The purpose of these studies was to reduce the mechanical heat load generated inside onboard superconducting (SC) coils. Fretting tests were performed at liquid-helium-cooled boundaries between the structural components of SC coils, the test results demonstrating that mechanical loss had occurred at the boundary between an epoxy-impregnated SC coil and coil fasteners. The correlation between SC coil vibration and increment of heat load in the eigen-mode of SC coil twisting was also investigated and discussed by performing vibration tests and dynamic harmonic analysis. The results suggested that frictional heat occurred in the areas subjected to the greatest degree of bending.