Carbon Graphite Water-Lubricated Bearings for Marine Propeller Shafts: Solutions for Seawater Corrosion and Sediment Erosion

Marine propeller stern shaft bearings are critical components of the ship's power transmission system; their performance directly relates to navigation safety. Traditional copper alloy bushings wear rapidly under seawater + sediment conditions and require regular dry-docking for replacement. With the advantages of wear resistance, corrosion resistance, self-lubrication, and zero oil pollution, carbon graphite water-lubricated bearings become the ideal alternative to copper bushings. Based on Huahao Sealing's application experience at multiple shipyards, this article introduces carbon graphite water-lubricated bearing selection points.

1. Service Characteristics

1.1 Typical Service Parameters

• Media: seawater (3.0-3.8% NaCl salinity, pH 7.5-8.4) + sediment (0.1-1.0 kg/m³ sediment content, up to 5 kg/m³ at Yangtze estuary)
• Speed: 2-14 knots (1-7 m/s), corresponding stern shaft linear velocity 3-15 m/s
• Load: propeller thrust + shaft dead weight + propeller eccentric excitation force, specific pressure 0.5-2.0 MPa
• Temperature: seawater 0-35℃, coastal vessels up to 40℃

1.2 Failure Modes

Conventional tin bronze (ZCuSn10P1) bushing failure statistics: sediment abrasive wear (55%, groove depth 1-3 mm), seawater electrochemical corrosion (25%, pitting pits 2-5 mm), poor lubrication shaft seizure (15%), fatigue cracking (5%). Average life is 1-1.5 years, requiring dry-dock replacement every 2 years at a cost of 200000-500000 CNY per occurrence.

2. Material Selection

2.1 Epoxy Resin-Impregnated Graphite

For marine water-lubricated bearings, epoxy resin-impregnated graphite M120E is preferred, with key properties:

• Seawater stability: 3.5% NaCl solution immersion for 3000 hours, weight change <0.1%, no swelling
• Self-lubricating: after water film rupture (e.g. ship grounding, start-up moment), still allows 30-minute dry run without shaft seizure, friction coefficient <0.3
• Wear resistance: sediment service wear rate 0.03-0.05 mm/1000h, 1/5-1/8 of copper alloy
• Oil-free zero pollution: uses seawater as lubricating medium, no lubricating oil consumption, zero discharge meeting MARPOL Convention

Compared to phenolic-impregnated graphite: epoxy is more stable in seawater (pH 8); phenolic hydrolyzes in weak alkali. Compared to furan-impregnated: furan has higher temperature rating but higher cost; epoxy offers the best cost-performance for ambient seawater service.

2.2 Carbon Fiber-Reinforced Type

For stern shafts >300 mm diameter or heavy vessels (tankers, bulk carriers), M120E-CF (carbon-fiber-reinforced epoxy-impregnated graphite) is recommended:

• Compressive strength: 180 → 280 MPa (+56%)
• Impact strength: 0.8 → 1.2 J/cm² (+50%)
• Cost increases ~25%, but life extends 30-50%

2.3 Mating Shaft Material

Stern shaft clad with 316L stainless steel or overlaid with cobalt-base alloy (Stellite 6), hardness above HV 350. Avoid ordinary carbon steel shafts (seawater rust + sediment wear).

3. Structural Design

3.1 3-Lobe/6-Lobe Water Chamber Structure

Water-lubricated bearings use a lobe-type structure, embedding graphite strips into a rubber matrix (or metal matrix + rubber damping layer), forming alternating graphite load-carrying strips and water chambers:

• 3-lobe: suitable for small diameters (<200 mm), simple structure
• 6-lobe: suitable for large diameters (>300 mm), 40% water chamber ratio, uniform load
• Water chamber ratio: 35-45%; too high lacks load area, too low cannot flush sediment
• Graphite strip width: 20-30 mm, specific pressure controlled below 1.0 MPa

3.2 Axial Water Flow Path

1-2 axial spiral water grooves (5-8 mm depth) on the bearing inner wall guide seawater entering from the front end and discharging from the rear, forming continuous cooling and sediment flushing. Flow velocity 0.5-1.0 m/s ensures no sediment deposition.

3.3 Rubber Damping Layer

3-5 mm styrene-butadiene rubber (SBR) damping layer between graphite strips and metal matrix absorbs propeller vibration and hull deformation, preventing graphite strip brittle fracture.

4. Installation Alignment Points

4.1 Bearing Press-Fit

• Dry-ice cool graphite strip OD to -40 ~ -30℃, shrink then press into bearing housing, interference 0.08-0.12 mm
• Direct hammering of graphite strips is prohibited; force-bearing face padded with wood board
• Graphite strip end face flush with bearing housing, tolerance ±0.2 mm

4.2 Stern Shaft Alignment

• Shafting flange offset <0.05 mm, deflection <0.05 mm/m
• Bearing ID larger than stern shaft by 0.6-1.0 mm (diametral clearance), ensuring water film thickness
• 2-hour trial run, check temperature rise <20℃, no abnormal noise

5. Huahao Sealing Case Study

A shipping company's 25000 DWT bulk carrier (stern shaft diameter 340 mm, shaft length 12 m, rated power 3000 kW) originally used tin bronze bushings averaging 1.5 years life, each dry-dock replacement costing about 350000 CNY (including dock fee, labor, spare parts). After switching to Huahao Sealing's M120E-CF carbon-fiber-reinforced epoxy graphite 6-lobe bearing + Stellite 6 overlaid stern shaft solution in 2018, as of 2026 it has operated for 8 years, accumulated 480000 nautical miles sailing, with inspection wear of only 0.25 mm. Estimated remaining life is 5-6 years. Over 8 years cumulative maintenance cost saving is about 2800000 CNY, while eliminating lubricating oil leakage risk.

Selection core: shaft diameter/load determines material grade (heavy-duty selects CF reinforced type), sediment content determines water chamber ratio (high sediment selects 40-45%), seawater environment determines impregnant (epoxy resistant to weak alkaline seawater).