Gearbox Analysis
Why an epicyclic gear train?
The reason Black and Decker went with an epicyclic gear train was mainly because of its compact size. The screwdriver can produce a gear ratio of 81:1 in a very small tool that can conveniently fit in a pocket or small bag, which is great for portability.
Gear train calculations
An epicyclic gear train consists of a sun gear, planetary pinions, a planetary carrier, and a ring gear. In this screwdriver, there are 2-stages to this gearbox. In other words, there are 2 identical epicyclic gear trains connected in series.
Pitch Diameters:
Planetary Pinions (Planets or P): 0.525 in
Sun gear: 0.181 in
Ring gear: 1.231 in
Planetary Carrier: 1.412 in
Gear Ratio: 81:1
Torque: T_out = 81 * T_in
Angular velocity: w_out = w_in / 81
Torque and angular velocity:
The gear ratio in this gear train is 81:1, which correlates to an 81:1 torque output/input respectively. This allows the gearbox to increase torque from the electric motor dramatically. However, this torque increase does come at the cost of speed, but given that the motor spins quite fast, the angular velocity output is relatively good.
