Hand Drill DFMA/Gearbox Analysis

Introduction

For the final project we were tasked with disassembling a Black and Decker battery powered drill. The primary object of the project was to analyze and model the gearbox on the device in Creo. The secondary objects were to note any clear cases of DFMA and to break down the primary mechanical functions of the device.

DFMA: Assembly Mistake-Proofing

The plastic motor casing has a specific shape that only fits into the gearbox case in a specific orientation. This reduces error during assembly as it is not possible to assemble the unity in the incorrect orientation.

The gearbox pin has a cutout on the shell of the front of the drill that lets the pin sit flush inside of it. The other side does not have this. This clearly shows the correct orientation of the pin and informs the assembler that there is a right and wrong direction for the pin to be inserted.

The rear external shell has indentations designed to lock both sides into play without any movement. This shows the assembler that they are in the correct orientation and also makes the assembly process easier by lining up all the screw holes without any movement.

Mechanical Explanation:

A) Electrical switch:

The electrical switch is integrated into the housing for the power contacts for the electric motor. The motor has two metal connection tabs which are connected to the positive and negative leads from the battery. The power switch moves a plastic ring that holds the connections from the battery and depending on the direction it is moved, moves the plastic ring in the direction that connects the motor to the the positive and negative battery leads. This means that when pushed in one direction the motor polarity in one direction and when pushed in the other it will flip to the other direction. The only mechanical change is which metal tab of the ring (carrying the battery power) touches the two tabs on the motor unit. It’s a simple mechanical element that solves a more complicated problem of switching motor polarity.

B) Locking handle:

The handle uses a locking mechanism that employs a spring to push the locking gears into place. When the user pushes on this mechanism it compresses the spring and allows the gears to disengage and rotate until the spring is uncompressed and the gears lock into the new orientation.

C) Automatic/manual switch:

The power/manual option uses a switch in the front of the drill that allows the user to lock the drill gearbox in order for the drill to be used as a manual screwdriver. It does this by pushing or pulling the second planetary carrier into the locked ring gear in the front of the drill assembly. The manual mode pulls the carrier in and locks it against the ring gear. This prevents the gears from moving and essentially turns the drill into a manual screwdriver that doesn’t spin. With the switch is set to automatic, the carrier is pushed forward and allowed to spin freely, letting the gears spin and the drill function as normal.

Product Structure

Gearbox Analysis

The gearbox consists of two sun gears with 6 teeth, six planetary gears with 19 teeth, two planetary carriers, and one ring gear integrated into the shell of the device with 48 teeth.

A.

Gear ratio calculation:

Npc = Nsun + Nring = 54

GR1 = Npc/Nsun = 54/6 = 9

GR2 = Npc/Nsun = 54/6 = 9

GRfinal = GR1 x GR2 = 9 x 9 = 81

Final gear ratio of 81 throughout the entire gearbox

B.

Center distance (C) between sun and planet = .35”

To find P (diametric pitch):

C = (Nsun + Nplanet)/2P

.35 = (6 + 19)/2P

P = 35.7

For sun gear PDsun:

Nsun/P = PDsun

6/35.7 = .168

For planet gear PDplanet:

Nplanet/P = PDplanet

19/35.7 = .53

For ring gear PDring:

– PDring = PDsun + (2 x PDplanet)

PDring = .168 + (2 x .53)

PDring = 1.22

Pitch diameters:

PDsun = 0.168

PDplanet = 0.53

PD ring = 1.22

C.

Black and Decker decided to use a epicyclic gear train for its ability to have a large gear reduction in a small packaging envelope. A traditional gear train would have to be much larger to achieve this, and for a product such a drill, packaging is an extremely important component of the design. This setup fits perfectly inside the body of a drill and can be built to similar dimensions as the electric motor so there is no wasted space inside the chassis. The ring gear is also part of the exterior shell, which minimizes individual parts required. Additionally design allows for an easy locking mechanism to be built into the drill for the manual mode feature to be implemented.

Gearbox drawings

real-time Animation

Project Takeaways

Modeling and animating the planetary gearbox in Creo a really compelling component of the project. Being able to see the gear reduction play back in real time after modeling the setup was a satisfying illustration of the progress we have made in Creo. The entire process of learning about how planetary gearboxes work and why they are beneficial, to going through the modeling and drill disassembly/assembly gave a nice overview of the topic. It was also interesting seeing how real products go about getting designed with manufacturing and assembly in mind.

Conclusion

This project gave us hands on experience at disassembling a real product in the context of CAD and mechanical analysis. Being able to model the gearbox in Creo let us use the skills we have developed over the semester and proved to combine everything together in one project. It was fascinating to learn about all the different elements that go into designing something that most people have used but never thought about mechanically. Seeing how powerful the gearbox of this little drill can make a small electric motor shows the importance of gearboxes. I’m looking forward to modeling future products with these same skills and gaining more insight into the mechanical workings of other machines.