HAND DRILL GEARBOX STUDY
The purpose of this project was to examine the driver gearbox of a B&D Li2000 screwdriver and use computer-aided design (CAD), specifically Creo 5.0 Parametric, to model its parts and create appropriate engineering drawings. This project also demonstrated how to deconstruct and reassemble the device, analyze the multi-stage epicyclic gear train of the gearbox in terms of its gear ratio and pitch diameters, and recognize specific features that function either as mistake-proofs or practical mechanisms.
The chart below displays the product structure of this screwdriver with emphasis on the gearbox. Note that the handle and battery pack was not disassembled.
GEAR BOX COMPONENTS
The following CAD drawings illustrate the overall gear train and its four unique components of the driver gearbox: the washer, sun gear, 6 x planet gears, and 2 x planetary carriers. Note that the models of the planet gears have been simplified without teeth.
An epicyclic gear train was chosen for this B&D screwdriver for several advantages:
the system is smaller in volume and can fit into the narrow cylinder case, thereby minimizes the product's overall size
the load applied to the gear train is shared among the multiple planets of the system, thereby increasing torque capability
the shared load and axis provides increased stability of the system.
Multiple calculations were made to analyze the multistage epicyclic gear train (gearbox). These include the gear ratio of each gear set, the overall gear ratio (product of the gear ratios of each set), and the pitch diameters of each gear (planet, sun, ring, and planetary carrier).
DISCUSSION #1: MISTAKE-PROOFING
Several elements of this screwdriver were designed specifically for mistake-proofing. The following features exemplify methods to eliminate assembly error and prevent misalignment or misconnection of critical parts.
1. Exterior Casing
Part of the assembly requires putting and locking the two halves of the exterior casing (orange) together to encapsulate the gearbox and connect them to the handle. To prevent the pieces from being attached on the wrong side of the handle, there is a gear-like groove engraved in the handle that must match that of the case piece to be attached.
2. Motor and Gear Train Connection
The upper body of the drill contains two parts: one that hold the motor and one that holds the epicyclic gear train. To ensure proper assembly of these two parts and prevent misconnection, the plastic case of the gear train contains two sets of grooves:
(Left) A wide single groove that fits the wide single protrusion on the motor casing
(Right) Two symmetric grooves parallel to each other that fit the two parallel protrusions on the motor casing
3. U-Pin Insertion
To ensure that the motor and gear train parts stay connected, a metal U-pin must be inserted. However, there are two pairs of holes on opposite sides of the gear train case through which this pin can be inserted. One of the pairs, however, contains a groove between the holes that allows the pin to fit into after insertion. This way, the pin will mold itself into the cylindrical shape of the case and allow proper encapsulation of the exterior casing around it. If the pin was inserted in the other pair, which doesn't contain the groove, part of the pin will stick out and its protrusion will interfere with the assembly of the exterior casing.
DISCUSSION #2: MECHANICAL DESIGN
Several mechanical elements were designed to interact with each other for various functions:
1. Tool Handle Pivot Lock
The handle and upper body (motor and gearbox) of the screwdriver are connected by a pivot lock where the user can adjust the angle between the two parts. To keep the desired position in place and prevent sliding of the upper body, a pin runs through the pivot hole. One end of this pin has a gear like segment with 9 teeth, one of which is wider and has an upside-down U shape to it, which will be called the major tooth. These teeth fit through the gear shape of the exterior casing (depicted below). During the adjustment of the screwdriver orientation, the pin gear slides above the casing gear and is "unlocked". When an orientation is set in place, the major tooth slides onto one of the teeth of the casing gear rending it "locked".
The image above depicts (from left to right):
1) how the pin looks from the bottom of its cylindrical shape to the gear segment, emphasizing the difference between the major tooth from the other teeth of the gear
2) how the gear segment pin looks relative to that of the exterior casing in the 'unlocked' position and
3) how the gear segment pin fits into that of the exterior casing in the 'locked' position
2. Forward/Reverse Switch
Depending on the position of the power switch, the screwdriver can operate in either clockwise (CW) or counter-clockwise (CCW) rotation. (Note that this CW/CCW rotations are observed when looking directly at the chuck). This switch piece has a center protrusion that the user can push and adjust the switch position either to the left or right. When the piece is pushed to the left and the right-hand triangle pointing down is shown, the screwdriver will operate clockwise. When the piece is pushed to the right and the left-hand triangle pointing up is shown, the screwdriver will operate counter-clockwise.
The position of the power switch dictates how the chuck operates. Note that squiggle lines are break lines that exclude a sketch of the upper body to emphasize the relation between the chuck and power switch.
The circuit of this mechanism is depicted in its own schematic below where the CW rotation operates when circuit A is closed and the CCW rotation operates when circuit B is closed.
The electrical circuit diagram of the forward/reverse switch mechanism
3. Power/Manual Option
A handy and practical feature of this screwdriver is a toggling option between power/automatic mode and manual mode. This can be done by rotating the top piece of the upper body. Mechanically, the different modes changes the position of a base gear. When the screwdriver is set to power/auto mode, the base gear is lowered and separated from the first planetary carrier of the gear train. This allows the gear train to rotate continuously with its speed controlled by the power of the motor. When the screwdriver is set to manual mode, the base gear is lifted to surround and fit around the first planetary carrier. The outer part of the base gear has teeth that fit into the ring so when the power switch is turned on, the base gear locks the planetary carrier in place and blocks rotation.
The image above depicts the base gear position relative to the first planetary carrier of the gear train depending on the mode set. Note that the shading denotes the interior of the upper body.
Overall, this project provided an extremely valuable hands-on experience on how to disassemble and reassemble a device as well as how to use computer aided design (CAD) to virtually build and assemble parts of the product. More specifically, this class taught us how to use Creo 5.0 to model the driver gearbox and how to analyze the multi-stage epicyclic gear train in terms of its gear ratio and pitch diameters.
From a personal perspective, the most interesting thing about this project was inspecting the interior mechanisms of a household item. Being able to see the parts that make up the function of a practical tool in person without losing the ability to reassemble them was a very fun task. Lastly, the ability to put together all the skills that was taught by this class and apply it on a product designed by our own professor was incredibly useful and relevant. 10/10 would recommend this class.
Thank you Professor Zink and all the LA's in ME 359 for teaching me these helpful skills!