(a) Why Gear Failures Occur Despite Proper Setup
Even with ideal static conditions, several dynamic factors degrade the integrity of main reduction gearing:
- Dynamic Loading and Hull Deflection: A ship is not a rigid structure. In heavy seas, the hull flexes, which can cause the gear casing to distort slightly. This shifts the tooth contact away from the intended "perfect" line, leading to localized high-stress areas.
- Thermal Expansion: As the turbine and gearbox reach operating temperatures, components expand at different rates. Static alignment often fails to account for the exact "hot" running position, leading to misalignment under load.
- Vibrations (Torsional and Axial): Propeller law and engine impulses create vibrations. If these synchronize with the natural frequency of the gearing system, they cause momentary tooth separations and "hammering," which breaks the oil film and leads to pitting.
- Oil Film Breakdown (Elastohydrodynamic Lubrication): Scuffing occurs when the oil film thickness drops below the surface roughness of the metal. Even with a "copious supply," if the local pressure is too high or the sliding speed too low (common during maneuvering), the lubricant cannot prevent metal-to-metal contact.
- Contamination: Microscopic particles (metal wear or sea water) act as abrasives. Water, in particular, reduces the load-carrying capacity of the oil and promotes corrosion-fatigue pitting.
(b) Viscosity in Marine Turbine Oils
Viscosity is arguably the most critical property of a turbine oil, as it determines the thickness of the lubricating film that prevents wear.
Significance of Viscosity
- Load Carrying: It must be high enough to maintain a hydrodynamic film between gear teeth and in journal bearings to prevent metal contact.
- Friction and Heat: If viscosity is too high, internal fluid friction increases, raising the operating temperature and reducing the efficiency of the turbine.
- Cooling and Flow: The oil must be thin enough to flow rapidly through sprayers to carry heat away from the gear meshes and bearings.
Control of Viscosity
Viscosity is primarily controlled by temperature regulation. Marine systems use L.O. (Lubricating Oil) Coolers with thermostatic bypass valves. By maintaining the oil inlet temperature (typically between 40°C and 50°C), the viscosity is kept within the design "sweet spot."
Causes of Viscosity Change in Service
- Oxidation: Constant exposure to heat and air causes the oil to "age," forming sludge and organic acids, which increases the viscosity.
- Contamination: * Water ingress (from gland steam or cooler leaks) can create emulsions, usually increasing the apparent viscosity and ruining lubricity.
- Fuel dilution (less common in pure turbines, but possible in combined plants) will decrease viscosity.
- Thermal Cracking: If the oil is localized-overheated (e.g., a hot bearing), the molecular chains break down, which can eventually lower the viscosity.