Ball Mill Performance & Efficiency: S. Description: Symbol: Formula: Example: No. 1: Arm of gravity: a: 0.666 * ( 1 – 4 * R 2) 1.5: h: 0.583 – TAN-1 [ 0.5 / R * (1 – 4*R 2) 0.5] – [ 2*R * (1 – 4*R 2) 0.5]: Deff: 4.18: h: Centre Distance (from mill centre to media top layer), mts