Introduction:
This reverse engineering project revolved around a Black&Decker Electrical Hand Screwdriver, shown in Figure 1. The processes in reverse engineering the screwdriver involved researching the tool's specifications, disassembly of the tool, examination of its parts, and a kinematic analysis to determine the motor specifications necessary to provide the desired torque output of turning a screw.
Figure 1. Exterior of electrical hand screwdriver tool
The following table of contents will direct you throughout the report on this reverse-engineering project.
Disassembly:
After removing the screws and pin holding the screwdriver's plastic casing, the exterior plastic casing covering the gear system was readily removable.
Figure 2. Interior of plastic upper plastic casing upon first disassembly.
Figure 3. Lower planetary gear set, visible upon first disassembly.
Immediately upon viewing the interior of the screwdriver, two planetary gear sets in series were visible as seen in Figures 2 and 3. The plastic casing serves as the stationary ring gear for the entire gear system, and the lower gear set's carrier also serves as the sun gear for the upper gear set. Upon further inspection, each component of the interior was heavily lubricated, which was a surprise to see in such a small, low power, household appliance.
Figure 4. Interior of plastic casing with upper planetary gears removed.
As seen in Figure 4, removing the planetary gears shows the arm that the three gears are attached to in the upper gear set. The arm and the check can be seen to rotate at the same speed when manually turning the chuck, showing that this arm is the final output of the system being driven by the gear system.
Figure 5. Lower gear set, after removing the carrier arm.
Removing the carrier arm allowed for a comparison of the lower and upper planetary gears, which revealed them to be identical, having the same number of teeth and being made of the same plastic material. This was in contrast to the higher torque screwdriver we observed the video deconstruction of in class, which had stronger, metal gears in the upper gear set in order to better withstand the higher torque that is transmitted near the output of the screwdriver. From this design choice alone we can see the difference in material considerations necessary between weak and power tools. The sun gear attached to the motor was also the same size as the carrier sun gear, with the same number of teeth.
-Note, check on the sun gear material to see if its a weak bronze or something so you can comment on that
After the initial disassembly, I could begin to start formulating what would be necessary for kinematic analysis. It was clear from observing the motor speed that it far exceeded the speed at the chuck during operation, indicating the tool's intended purpose is to transfer a high input angular velocity to a high output torque.
Kinematic Analysis
| Gear Designation | Number of Teeth |
|---|---|
Sun | 6 |
| Planetary | 19 |
| Ring | 48 |
Figure 6. Number of teeth for each gear in the planetary gear system. Sun gear corresponds both to the motor and carrier sun gears.
To begin analyzing the transmission of torque and angular velocity throughout the gear system, the teeth numbers were obtained during disassembly and recorded above in Figure 6.
