To provide a smooth chain traveling surface and to reduce noise by constituting a tension arm by a synthetic resin molded body and forming a chain traveling surface forming part into a hollow shape under the application of a gas assist method.
A tension arm 9 is a synthetic resin molded body of polyamide 6, 66 and is constituted of a main body part 12 and a chain traveling surface forming part 13. On the substantial whole of one side surface of the main body part 12, a deep thinned recessed part 14 is formed. On the substantial whole of one side surface of the chain traveling surface forming part 13, a shallow thinned recessed part 15 is formed. A plurality of reinforcing ribs 18 are provided at prescribed intervals between projecting lines 16, 17. In the molding of the arm 9, both of an injection molding method and a gas assist method are used, the chain traveling surface forming part 13 is formed into a hollow shape and a cavity 22 is provided on the substantial whole. Because the chain traveling surface forming part 13 is hardened because the chain traveling surface 21 is strongly and uniformly pressed on a cavity surface by gas pressure acting within the chain traveling surface forming part 13 in a molding process, the arm 9 has a smooth chain traveling surface 21 and noise can be reduced.
YAKUBO KAZUE
SHIRAKAWA ATSUSHI
