On January 1, 2000, the Amateur Softball Association of America (ASA) imposed maximum bat performance limitations on commercial softball bats. The ASA adopted a testing standard defined by the American Society of Testing and Materials (ASTM) to determine the bat performance factor (BPF), a normalized coefficient of restitution that must be less than 1.2 for the bat to be eligible for ASA sanctioned events.
The ASTM standard requires that the softball strike the bat, which is free to rotate in the horizontal plane, at 26.8 mfs ± 0.3 mfs (88 ftfs ± 1 ftfs) with little or no spin. The central project goal was to develop the ASTM test apparatus, which consisted of a precision ball launcher, a pivoting stage for the bat, and instrumentation for velocity measurements. The key feature of the testing apparatus developed in this project was the ability to measure the rebound velocity of the ball directly-ASTM method derives the ball rebound velocity by assuming the bat behaves as a rigid body and applying conservation of angular momentum.
Tests revealed a discrepancy in the BPF between the ASTM method and an alternative method,. termed the direct method, which uses the direct measurement of the ball rebound velocity. Furthermore, the ASTM method proved to be very sensitive to parameter errors, demonstrated by magnification factors between 2.0 and 3.0. The direct method was insensitive to parameter variation with magnification factors between o and 1.0.
The ball rebound velocity discrepancy was also analyzed with mechanism simulation software. A three-degree-of-freedom model of the bat was used to test the effects of elasticity and pivot friction. The analysis determined that applying conservation of angular momentum on an elastic body caused transient errors in the derivation of the ball rebound velocity; and pivot friction significantly affected the motion of the bat and thus, the derived ball rebound velocity.
The experimental results show that the direct method was more accurate than the ASTM method in calculating the BPF; and the conclusion of the analytical model shows that the ASTM method can be corrected by precisely identifying external moments in the system.
| Identifer | oai:union.ndltd.org:pdx.edu/oai:pdxscholar.library.pdx.edu:open_access_etds-2726 |
| Date | 09 May 2001 |
| Creators | Lee, Danny V. |
| Publisher | PDXScholar |
| Source Sets | Portland State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Dissertations and Theses |
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