Gearbox Analysis
The gearbox for the Black & Decker LI 2000 is comprised of a two stage epicyclic gear train, and thus contains multiple planetary pinion gears, a sun gear, and ring gear, and a planetary carrier. The gearbox analysis shown below contains calculations for output torque, gear ratios, and pitch diameters of the gears.
Black & Decker likely chose the epicyclic gear train for several reasons. First, the gears used in the screwdriver, such as the pinion gears and the sun gear, are standard in design. Thus, they are easy to manufacture, and cost effective. The gears within the epicyclic gear train are centered about a single axis, and this allows for a change in the direction of rotation. This is a very important aspect of a screwdriver, so it was likely thought of when deciding whether or not to use an epicyclic gear train. An interesting design in this particular screwdriver is that the ring gear is stationary, attached to part of the screwdriver cover. Since it is stationary, it keeps the motor more stable. Lastly, through calculations below, it is shown that the product has a high gear ratio, making it more efficient.
First, the gear ratio is calculated using the equation:
Gear Ratio=Noutput/Ninput
*N represents the number of teeth
The following information is also needed for the calculation:
N(sun)=6
N(ring)=48
N(planetary pinion)=19
N(planetary carrier)=N(sun)+N(ring)=6+48=54
Thus the gear ratio is: GR=N(planetary carrier)+N(sun)=54/6=9
Since this is a two-stage epicyclic gear train, then the gear ratio is squared, thus we have GR=9*9=81
Next, the pitch diameter is calculated, using the equation: N/Diametral Pitch
To calculate the diametral pitch the equation used is: DP=(N+2)/Outer Diameter
Pitch Diameter (planetary pinion)=N/(N+2)/OD=19/21/0.565=0.51
Pitch Diameter (sun gear)=N/(N+2)/OD=6/8/0.20=0.15
Pitch Diameter (ring gear)=N/(N+2)/OD=48/51/1.387=1.31
Then, a function is derived for the output torque and RPM in relation to the motor torque and RPM (w-motor):
GR=w-in/w-out=torque-out/torque-in,
w-out=w-in/GR=w-in/81
torque-out=GR*torque-in=81*torque-in
Product Structure
Drawings:
including
Components and Assembly
Discussion
DMFA: Assembly Mistake Proofing
Black & Decker utilized mistake proofing in their design of the screwdriver, in order to make it easier to manufacture and assemble. Let's look at 3 specific instances where Black & Decker used mistake proofing:
1. The Black & Decker screwdriver uses vastly different coloring and unique shapes of parts to differentiate whats parts go where. For example, the upper part of the screwdriver includes a casing that is bright orange, along with parts that incorporate this orange color to signify to the assembler that these parts go together. We can see this with the lock button. Although it is black, there is bright orange coloring on it, thus one can infer where it belongs. The lock button assembly also acts like a puzzle: the unique shapes allows the assembler to more easily figure out which parts connect where, as seen in the photo below
2. Black & Decker also simplified assembly by combining parts. For instance, there are two main parts of the screwdriver that hold other minor parts: the handle and the top. The top of the screwdriver includes the lock button, the switch, and even the gearbox all in one area. The handle holds the battery and its components. This was, the assembler can focus on these two parts one at a time, while also dealing with less external parts, ultimately making the assembly much easier.
3. Finally, Black & Decker simplifies the gear box itself. It attaches the ring gear to the casing of the motor, making it stationary and immovable. Therefore the assembler cannot accidentally misplace it, or insert it in a different area. By removing this choice of placing the ring gear, it is clearly known where the gearbox belongs, making it less likely that it will be assembled incorrectly.
How does forward switch work?
A battery powered DC motor within the screwdriver creates a source of energy. As charge flows around the circuit of the DC motor, it is attracted and repelled by the two different poles of the magnetic field, causing the loop to spin. This is how the forward switch works. When the switch is flipped, the orientation of the magnetic field and direction of charge is also flipped. So, the loop will instead spin in the other direction, which also switches the direction the screwdriver spins.
How does tool handle work?
The pivot lock is a plastic rod with a gear shaped end that enters the orange casing on one side, and then exits through the other side. There is a spring around the plastic rod, and it is a bit longer than the casing, so when it is pushed all the way in, the handle is allowed to freely rotate. When the rod is pushed to its outer position, the gear shaped part on the rod locks into place. The handle then locks into place and therefore cannot rotate.
How does the power/manual option work?
The screwdriver lets the user choose between battery power mode and manual power mode to rotate the drill bit. It does this by restricting the movement of the planetary carriers. When the screwdriver is in power mode, the DC motor rotates and causes the components of the gearbox to move. However when the screwdriver is in manual mode, the planetary carriers in the gearbox are not moving, because the DC motor is not being utilized to rotate it. Thus, the axis of the planetary carriers are lined up with the axis of the hex spindle. This allows the user to manually turn the screwdriver in order to screw objects into place.
Most interesting/useful thing you learned from project?
I enjoyed deconstructing the screwdriver, and figuring out how exactly it works. I also enjoyed applying the material learned in the course to conduct analysis in the screwdriver. The calculations and equations disccussed in class, such as gear ratio or pitch diameter, are much better understood when they are applied to a physical product. I also enjoyed learning about mistake proofing, because although I knew much thought went into product design, I did not realize that this included specific ways for easy manufacturing and assembly. Often when people think of product design they think "how can I design this product so it does what I want it to while be efficient and cost effective?". So it was nice to see the other side of it. I look forward to taking these skills and applying to my own designs in the future, or any other personal project I may create.
