Drive pulley 1 consists of three rigidly attached (or integral) steps a, b and c of radii r_a, r_b and r_c. This pulley is the carrier of a planetary mechanism and is connected by turning pairs to planet gears 3 and 4 which are rigidly attached together. Planet gear 3 meshes with fixed sun gear 2 on whose hub d pulley 1 rotates. Planet gear 4 meshes with gear 5 which is rigidly attached to gear 6. Gear 6 meshes with gear 7 which rotates about fixed axis A. The speeds n₁ and n₇ of pulley 1 and gear 7 (in rpm) are related by the equation n₇=-n₁(z₆/z₇)((z₅z₃-z₄z₂)/z₅z₃) where z₂, z₃, z₄, z₅, z₆ and z₇ are the numbers of teeth of gears 2, 3, 4, 5, 6 and 7. Gear 7 may rotate in either direction depending upon the numbers of teeth of the gears. The driving belt can be shifted to step a, b or c of the pulley, obtaining angular velocities of ω_a, ω_b or ω_c for gear 7 which are related by the condition (ω_a)(r_a)=(ω_b)(r_b)=(ω_c)(r_c).
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