Assignment 3: Mechanisms

There is something very pleasing about watching gear mechanisms work in an infinite loop!

Browsing through Cornell's Kinematic Models for Design Digital Library, there were so many interesting mechanisms.

I found Model: O05 Planetary Gear, Slider-Crank Kinematic Chain from Cornell's Reuleaux Collection very compelling.

Planetary gear trains (or epicyclic gear trains) are not attached to fixed bodies.  Instead, the links holding the center of the gears move.  Since all the links can move, the train value can be changed by using different gears or carriers. This functionality makes planetary gear trains useful in automotive transmissions.

Below is an image of a basic planetary gear train.

https://memechanicalengineering.files.wordpress.com/2014/02/machines-and-mechanisms.pdf

The central gear (1) is called the sun gear.  The planet gear (3) revolves around the sun, hence the name planetary gear.  The carrier (2) holds the planets in orbit around the sun.This sun/planet gear system is usually inclosed in a ring gear (4).

http://kmoddl.library.cornell.edu/stillImages/small/O05.jpg

In Model O05 from Cornell's library, "there are five links. The fixed base holds two revolute joints. The sliding guide is constrained to the base by one of the revolute joints and constrained to the rod that is fixed to the larger gear. The pinion gear is affixed to another link that rotates on a revolute joint at the base. Finally, there is a link connecting the two gear wheels by two revolute joints. The model is designed so that it can be removed from its base and the large gear fixed as a sun gear and the pinion roll as a planet gear. The pinion has 26 teeth and the larger gear as 52 teeth."

Another gear mechanism I found very beautiful involves its application to art, which can bee seen in this video:

Assignment 2/Part 2 of 2: Windlass

We started by brainstorming general designs:

Initially, we decided on a triangular design:

Professor Banzaert recommended an A-frame, explaining that our triangle structure may not hold up to the weight of the bottle.  We began out calculations, keeping several A type frames in mind:

While designing our parts on SolidWorks, we felt that a full semi-circular plate would make our design strongest in terms of foundation.  The stronger the better! This would decrease the risk of any unforeseen accidents and in turn decrease our number of iterations.  

Our design involved using bushings, pegs, and notches.  We made three iterations of each method to ensure the tightest fit.

We knew any initial paper calculation would change once we began designing our parts in SolidWorks and so to save time, we made all our calculations with the program.

Yay, time to print!

Immediately after assembling our parts, we tested it with the 1 liter bottle. 

Although the foundation was strong and did not collapse inward, we found the weight of the bottle turned the four outer rods in a direction that was opposite to the direction the center rod was turning.

To fix this issue, we piano wired the two center plates (holding five rods) to the center  rod.  In order ensure that the center rod did not turn in a direction that was opposite to the direction the wheel was turning, we piano wired the wheels to the center rod as well.  Just to be extra safe, (and also because we wanted to use the machine!) we heat staked the outermost bushings to the center rod to ensure our wheel would not move or fall off.

The most difficult part of this project was getting the drill press to align with the center plates, wheels, and with the rod, and drill completely through from the top, through the rod, and to the bottom.

We were SO HAPPY it worked!  Out of excitement we accidentally pushed the windlass off the table TWICE but it STILL held up nicely!

There were two requirements we forgot to check while going over our design:

1. The bottle must rise 10cm above the table. 

2.  We may use only one handle.

    Since we gave ourselves a lot of room to make changes by staying well under the limit of 500cm^2, a simple change it plate size will fix the first issue.

Technically speaking, our windlass does work with one handle! Two handles make sure the structure is well balanced on both sides and also makes pulling the bottle  up a faster process.  In a real world situation, individuals would be able to use the well from both sides or even pull larger loads of water when two people are turning the wheel together.  This makes for a very practical and efficient windlass.

Exact measurements coming soon!

Assignment 1: Bottle Opener

I joined this class a little late but I am so glad I did!

Our first assignment was using Solid Works to create a bottle opener.

My first instinct was to do an online google search on bottle openers and a certain design caught my eye.

http://www.sothatscool.com/imgs/brass-crescent-bottle-opener-1.jpg

I found this design aesthetically pleasing, so I decided to go with it and print one myself!

The dimensions were not all that difficult.  The diameter of my outer circle was 2.5in and I simply placed another circle along the same axis of radius [INPUT RADIUS].

I sanded down the edge that would catch the bottom of the bottle cap so it would fit in there better.  As soon as I printed this design, I realized my mistake.  The delrin was only 1/8th of an inch thick, making it too flimsy for the top that would be pushing down on the bottle cap to stay in place.

I decided to alter my design into something less risky that I knew would definitely push down on the bottle cap and stay there.   I also decided to keep the circular element as it would give the user better leverage than a handle.

This was my second iteration.  It kind of looks like a snake about to eat itself!  Before printing I immediately realized that the extended tail of the snake would not let me catch the bottle cap, so I cut his tail right under the bottom jaw.  

The thickness of this delrin was 1/4th of an inch, already making this design stronger than the first.  The snake's top jaw pushed down on the top edge (instead of the center) of the bottle cap and his lower jaw caught and pulled up the opposite bottom edge.  Instead of making a handle, I wanted to force the user to hold their hand perpendicular to the bottle opener, forcing their entire arm up to their elbow to become the handle, and thereby decreasing the force required to open the bottle. (referring to the equation Torque=Force*Radius)

I'm glad it was a success! 

