Gear box oil is a high viscosity oil and mostly contain EP (extreme pressure) additives and AW (anti wear) additives to counter the sliding action of the gear teeth.
The functions of oil in a gear box are: lubrication, transfer of heat, preservation against corrosion and carrier of contaminations. The most important criteria for selecting gear oil is viscosity. In general, the oil viscosity of a gear box is determined by the gear box manufacturer. Deviation from the default oil specification must be by approval and in conjunction with the manufacturer. Replacing or adding incorrect oil type in a gear box can destroy the equipment. Furthermore, nowadays many gear box manufacturers also recommend the oil cleanliness in the gear box, before adding it to the gearbox, in the commissioning phase and during service.
ISO4406 cleanliness code is easy to recognise and understand, the higher the numbers the more particle contaminations are in the oil. Every increase of ISO-class means double the amount of counted particles in the oil. For example, ISO16/14/11 contains 60.000 of 4micron particles, whereas ISO17/15/12 contains 120.000 of 4micron particles in an 100ml oil sample.
Another important parameter for oil cleanliness is water content in the oil. Dependent on the oil type the recommended water content is between 300-900 mg/kg(ppm) water-in-oil. In all cases keep the water content below the saturation level of water-in-oil, above the saturation level water will be present as free-water in the oil system. Free-water will create havoc in the gearbox, it will promote corrosion, washout oil additives and accelerate oxidation process of the lubricant.
Furthermore, the operational temperature of the gearbox oil is an important factor. According to Svante Arrhenius, a Swedish chemist who won the Nobel prize in 1903 for the relationship between temperature and most chemical reaction rates, stated: “Every 10°C increase of temperature above the baseline will halve the life time of the oil”. This equation is also known as the Arrhenius rate rule.
Too much heat in a gearbox create ranges of problems, such as: acceleration of oil additives depletion, thinner oil-film which in turn causing higher wear, together with the presence of water it will create a good habitat for microbial growth, it will damage seals and gaskets when the temperature exceeds the design temperature and it accelerates the formation of resins or varnish.
Above: Shell Omala150, oil sample from an azimuth thruster.
To ensure good cleanliness of gearbox oil the first is to measure the cleanliness by periodical oil sampling and study the trend. In modern time, there are many Oil Condition Monitoring (OCM) devices which measures the oil cleanliness in real-time from and can be observe from a distance. These devices are very useful to reach difficult places and as primary important equipment, which are indispensable and of primary importance. Lord Kelvin: “To measure is to know - if you can’t measure it, you can’t improve it”.
For a good housekeeping of the gearbox oil the following is recommended: preventative equipment to keep dirt & moisture out of the gearbox, measuring devices of the oil conditions regarding cleanliness & temperature and additional equipment to enhance the best conditions of the gearbox. The additional equipment is: desiccant air-breathers, oil filters full flow and side stream (off-line) adequate to the requirements, oil purifiers and if needed oil coolers.
Click on the button “more info” to learn more about side stream (off-line) oil filter and oil purifier.