LOSS-OF-LUBRICATION OPERATION OF HELICOPTER TRANSMISSIONS

An important parameter in helicopter design criteria has become the ability to complete the mission after loss of drive system lubrication. Loss-of-lube operation has become a basic military requirement to improve survivability after receiving combat damage; it is also applicable in the commercial field for offshore oil rig operation, where an emergency or forced landing would be extremely hazardous. This paper traces some of the history of loss-of-lube operation, and, in particular, it presents the most recent developmental tests conducted on present-day production helicopter drive systems. Previous efforts have isolated the major problem areas with emphasis on gears and bearings of the main transmission, because their duty cycle and heat generation are usually more severe as compared to accessory and tail rotor drive gear-boxes. The first reduction stage between the engine and the transmission is usually the primary problem area due to the high operating speeds and high temperatures generated. Normally ball and roller bearing sets or tapered roller bearings support a bevel pinion and gear in this area of the transmission system. Heat generated after loss of lubricant is very high, which leads to failure from seizure of the bearings and/or loss of gear backlash as a result of thermal expansion. Ultimately, total failure is caused by high heat load, reduced hardness, plastic deformation, and high frictional wear. Previous transmission tests have indicated that planetary assemblies are also a major life-limiting area following loss of lubrication. Often planetary assemblies contain bearing retainers, which are severely limited by the strength/melting point of the material. During dry operation, loss of clearance and plastic deformation in the sun gear/planet pinion mesh, caused by thermal expansion, can also occur and result in early transmission failure. These are the main areas of component improvement toward which the design for improved survivability must be directed. In recent years, the United States Helicopter Industry has contributed significantly to the engineering understanding of loss-of-lube phenomena through United Stated Army sponsorship of scientific evaluations of several helicopter main transmissions and the requirement for no-lube operation on both the UTTAS and AAH systems. This paper will discuss these investigations, including failures modes, recent test results from highly instrumented transmissions, and will set forth design suggestions to improve transmission operation after loss of lubrication.