ME359 SCREWDRIVER
DECONSTRUCTION PROJECT
Sitthipak Snidvongs
BU ID: U95482633
Date: 24th April 2018
Professor Peter A. Zink
Project overview
The best way to understand how something works is to take apart the components and investigate how each component contributes to one another. The purpose of this project is to understand how the Black & Decker Li2000 screwdriver works. This report will include all the necessary artifacts to help develop an understanding of how the gear ratio and gearbox works, which also includes the functionality of this particular screwdriver.
The product structure shows the overview of various components contained within the screwdriver. From that, calculations for the gear ratios, pitch diameter, torque and RPM of gears will be shown to investigate why the product was designed in such a way. Lastly, the gearbox will be modeled using PTC CREO where individual gears will be drawn to further develop an understanding of the product itself.
Product Structure
Gearbox Analysis
Number of Teeth
Gear Ratio
Torque and RPM Analysis
Pitch Diameter
Calculations for the Planetary Gear Set
Calculations for Combined Gear Set
Calculations for Pitch Diameter
Why the Epicyclic Gear Train?
1) POWER
Can produce a higher power because the loads are distributed among the planetary gears, this increases the torque
2) EFFICIENCY
From the calculations, it can be seen that the epicyclic gear train can produce an output torque 81 times the input torque with 1/81 of the input speed from the motor. Generally, larger torque is achieved by larger screwdrivers but designs from B&D delivers a reasonable amount of torque.
3) LIGHTWEIGHT
Screwdriver is compact and lightweight and does not require a large area to store the battery which makes it very convenient
Engineering Drawings
Assembly Drawing
Planetary Carrier Drawing
Ring Gear Drawing
Planetary Gear (Pinions) Drawing
Sun Gear Drawing
Discussion Questions
DMFA: Assembly Mistake Proofing
1. Elimination
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Design is simple and easy to follow
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Product can be broken down into 3 main parts (shown in the product structure)
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Screwdriver is designed in a way that there is one common axis that the gearbox and motor revolves around
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Design is cost-effective because it minimizes the assembly operations of individual parts, reduce chance of failure while trying to assemble the screwdriver
2. Facilitation
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Minimizes the number of individual parts
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Design has many common features
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Planetary gears have the same number of teeth & similar pitch diameter
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Planetary carrier gears were also made of the same material and pitch diameter
3. Mitigation
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Design is Compact
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All parts were relatively easy to assemble and disassemble
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Screwdriver is composed of only 5 screws, indicating that the assembly operation was minimized
How the Screwdriver functions
The Forward, Reverse, and Lock Switch
For both forward and reverse operations, the screwdriver must be operating in a power mode (the current symbol in the figure on the left)
Forward Operation:
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Switch (1) must slide towards the right side, completing the circuit which allows current to flow in one direction
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Press the switch to begin the forward operation and the screwdriver will begin rotating towards the right (direction of motion of sun gear).
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Release the button to stop the rotation
Reverse Operation:
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Switch (1) must slide towards the left to complete the circuit and allow current to flow in one direction
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Press the switch to begin the reverse operation and the screwdriver will begin rotating towards the left (sun gear rotates towards the left)
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Release the button to stop the rotation
Lock Operation:
When switch (1) is in the central position, the lock will prevent battery discharge because the circuit is incomplete
The Tool Handle Pivot
The tool handle can be changed to three positions to adapt into the needs of various uses. To pivot the handling tool, press the button labeled as (2) and release the button anywhere of the positions that provides the most versatility.
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The tool handle pivot works by interlocking the teeth mechanism in the handle
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The spring allows it to rotate to three different positions shown in the figure above
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Rotation will be fixed once the teeth interlocks with the other
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Once button is pushed, teeth will no longer interlock and it will be viable to change positions
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A screw must be installed for the pivot handle to function properly
The Power/Manual Option
Power and Manual mode can be switched using the collar of the screwdriver
Power mode:
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When switch (4) is rotated to the left (current symbol), the screwdriver will operate from due tot he motor
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The gearbox will not be locked and will perform based on the calculations of the gear ratio, torque and speed
Manual mode:
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The screwdriver acts like a regular screwdriver (non-powered)
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It will turn for a very short period of time (less than a second) and the motor gets turned off when button is pressed
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Screwdriver locks the last carrier gear with the hex spindle, which is connected to the whole gearbox
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User can manually operate the screwdriver through torque and moments
Electrical Circuit Diagram
Circuit diagram 1
Circuit diagram 2
It can be seen from circuit diagram one that the circuit is currently open (no current flow). This corresponds to circuit diagram two which shows that the circuit is open when it is located at the lock switch, middle. However, once the switch is rotated to the right (clockwise), as shown in circuit diagram two, it will cause the plates to interact with the motor prongs which carries current from the battery to rotate the screwdriver in the clockwise direction (forward). Furthermore, this causes the current in circuit diagram one to flow to the forward direction and into the motor circuit.
Similarly, once the switch is rotated to the left (anti-clockwise), shown in circuit diagram two, the plates will interact with the motor prongs which allows current to flow from the battery to rotating the screwdriver in the anti-clockwise direction (reverse). From circuit diagram one, it can be seen that current will flow through the reverse direction and into the motor circuit.
In addition, the direction in which the motor spins will change based on the polarity. In other words, the motor will rotate in one direction when the positive terminal was connected to a negative terminal and will rotate in the opposite direction when the terminals are swapped. This is because the direction in which the current flows through the motor's coil is different.
Interesting/Useful Things from Project
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Getting students involved in hands on experience
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Learning how mechanical gearboxes work
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Combining our knowledge from CAD modeling with gears
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Gained insight about the importance of gear ratios, especially in a screwdrier
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Learning that an epicyclic gearbox could increase the amount of torque while decreasing the speed
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This project was a great way to incorporate our understanding to real world applications
Takeaway: The best way to really understand how something works is to take it apart and learn how each component affects the other
Conclusion
The main purpose of this project was to understand how the gear train for the B&D Li2000 worked by deconstructing, analyzing and modeling it. The most interesting part was determining the combined gear ratio which led to analyzing the combined speed and torque. The most astonishing part was that the result indicated the the output torque is 81 times that of the input torque and that the speed for the output was that of the input speed. This indicates that the gearbox is very simple and efficient because without it more power would have been required as an input from the battery to achieve the same torque and speed.
I have learnt many valuable things from this project, from being able to take apart, analyze, model and assemble the product back together, I have also understood how gear ratios work. I am certain that I will use the screwdriver again in future projects. Overall, this project has helped me develop a better understanding of how the electric screwdriver works which will definitely benefit me in future projects.