Black & Decker Li2000 Screwdriver Disassembly
by Ilyas Nazarov
Black & Decker Li2000 Screwdriver
The goal for this project was to disassemble the B&D Li2000 screwdriver in order to gain an understanding of how the internal mechanisms worked, namely the gear assembly within the gear box. During disassembly and reassembly, I also took note of DFMA (Design for Manufacturing and Assembly) examples as well. I also observed how the various other assemblies worked, such as the handle pivot lock and the power/manual option.
The first component of the Li2000 screwdriver disassembly project was to analyze the gearbox assembly. The calculations below show the process I used to obtain the gear ratios for the entire system and the pitch diameters for the planet and sun gears.
Screwdriver graphical representation:
Below are sketches of the sun gear, planetary gear, planetary carrier, and ring gear, along with an assembly of the entire gearbox.
Here’s an exploded view of the assembly:
DFMA, or Design for Manufacturing and Assembly, is an important method of design which helps to prevent assembly mistakes. There are several instances of DFMA in the B&D Li2000 Screwdriver.
The first example is the external shell itself. The two images below show the motor correctly seated in the case and then the motor misaligned from the case. In the second misaligned photo, it is clear to see how the bottom of the motor does not line up with an extrusion in the case.
The second example of DFMA is the shape of the handle pivot washer. In the picture below, I have improperly inserted it into the case, and as a result protrudes significantly when compared to its proper placement in the second picture.
A third example of DFMA is the fitment of the switch to the internal metal contact rocker. As shown in the picture, the switch can only be put on one way since there is a T-shaped extrusion on the contact carrier and a corresponding hole in the switch.
A fourth example of DFMA is the U-pin on the exterior of the gearbox assembly. The casing has an indent for the pin to sit flush on only one side as shown below. If the pin were to be inserted incorrectly, it would stick out significantly from the case.
Elaboration on the function of mechanical elements:
Aside from the gear box, there are three other features of the Li2000 worth noting: the forward/reverse switch, the tool handle pivot lock, and the power/manual option.
The forward/reverse switch works by switching the polarity of the flow of electricity from the battery to the motor. The electrical circuit below offers some insight into how it works:
Position A shows the switch in the “forward position” and position B shows the polarity switched when the switch is put in the “reverse position.” The image below shows a sketch of the metal contact carrier within the casing onto which the switch connects.
In one position, the motor leads connect metal contacts (1) and (3) and in another position, the motor leads connect with metal contacts (2) and (4), which have a reversed polarity.
The tool handle pivot lock is also an important mechanism within the Li2000. It allows the user to adjust the angle of the handle for optimum comfort during operation. The pivot lock works by having the user push in from one side, which disconnects the gears and allows the user to move the handle. When a desired position is found, the spring automatically retracts the assembly and the gears are locked into a new space in the casing. The diagram below shows the locking tooth in the gears:
The image below shows one of the gears within the casing:
The Li2000 also has a feature where the user can toggle between the normal powered mode and a “manual” mode, which locks the gearbox and allows the user to manually turn the drill like a regular screwdriver. This feature works by having a stationary white gear within the gear box. In power mode, the lower planetary carrier is disconnected from the stationary gear, and the gearbox as able to spin the bit socket. However, in manual mode, the lower planetary carrier is embedded into the stationary gear, which locks the entire gear system, allowing the user to manually move the drill. The figure below shows both the power and manual mode positions.
Overall, this teardown of the B&D Li2000 was very interesting. I was able to learn how the various components of the drills works exactly, and the thought process of the engineers who designed the device. I found the instances of DFMA very interesting in that they were extremely effective in preventing incorrect assembly, while at the same time being very simple in nature. It was also interesting to see how Black and Decker used some tactics to cut costs, such as using a switch which manually changed the polarity rather than an electronic switch. As a result, although it was much cheaper to design the switch that way, over time, the switch will most likely fail.