Rudder Bearing Bushing Material (Synthetic)

What sets synthetic rudder bearing material apart

A rudder carries large side loads and moves slowly, with water or grease as the lubricant and sand or silt in the water. Traditional materials such as bronze or lignum vitae wear, corrode or swell, and they tolerate misalignment poorly. Synthetic materials, typically fabric-reinforced thermoset laminates or engineered thermoplastics, give low friction, good resistance to seawater and a long wear life with little maintenance. They also behave differently from metal: they expand more with temperature, many absorb water and swell slightly, and they are sensitive to edge loading. That is why the installation clearance is calculated for the material and not copied from a metal bearing.

Main designs

• Laminated composite bushings, with fabric layers in a thermoset resin matrix, for rudder stock and pintle bearings
• Thermoplastic bushings, machined or moulded, for lower loads or where a self-lubricating behaviour is wanted
• Split or segmented bearings that can be fitted without lifting the rudder, used in repair situations
• Bearing plates and thrust washers of the same material for axial loads
• Grease-lubricated and water-lubricated designs, with the grooves and the lubricant supply adapted to each

Properties that decide the choice

• Permissible bearing surface pressure, as stated in the approval for that material
• Friction behaviour in water or with grease, and its dependence on speed and load
• Water absorption and swelling, which changes the clearance in service
• Thermal expansion compared with the steel stock and the housing
• Resistance to wear from sand and silt, and to chemicals in the lubricant or the surrounding water

Selection and sizing

• Bearing load and projected area, kept below the permissible pressure for the material
• Running clearance, set from the manufacturer's installation instructions with allowance for swelling and temperature
• Fit into the housing, using an interference fit or a retaining arrangement as the manufacturer prescribes
• Bushing length and wall thickness, to resist edge loading and allow for the later wear-down
• Lubricant arrangement and drainage, so sand does not collect in the bearing

Regulations and class

Classification societies treat synthetic rudder bearing materials as approved products. The approval states the material, its tested properties and the surface pressure the society accepts for design calculations. Rudder clearance and bearing wear are checked at dry-dockings, and an excessive clearance can be a reason to renew the bushing. Replacing a bearing with a material that does not hold the approval changes the basis of the rudder design and must be agreed with the society beforehand.

Typical faults

• Excessive wear and rudder play, noticed as noise, vibration or a worsening clearance reading
• Tight or seized bearing after the bushing swelled more than the installation clearance allowed
• Edge cracking or crushing from misalignment between stock and bearing
• Abrasive damage after silt or sand entered a bearing with a failed seal
• Loosening or movement of the bushing in its housing, if the fit or retention was not correct
• Chemical or heat damage from an incompatible lubricant or from friction under dry running

What to look for in a supplier

• A class approval that names the material and gives the permissible bearing pressure
• Installation instructions with clearance formulas for the specific material and shaft diameter
• Machining to drawing, including grooves, chamfers and tolerances, from semi-finished stock
• Records of batch and material properties, so a repeat order matches the earlier one
• Service support at the dry-docking, with measuring and fitting guidance

Record the rudder clearance at every dry-docking and plot it. A gradual trend can be planned for, while a sudden step usually means damage to the bushing or the stock.