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Black + Decker Li2000 Screwdriver Reverse Engineering
ME357 2020 Boston University College of Engineering
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INRODUCTION
The purpose of this project was to reverse engineer the Black and Decker Li2000 Screwdriver by disassembling it and analyzing its components. On this website, you will find information regarding the physics of its user functions, analysis of the gear box, and an overview of mistake proofing assembly principles. This project combined course material and engineering design knowledge by allowing students to utilize CAD skills within Creo to create parts, an assembly, and a mechanism of the gear box.
B+D Disassembly Parts and Product Structure
Gearbox Analysis
The B+D Li2000 Screwdriver utilizes an epicyclic gear train consisting of a ring gear, two planetary carriers, one sun gear, and two sets of three planet gears. The screwdriver used an epicyclic gear train for several reasons including compactness, increased load distribution along a single axis, and an increased lifespan. Additionally, the output torque and angular velocity are optimized from the two stage gear train and accompanying gear ratios. These ratios allow the output torque at the screwdriver hear to be increased by a factor of 81 from the input, while the angular velocity is decreased by a factor of 81. The gear ratio calculations and pitch diameter calculations are displayed below.
Stage 2 of Gear Train
Stage 1 of Gear Train
CAD Drawings and Gearbox Animation
Below are CAD drawings of each component of the epicyclic gear train modeled in Creo Parametric, a CAD assembly with an exploded view, and an animation of the gearbox mechanism. Although the input angular velocity is 14, 580 RPM, this was animated with a slower angular velocity in order to better observe the details of what is going on. As you can see, the gear stage closest during the animation (the gear closer to the motor of the screwdriver) has a larger angular velocity, and it is reduced to a smaller angular velocity at the output.
CAD Drawings of Each Component
Assembly Drawing
Animation
Important Mechanical Features
Power/ Manual Option
This feature allows users to switch between power and manual modes based on preference. In the power mode, the motor drives the output, while in the manual mode, the user turns the screwdriver by hand. This is done by rotating the head of the screwdriver to the preferred option as shown by user-friendly symbols. The action of rotating the head raises and lowers the planetary carrier into and out of a stationary ring gear that locks it into place. When the screwdriver is set to manual mode, the teeth on the circumference of the planetary carrier mesh with the teeth of the ring gear, thus locking it into place and preventing power drilling. When the screwdriver is turned to power mode, the planetary carrier is not locked into place, allowing the gears to rotate freely and the gear train to operate.
Forward/ Reverse Switch
This feature allows users to choose one of three modes by means of a sliding switch. Sliding the tab to the center puts the screwdriver into neutral mode because it aligns the two terminals of the motor so that neither circuit is complete. Sliding the tab to either side flips the polarity of the terminals, completes a circuit, and causes the motor to operate either clockwise or counterclockwise.
Handle Pivot Lock Mechanism
The handle of the tool can be pivoted into the three positions for optimum versatility. These positions are different angles pictured in figure D from the Black and Decker Lithium Pivotdriver Instruction Manual. In order to pivot the handle, users must press the pivot release button, and may choose to release it into a new position. The pivot release pin has teeth that lock it into place with teeth on the orange casing and black handle of the screwdriver. As seen in the pictures, there is a gear tooth that is wider in all components of the screwdriver. When everything aligns, free rotation is allowed until it locks into one of the three positions.
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The wider tooth on the locking pin highlighted in orange to the left allows pivoting when pressed as a result of proper orientation with the teeth on the casing.
Principle of Design for Manufacturing and Assembly
Mistake Proofing is an important principle of design for manufacturing and assembly (DFMA) because it ensures the components are put together properly because there is only one way to do so. A common type of mistake-proofing within the B+D screwdriver is prevention, which employs techniques of part symmetry leading to correct part insertion. This process prevents the engineer from making a mistake in relation to assembly. Examples of mistake-proofing in the screwdriver include:
Ring Gear to Motor Housing Assembly
addition, a U-pin secures this connection without requiring screws, and the idea of mistake proofing is demonstrated by the pin fitting perfectly in only one orientation.
There are indents along the top edges of the ring gear that fit with notches on the casing surrounding the motor so that it can only fit together properly. This allows them to “snap” into place in only one possible orientation. This ensures that the ring gear housing the gear train stays rigid with respect to the motor. In
Main Body Casing
The orange casing of the main body demonstrates the DFMA of mistake proofing because it snaps into connection with the motor housing in only one orientation. The holes for the screws guide this assembly, and the same size screws are used to secure the casing into place.
Removable Sun Gear
The sun gear is designed so that the hole in the center has an extra piece of material covering a portion of the hole as shown in the picture to the right and the CAD drawing in the CAD section. This allows it to fit into place on the motor that has a flat piece removed from the connection axis. This is important because the motor drives the sun gear that operates the gear train, and by staying fastened into place on the motor, the sun operates with the correct angular velocity rather than spinning freely. This is a mechanical requirement
Wiring
The positive end of the motor is connected to the red wire, and the negative end to the black wire, and the motor receives power from the battery. This coloring allows the assembly and repair to be easier and mistake-proof because engineers and users can connect the wiring properly. The red and black coloring related to positive and negative terminals is universal, making it accessible and straightforward.
Conclusion
This project allowed me to reverse engineer a relatively cheap household item, and analyze the complex mechanical structure of a seemingly simple tool. Reverse engineering techniques are important because reverse engineering allows you to understand how different machines operate. This project also provided me with an opportunity to build a portfolio demonstrating the Computer Aided Design skills that I have acquired this semester. The most useful thing that I have learned from the screwdriver is how to analyze a gear train because epicyclic gear trains are common among a variety of machines. The most useful thing that I have learned from the overall project is how to build a website, and I look forward to using this skill for future projects.
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References
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