Abstract
This article presents the bolt self-loosening mechanism in curvic coupling due to the structural ratcheting under cyclic loading. Finite element simulations are carried out under several loading conditions with different preloads and cyclic torque loadings on curvic coupling. To avoid relative rotation between bolt and nut in simulation, the surface between the threads of bolt and nut are connected together. The finite element analysis results reveal that the local cyclic plasticity occurring near the roots of the engaged threads resulted in cyclic strain ratcheting. The structural ratcheting causes the gradual loss of clamping force with loading cycles in bolts and the stresses to redistribute in the curvic coupling. Either the structural ratcheting behavior or the fatigue strength of the curvic coupling is more sensitive to the magnitude of the torque load in contrast to the magnitude of the preload of the bolt. Comparatively, the large preload of the bolt or the low magnitude of the torque load is benefit for improving the structural integrity in curvic coupling under the bolt self-loosening.
Highlights
Curvic couplings are widely used in large power equipments, such as aero-engine and heavy-duty gas turbine
A torque load should be applied on one disk to loosen the bolts in curvic coupling, and another disk should be fixed at the bottom base plate
The wire rope connected to the loading nut and bolt as the loading device is to apply the torque load on the curvic coupling to actuate the rotation of the twisting plate
Summary
Curvic couplings are widely used in large power equipments, such as aero-engine and heavy-duty gas turbine. Owing to the special feature of the curvic teeth shape, the curvic couplings have lots of advantages, such as reliable positioning, precise centering, excellent structure stability, strong loading, and bearing ability, meeting the requirements of strength, vibration, and fatigue life. The centering ability can be improved by repeated preload and running-in. It is being used more and more widely. Because of the complex shape of the curvic teeth, the stress distributions and part stiffness in curvic coupling are difficult to be studied in theory. In order to analyze the rotor character in dynamics, NL Pedersen and P Pedersen[2] did some works to determine the equivalent flexural stiffness of a curvic coupling.
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