Taking things a part is the best way to figure out how something works. The internal workings of an object may seem like a mystery, especially when encased in fancy coverings, however a lot can be learned by taking a look on the inside. This is called reverse engineering and is a powerful tool in learning how things work, if done correctly. As an engineering candidate at Boston University, this has been stressed over and over again; some classes I have taken spend entire weeks on just teaching how to do this properly. And for good cause, by being able to disassemble and reassemble something, you gain a more dynamic understanding of the engineering that went into making something. And so to get hands-on experience with this, we practiced what we learned with a Black&Decker Electric Screwdriver.
Project Goals: The main goal of the project is to disassemble the gearbox of a Black&Decker Li2000 electric screwdriver. Once this was done, the gearbox should be analyzed and modeled on PTC Creo.
Learning Objectives: Completing this project required a lot of analytical skills in order to effectively disassemble and analyze the internal gear box. These skills include:
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Disassembly
The first step in this project was taking it a part. This was the easy part, by finding a few hidden screws the outer shell came right off. From there, each part was removed and kept track of until every part was removed from the screwdriver and laid out in front of me.
With all of the internal pieces in front of me, the details of how the product functions became clear to me. Each part served a specific purpose, and interacted with other parts in order to make the tool work effectively. With this in mind, as well as having the process of disassembling the tool fresh in my head, I created a structure chart of the screwdriver:
The first step in this project was taking it a part. This was the easy part, by finding a few hidden screws the outer shell came right off. From there, each part was removed and kept track of until every part was removed from the screwdriver and laid out in front of me.
With all of the internal pieces in front of me, the details of how the product functions became clear to me. Each part served a specific purpose, and interacted with other parts in order to make the tool work effectively. With this in mind, as well as having the process of disassembling the tool fresh in my head, I created a structure chart of the screwdriver:
The rest of the project will just be focusing on the gearbox branch of the structure chart.
Analyzing the Gearbox
Once the entire screwdriver was taken a part, it was much easier to access and analyze the gearbox. Not only was the gear ratio of the gearbox determined, but a particular control feature of the gearbox was explored. By turning the head of the gearbox, the screwdriver could be toggled between manual and powered operation. This was explored first.
Once the entire screwdriver was taken a part, it was much easier to access and analyze the gearbox. Not only was the gear ratio of the gearbox determined, but a particular control feature of the gearbox was explored. By turning the head of the gearbox, the screwdriver could be toggled between manual and powered operation. This was explored first.
With this feature of the gearbox figured out, the gear system inside of the gearbox could be analyzed. First, information on each of the gears was pulled from analyzing the gears individually. It is at this point when I learned that the gearbox inside was comprised of two sets of planetary gears. Once this was completed and numbers were found, calculations of the gear ratio and pitch diameter could be carried out. Below is an image of the inside of the gearbox, and below that the calculations from the gearbox analysis. Note, there is an error with my overall gear ratio calculation. Equation 3 should be "Npc = Nring * Nsun" which would result in an overall gear ratio of 81.
Modeling
All of the calculations about the gearbox are completed, now it was time to model the gearbox in Creo. The gearbox has 10 parts in total, 4 of which are unique. They are as follows:
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Below are the drawing files for the gearbox components and assembly:
Take-Aways
Design for Manufacturing and Assembly:
Black and Decker incorporates DFMA in its products for the simple reason of making the manufacturing and assembly process simple and efficient. This is evident via analysis of the parts of their electric screwdriver. One instance of this is in the screws used to fasten the casing together. A majority of the screws are the exact same. This is a textbook example of DFMA, using similar screws throughout a product makes it easier to replace and install screws as the product is being assembled because there is only one type of screw to worry about. Further, it allows for easy manufacturing because the machinist only has to create on type of hole for these screws, just in different locations. Another instance of DFMA is in that the six planet gears in the gear box are all the same type and made of the same material. Again, similar to the screws, it allows for easy installation and replacement of the internal components because there is only one type of planet gear to focus on. A third instance of DFMA is in the design of the plastic case. Most likely, this casing was injection molded, however it had to be designed in a way that would allow it to be manufactured using this process. Under close inspection, the case has a simple design: simple curves, regular bored holes, and simple extrusion features. The case even comes in two parts that must be screwed together. All of these factors allow for it to be made using an available manufacturing method that can be used to create a lot of this component quickly and cheaply.
Choice of an Epicyclic Gear Train:
The gear train that Black and Decker chose to use in its screwdriver was an epicyclic gear train. This gear train layout has many advantages that are useful for the design and operation of this screwdriver. Using this gear train is very compact. The coaxial arrangement means that there is no need for additionally shafts for an output or other gears, allowing for a gearbox that is small yet still performs as needed. Further, this design is very modular as individual planet gear stages can be stacked until the required output is achieved. Additionally, an epicyclic gear train has a high power density, meaning it can withstand a higher torque. In this arrangement, the planet gears share the load rather than a singular gear being subjected to the entire load. This increases the lifetime of the gear allowing for longer operation and a higher overall product lifetime. Finally, there is very low backlash and high efficiency with epicyclic gear trains, two qualities that are very desirable in any system involving gears. Given that a screwdriver needs to be able to withstand and deliver a reasonably high torque yet needs to be able to fit in a hand comfortably, it is very clear as to why Black and Decker choose to use this type of gear train. From a design and mechanical standpoint, it is the most advantageous option.
Design for Manufacturing and Assembly:
Black and Decker incorporates DFMA in its products for the simple reason of making the manufacturing and assembly process simple and efficient. This is evident via analysis of the parts of their electric screwdriver. One instance of this is in the screws used to fasten the casing together. A majority of the screws are the exact same. This is a textbook example of DFMA, using similar screws throughout a product makes it easier to replace and install screws as the product is being assembled because there is only one type of screw to worry about. Further, it allows for easy manufacturing because the machinist only has to create on type of hole for these screws, just in different locations. Another instance of DFMA is in that the six planet gears in the gear box are all the same type and made of the same material. Again, similar to the screws, it allows for easy installation and replacement of the internal components because there is only one type of planet gear to focus on. A third instance of DFMA is in the design of the plastic case. Most likely, this casing was injection molded, however it had to be designed in a way that would allow it to be manufactured using this process. Under close inspection, the case has a simple design: simple curves, regular bored holes, and simple extrusion features. The case even comes in two parts that must be screwed together. All of these factors allow for it to be made using an available manufacturing method that can be used to create a lot of this component quickly and cheaply.
Choice of an Epicyclic Gear Train:
The gear train that Black and Decker chose to use in its screwdriver was an epicyclic gear train. This gear train layout has many advantages that are useful for the design and operation of this screwdriver. Using this gear train is very compact. The coaxial arrangement means that there is no need for additionally shafts for an output or other gears, allowing for a gearbox that is small yet still performs as needed. Further, this design is very modular as individual planet gear stages can be stacked until the required output is achieved. Additionally, an epicyclic gear train has a high power density, meaning it can withstand a higher torque. In this arrangement, the planet gears share the load rather than a singular gear being subjected to the entire load. This increases the lifetime of the gear allowing for longer operation and a higher overall product lifetime. Finally, there is very low backlash and high efficiency with epicyclic gear trains, two qualities that are very desirable in any system involving gears. Given that a screwdriver needs to be able to withstand and deliver a reasonably high torque yet needs to be able to fit in a hand comfortably, it is very clear as to why Black and Decker choose to use this type of gear train. From a design and mechanical standpoint, it is the most advantageous option.