Assignment 2/Part 1 of 2: Fastening and Attaching

We learned four methods of fastening and attaching, all of which we plan to use on our windlass!  Practicing these methods gave us a very concrete understanding of their benefits and disadvantages.


1.  Piano Wiring involves drilling a hole into your material and putting piano wires through them.  The screw used to drill the whole varies in size so that you may vary your fit (running or press).

• Function: Attaches separate parts together in numerous ways, limited only by your objective, materials, and creativity.

• Advantages:  

1. The piano wire is relatively strong, so it will hold your parts in place, depending on the force applied to the wired area. 
2. Not permanent, so you can remove the wire and separate your parts, if need be.
3. Several pieces can be drilled together like a hinge, allowing mobility.

• Disadvantages: 

1. Drilling down a thin piece of delrin requires a lot of precision (that was almost impossible to achieve without luck!).  The screw and your piece must be properly aligned.  Anything in the way could potentially bend the screw and drill in the wrong direction, instead of drilling through it.  
2. Drilling down a thin piece of delrin could potentially make your part prone to breakage. 
3. If you do not drill carefully, you run the risk of damaging your part to the point that it is unusable.
4. The piano wire must be completely straight, or it will not go down the drilled hole.
5. If the screw is not long enough, or if your part is too long, the drill press will not be able to run down the entire desired length of the part.  If possible, you can turn your part around the opposite side but there is a very low chance your two drilled paths will meet.

2.  Pegs & Notches involve cutting out your parts so they fit together like legos/puzzles. You can  vary the size of your notch and peg depending on your desired fit (press, running, or in between).

• Function:  Attaches separate parts together perpendicularly.  

• Advantages:

1. Not permanent, so you can take your attached parts apart. (Try saying "attached parts apart" really fast 10 times! xD)
2. Several parts can be attached together.
3. Depending on your material, you can sand down the peg or the notch to adjust your fit.

• Disadvantages:

1. Several iterations are required to get your desired fit, which can be wasteful of time and material.
2. Your calculations may be perfect, but the laser cutter is not and may cut your material in a less than desirable way. 
3. If your material is even slightly warped, there is a higher chance you will not achieve your desired fit.
4. Does not allow for mobility where the parts are connected.

Tolerance and Measurements using a caliper*: (For Peg and Notch Set #1)

Peg width: ~0.124in 

Loose notch width: ~0.164in

Tight notch width: ~0.1245in

SolidWorks vs. Our Measurement using a caliper*:  (For Notch Set #2)

SolidWorks measurement for notch #1: ~0.135in

Our measurement for notch #1: ~0.141in

SolidWorks measurement for notch #2: ~0.125in

Our measurement for notch #2: ~0.134in

SolidWorks measurement for notch #3: ~0.115in

Our measurements for notch #3: ~0.112in

3.  Bushings are hollow cylindrical pieces of material with variable height, inner radius, and outer radius.  This allows for adjustment depending on your desired fit (tight, running, or in between).

• Function: Holds parts in place on the rods.

• Advantages:

1. Can be moved any time with variable force to desired place on rod.

• Disadvantages:

1. Same disadvantages as those for pegs and notches (1-3).

Tolerance and Measurements using a caliper*:

(Each of the following measurements is an average of three trials)

Rod diameter: ~0.25in 

Tight bushing inner diameter: ~0.256in

Loose bushing inner diameter: ~0.26in

Running bushing inner diameter: ~0.262in

4.  Heat Staking involves melting materials together.

• Function: Attaches parts by melting them.

• Advantages:

1. Melting pieces together allows for a strong attachment.
2. Does not require perfectly calculated precision or rely on luck and therefore, simplest of the four methods.

• Disadvantages:

1. Attachment is permanent.
2. Different materials may not melt together and create a strong attachment since their melting rates may vary at a fixed temperature.
3. Forms a round bump where the heat is applied and destabilizes the structure if the round top is touching the surface your structure stands on.
4. Difficult to heat stake parts that that do not fit in the circular opening of the heat staking machine.

*We found that caliper measurements are delicate and change easily with variable force used to separate the measuring bars on the instrument.

Laser printing adds to precise measurement error.

About Me

Hello!

My name is Sabrina Ahmed and I'm currently a sophomore at Wellesley College.

I'm an Applied Mathematics major (maybe even a studio art minor) and wondering if mechanical engineering can be a possible focus. I'm a little intimidated since I feel succeeding in engineering demands ingenuity and a supreme grasp on many physics concepts, and I'm not yet confident that I can rise to the challenge.  Despite my personal inhibitions, I'm so excited to start thinking like an engineer (and use fun power tools)!  I want to be able to look back and think, "Wow, I helped make that...and I actually understand why it worked!"

I hope a few of the many things I get out of this class include becoming more organized, creative, and aware in my thinking and being able to work productively in teams with peers I just met.

Thanks for visiting!

This is my first painting at Wellesley and I'm very proud of it! 

(ART 218 with Professor Daniela Rivera - Spring 2015)