| 摘要 |
PROBLEM TO BE SOLVED: To provide a hollow crank for a bicycle lightweight and having sufficient strength as well as improving freedom of design by forming the interior of a longitudinal center part of the crank into hollow structure, and forming this hollow structure by eliminating sand, filled in a hollow part, from an opening part after casting. SOLUTION: At the time of manufacturing a crank 1, a core formed by coating reclaimed sand with resin such as synthetic resin serving as hardening material in order to maintain the shape of the core is insert-arranged in fusion space between a pair of dies, and positioned and fixed. A fused aluminum alloy, is filled in fusion space between the core and the dies to cast crank material with the core embedded inside. This crank material is placed in a semiclosed die for semiclosed die forging and subjected to hot forging. A prepared hole for a pedal fitting hole 6 is then opened, and core sand is eliminated from an opening part 15 opened at this time. Required parts are suitably machined to complete the crank 1. |
