Design charts were developed to determine minimum and maximum face-gear inner and outer radii. Topics included tooth generation, limiting inner and outer radii, tooth contact analysis, contact ratio, gear eccentricity, and structural stiffness. Face-gear design and geometry were investigated. A light-weight, split torque transmission design utilizing face gears was described. The use of face gears in helicopter transmissions was explored. In particular, the use of torque-splitting gear trains is proposed as a practicable means of improving the effectiveness of helicopter main gearboxes.Īpplication of Face-Gear Drives in Helicopter Transmissions Helicopter transmission arrangements with split- torque gear trainsĪs an alternative to component development, the case for improved drive-train configuration is argued. The complete system stiffness as represented by the stiffness matrix used in this analysis must be considered to precisely determine the optimal tooth indexing position. For the design studied, the balance beam is not an effective device for load sharing unless the coefficient of friction is less than 0.003. Friction must be considered to properly evaluate the balance beam mechanism. Although the gearbox has symmetric geometry, the loads and motions of the two power paths differ. The phenomenon of sidebands spaced at shaft frequencies about gear mesh fundamental frequencies was simulated by modeling total composite gear errors as sinusoid functions. Cornell's method for calculating the stiffness of spur gear teeth was extended and applied to helical gears. The mathematical model includes time-varying gear mesh stiffness, friction, and manufacturing errors. The Lagrange method was applied to develop a system of equations of motion. This report presents the results of an analytical study of the system dynamics and performance of a split torque gearbox that uses a balance beam mechanism for load sharing. Split torque designs, proposed as alternatives to traditional planetary designs for helicopter main rotor transmissions, can save weight and be more reliable than traditional designs. Also, results show that shaft location and mesh stiffness tuning are significant design parameters that influence the motions of the system. Results show that although the gearbox has a symmetric configuration, the simulated dynamic behavior of the first and second compound gears are not the same. The model was demonstrated with a test case.
The effects of time varying gear mesh stiffness, static transmission errors, and flexible bearing supports are included in the model. A mathematical model was developed to study the dynamics of the system. When the thrust loads are balanced, the torque is split evenly. The design studied in this work includes a pivoting beam that acts to balance thrust loads produced by the helical gear meshes in each of two parallel power paths. A split torque design allows a high ratio of power-to-weight for the transmission.
In this way the test transmission will provide a base for several years of analytical, research, and component development effort targeted at improving the performance and reliability of helicopter transmission.ĭynamics of a split torque helicopter transmissionĪ high reduction ratio split torque gear train has been proposed as an alternative to a planetary configuration for the final stage of a helicopter transmission. Progressive uprating of engine input power from 3600 to 4500 hp twin engine rating is allowed for in the design. One necessary change to the test stand involved gear trains of different ratio in the tail drive gearbox.
The transmission fits within the NASA LeRC 3000 hp Test Stand and accepts the existing positions for engine inputs, main shaft, connecting drive shafts, and the cradle attachment points. It is demonstrated that in comparison with conventional helicopter transmission arrangements the split torque design offers: weight reduction of 15% reduction in drive train losses of 9% and improved reliability resulting from redundant drive paths between the two engines and the main shaft. The 3600 hp split- torque helicopter transmissionįinal design details of a helicopter transmission that is powered by GE twin T 700 engines each rated at 1800 hp are presented.
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