Fastening devices between ground coils and guideways used in outdoor environments for long periods of time under conditions of vibration by electromagnetic forces during the running of linear-motor railway vehicles are important components that affect maintenance management costs and system reliability. However, the bolt fastening method (a promising way for fastening ground coils) has the problem of possible damage resulting from compressive creep and stress concentration near fastening portions due to fastening axial force in the resin portion.

As a method to solve these problems at the same time, the RTRI developed a laminated FRP bushing for bolt fastening portions using sheet winding (a manufacturing method consisting of impregnating glass cloth with resin, winding the half-hardened sheet around a metal mold while pressurizing it, and then subjecting it to heat curing)(Fig. 1). The laminated FRP bushing is such that its level of compressive creep can be reduced to a negligible level in effect by arranging reinforced fiber in the axial direction of the bolt on the reinforcing layer of the laminate. Further, stress concentration near the fastening portion can be significantly moderated by arranging a stress-moderating layer in which the coefficient of elasticity is suppressed to about 1/3 that of resin for coil molding on the outermost layer.

The RTRI performed comparative verification using a method to simulate steps on fastening surfaces using a fastening portion model to forcibly perform bolt fastening, and confirmed that stress generated near the fastening portion can be reduced to about 1/3 of the value found with conventional metal bushing, and to the order of about 1/2 compared to cases when improved metal bushing was used (Fig. 2). Further, through electromagnetic vibration tests representing 35 years of commercial-line usage for coils with actual trains, the RTRI confirmed that there were no abnormal conditions (such as looseness of bushings and generation of cracks in bolt fastening portions), and that the durability of the laminated FRP bushings is superior to that of metal bushings.