ダケカンバ材の野球バット適正の評価

Abstract

ダケカンバは北海道の先駆種でもあり蓄積量も多いが十分な活用がなされてこなかった。そこで価値の高い活用方法としてバット用材としての評価方法とその適正を検討した。バット用材に最も要求される物性を衝撃曲げ強度と考え，ダケカンバ材の繊維傾斜と衝撃曲げ強度および衝撃曲げ破壊エネルギーの関係を調べた。既存の報告に近い傾向が衝撃曲げ強度でも確認でき，ハンキンソン式の適用が可能であった。バット用材のグリップを模した試験体で衝撃曲げ試験を行い，既存のバット用材と比較した。密度および繊維傾斜を補正した結果，ダケカンバは既存のバット用材と破壊エネルギーと強度の両方で同等の性能を示した。実際に硬式野球バットを試作し，大学野球部で試打を行った。一週間の試打で破損は見られず実用に耐えうることが確認できた。アンケートの結果では打球感や飛距離などでも「良い」や「普通」の回答が大半をしめ，実用上問題がないことが確認できた。

Dakekanba (Erman's birch) is a pioneer species that has not been used sufficiently in practical applications even though it is the most abundant wood source in the Hokkaido prefecture. We believe that it has potential as a valuable material for use in baseball bats. Assuming that the critical factor for baseball bats is an impact strength, we elucidated the relationships between slope of grain (SoG) and impact strength or impact fracture energy of dakekanba wood by conducting impact bending tests. Results revealed that the Hankinson equation could be used to calculate these relationships to impact strength because they were similar to those considered in previous studies of impact fracture energy. Cylindrical wood specimens such as bat grips were assessed in the impact bending tests, and dakekanba wood was compared with woods used for commercial bats. After impact strength were corrected based on SoG and density, both strength and fracture energy of dakekanba wood were found to be nearly the same as those of commercial wood species for baseball bats. Prototype bats of dakekanba wood were manufactured and experimentally used by a baseball club at Kyoto University. The prototypes were confirmed to have sufficient durability, as no bats broke during their one-week trial use. On a questionnaire survey, players responded with “good” or “normal” regarding bat feeling, flown distance, etc. after hitting with the prototypes. Our study reveals that dakekanba wood has good potential as a baseball-bat material.