wall Hydrodynamic Wedge
Contact (Bottom) Oil Drag Begins High Pressure Peak
Shaft Speed and Oil Viscosity Interaction (Stribeck Curve)
The stability of the hydrodynamic film is governed by the dimensionless Hersey Number (µN / P), where µ is dynamic viscosity, N is rotational speed (RPM), and P is projected bearing load per unit area. Higher speeds (N) or higher viscosity (µ) strengthen the fluid wedge. However, excessively high viscosity causes severe parasitic fluid friction, elevated operating temperatures, and thermal oxidation of the oil. Low speeds or extreme loads require higher viscosity oils or hydrostatic oil jacking pumps during start-up.
Bearing Clearance Rules and Precision Calculations
Correct radial and diametral clearance is essential. Insufficient clearance leads to oil starvation, thermal expansion binding, and metal wiping. Excessive clearance causes oil leakage, loss of hydrodynamic pressure, vibration, shaft whirl, and destructive oil whip.
The Industrial Clearance Rule of Thumb
For standard industrial sleeve bearings operating under moderate speeds (1000 to 3600 RPM), the baseline diametral clearance rule is:
Diametral Clearance (Cd) = 0.001" per 1" of Journal Diameter (D)
For metric applications:
Cd = 0.001 mm per 1 mm of Journal Diameter
- Base Minimum Clearance Allowance: For small journal diameters (under 1.000"), a baseline minimum allowance of 0.001" to 0.0015" (0.025 to 0.038 mm) must be added to accommodate thermal expansion differentials between the inner journal and outer housing.
- Precision Calculation Example: A millwright fits a plain babbitt sleeve bearing onto a 4.500" (114.30 mm) pump shaft. Using the standard 0.001"/in rule: Cd = 4.500 x 0.001" = 0.0045" (0.114 mm) If the operating temperature differential between shaft and housing exceeds 30°C, clearance is increased by an additional 0.0005", setting total assembly clearance to 0.0050" (0.127 mm).
Split Sleeve Bearing Assembly, Scraping & Fitting
Heavy industrial equipment (e.g., heavy crushers, paper machine dryers, turbines) employs split two-piece sleeve bearings. Proper fitting requires precise blueing, hand scraping, and crush verification.
Prussian Blue Contact Testing & Hand Scraping
- Cleaning & Deburring: Thoroughly clean housing bores and split shells. Remove burrs along split mating faces using a fine India stone.
- Applying Engineer's Blue: Apply an ultra-thin, uniform layer of Prussian Blue to the journal surface.
- Lower Shell Lay-In: Place the lower bearing shell into the housing base. Lay the journal into the shell and rotate the shaft back and forth through a 45° arc.
- Inspecting High Spots: Lift the shaft and inspect the blue transfer pattern on the babbitt lining. High spots appear as bright blue islands.
- Hand Scraping Procedure: Use a sharp, curved triangular babbitt scraper held at a 30° rake angle to gently scrape high spots. Repeat the process until a minimum of 75% to 85% uniform contact area (10 to 15 contact points per square inch) is established across the bottom load-bearing sector (90° to 120° arc).
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| MEASURING CLEARANCE & CRUSH |
+---------------------------------------+-----------------------------------------------------------+
| Measurement Technique | Step-by-Step Millwright Procedure |
+---------------------------------------+-----------------------------------------------------------+
| Plastigage Method | 1. Wipe journal and babbitt shells completely dry. |
| (Precision Deformable Plastic Strip) | 2. Lay a strip of Plastigage (e.g., Green 0.001-0.003" or |
| | Red 0.002-0.006") axially along top of journal. |
| | 3. Install top shell and cap; torque cap bolts to spec. |
| | 4. DO NOT ROTATE SHAFT while Plastigage is installed. |
| | 5. Remove cap; measure squished strip width against scale.|
+---------------------------------------+-----------------------------------------------------------+
| Soft Lead Wire Method | 1. Place strands of soft lead wire (0.030-0.050" dia) |
| (Heavy Industrial Turbines/Crushers) | transversely across journal crown and split lines. |
| | 2. Torque bearing cap bolts to full manufacturer spec. |
| | 3. Remove cap; measure flattened lead wire thickness |
| | using an outside micrometer accurate to 0.0001". |
+---------------------------------------+-----------------------------------------------------------+
| Bearing Crush Verification | 1. Clean housing split lines and install bearing shells. |
| (Positive Interference Fit) | 2. Place shim stock under one split line face; tighten |
| | opposite side bolts to specified torque. |
| | 3. Measure split line gap with feeler gauge. Stand-off |
| | must equal 0.001" to 0.003" (0.025-0.075 mm) to |
| | guarantee positive radial clamping (crush). |
+---------------------------------------+-----------------------------------------------------------+
Oil Grooves and Entry Reliefs
- Chamfered Entry Reliefs: The split line edges of upper and lower bearing shells must be chamfered (relieved) axially to within 0.25" (6 mm) of the bearing ends. Entry reliefs act as oil reservoirs, distributing incoming lubricant across the entire axial length of the journal.
- Oil Groove Placement Rules: Axial oil distribution grooves must be located strictly in the non-loaded, low-pressure zone (typically at the 12 o'clock position in top-fed downward-loaded bearings). CRITICAL MANDATORY RULE: Never cut axial or circumferential oil grooves through the lower 180° load zone or hydrodynamic wedge area. Cutting a groove in the pressure wedge bleeds off fluid pressure, collapsing the oil film and causing immediate metal-to-metal wiping.
A millwright is fitting a plain babbitt sleeve bearing onto a 4.000-inch (101.60 mm) diameter industrial pump shaft. According to standard millwright clearance rules, what is the target diametral running clearance?
Which statement accurately describes the sequence of events during the formation of full fluid film hydrodynamic lubrication in a plain journal bearing?
What is the primary function of establishing correct bearing crush (0.001" to 0.003" shell stand-off height) during the assembly of a split sleeve bearing?