Finite element analysis of residual-stress-induced flatness deviation in banded carbon seals

Carbon face seals, used in the aircraft engines manufactured by AlliedSignal Engines, are assembled via a shrink-fit procedure which generates residual stresses in both components, resulting in 30-50% of the seals not meeting the required flatness tolerance of three helium light bands (3 HeLB, equal to 0.762 μm). A two-dimensional, axisymmetric finite element analysis is carried out here to provide a means for better understanding the geometric and processing parameters of the face seals in order to improve their flatness. Results indicate that to obtain a lower value of f_d, the flatness deviation, the seal should have a larger inner radius (R_i), a smaller initial radial interference (δ), a larger thickness of the steel ring (t_s), a smaller thickness of the carbon ring (t_c), and a smaller height (h_t). Also, it is preferable to have a slower cooling rate during the assembly process. It is observed that the coefficient of friction at the interface, μ, does not have any effect on f_d. Finally, the process of an "expansion" fit does not contribute towards a reduction in f_d, and hence is not worth the effort and cost.