Many years ago the US Army wanted to consolidate the lubricants used on their helicopter transmissions. The result was the change from mineral based oils with a relative high viscosity and higher film strength to the oil used in the gas turbines powering the helicopter and these oils have a lower viscosity and lower film strength as compared to the mineral based oils previously used.
With the advent of this change there were a lot of transmission failures due to inadequate lubrication. The use of gas turbine oil in transmissions was later adopted by all of the military and carried over to the civilian sector.
The gas turbine oil was designed to operate in a gear train and on bearings that had minimal point contact loading. In the transmission application the point contact loading in the gear mesh was considerably higher and the oil was operating at a point where lubrication film strength was at a minimum.
Unlike mineral based oils, which have a higher viscosity at operating temperature, and multi grade oils, which increase both viscosity and film strength at operating temperatures the gas turbine oil drops in viscosity and film strength at transmission operating temperatures. This required the development of two types of oil. One for warm theater operations and one for winter. This is good for the gas turbines but does not provide the same results when used in a helicopter transmission. Newer helicopter designs cater to this oil application using closer tolerances but the point contact loading of the gears is still the same if not higher with increased torque on larger helicopters.
This use by the military resulted in a higher maintenance rate with the decreased frequency between oil changes, which effected operating costs for the helicopter. Civilian operators were also effected in the same way.
