Mechanisms

Now that the class is working on our Lego cars, we've been learning about gears and mechanisms.  There are obviously hundreds of different mechanisms used in engineering and for this assignment, we explored Cornell's Kinematic Models for Design Digital Library (KMODDL) http://kmoddl.library.cornell.edu/index.php

There were so many mechanisms to explore that it was hard to choose one to talk about. One of the mechanisms from this collection that I found interesting was the Slider Crank Mechanism.  

http://media-3.web.britannica.com/eb-media/90/4190-004-4DAAB8E9.jpg

The description said this mechanism "may be the most ubiquitous mechanism in the world" because it is found in every internal combustion engine.  Now, I'm not really a car person at all, so the extent of my knowledge about combustion engines is that the piston moves up and down because of pressure from the sparks created.  I know now that the up and down of the piston is aided by a slider crank mechanism.

http://www.allsubjects4you.com/reciprocating-engine_files/image002.jpg

First, I'll explain how this mechanism works on its own.  Basically, it changes rotational motion into linear motion.  The wheel or handle is rotating, and in the image above, point A is fixed and the rod from A to B moves around point B in a circular motion.  The other end is not fixed, but slides back and forth in some type of guide.  Now, this crank has four links, and it can be modified four different ways, changing the part that is grounded or fixed.  So hopefully you can see where the combustion engine comes in. The sliding part of the mechanism is the piston on top and the rotational motion is below the piston. It's clear that this mechanism is extremely important for engineering because of it's many uses.

Well Windlass

Our second project was to build a small version of a well windlass.  There were many requirements:

• The design has to span 12 cm, the width of the "well".
• Limited to 500cm^2 Delrin sheet, 50cm Delrin rod, and 120 cm string.
• The windlass must lift a 1L bottle 10 cm above the table without buckling, breaking, wobbling, or shaking.
• One hand may be used to do the winding, and the crank or other mechanism must be positioned so it is not over the well.

1. Design Process
2. Engineering Analysis
3. SolidWorks and Construction
4. Final Product Testing
5. Accounting of Materials
6. Reflection

Design Process

My partner, Magdalena, and I began by sketching lots of ideas for windlasses. Our ideas included sides shaped like X's, triangles, rectangles, and more.  We brainstormed about ways to support the sides, ways to hold the rod, ways of attaching pieces together, and ideas for the crank.  

We eventually narrowed it down to a combination of rectangular sides shaped into a triangle, similar to the drawing at the very top of the image to the left.

Maggie and I decided on an overall triangular design because we thought it would stronger than a rectangular shape.  The design we chose combined multiple ideas.  The basic structure had two flat sides, two supports, the rod, and the crank.

The side pieces were rectangles that would lay together to form a triangle with notches to connect them at the top.  There would be two slots on each side piece for the supports to fit into. There was also one hole in the center of each for the rod. 

We had to figure out how tall the structure needed to be to span the 12 cm and also bring the bottle 10 cm above the table. We went through some math calculations, as you can see in the scribbles on our sketches 
(geometry was a long time ago). We approximated about 15 cm for the height of the triangle because we needed the rod to be at about 12 cm to 13 cm height to ensure the bottle would reach 10 cm.

We originally approximated the triangle as a 45-45-90 triangle, so if the height was 15 cm, the side would be approximately 21 cm.  (Later when we cut the design out of foam, we concluded that the triangle would be more little more upright.  The approximate angle measurements are in our detailed sketches in the next section).

One of our original sketches above had the supports above the rod because we didn't want the supports to get in the way of the bottle as came up.  We later switched the supports and the rod, like the picture to the left, so that the rod was higher, but we made sure the supports were far enough apart to allow the bottle to fit between.

Engineering Analysis

We chose this design for many reasons.  The rod is at the top of the triangular design, so there is only a small part of the rod that supporting the force of the string pulling the bottle up.  The sides support the rod so it won't bend as the strings puts force on it (if the rod was lower, it would bend more and wouldn't be as strong because the sides are further out). We also thought the triangular shape would be more stable than most of our other ideas. 

The horizontal support bars hold the two sides together, so that they won't collapse.  The supports are put through tight slots on the sides, and then fitted on the outside with very tight rectangular bushings to ensure they will stay.  We originally thought about making a notch and heat staking the end of the support, but eventually decided on rectangular bushings instead.  This way the windlass could be disassembled if we needed to change a dimension or rebuild it.

After sketching everything out, we created a foam mockup of our windlass. Even with just foam, it was fairly stable.  We didn't cut bushings out of foam because they are so small, and instead just built them in SolidWorks.After seeing the foam mockup, we made a few small dimension changes.  We increased the length of the sides from 21 cm to 22 cm and we shifted the rod up about 1 cm more.  These dimensions changes are reflected in the sketches above, and are the dimensions we used for the SolidWorks design. 

SolidWorks and Construction

After we constructed our foam mockup, we started on creating SolidWorks parts and drawings.  Before creating the large final pieces, we made test pieces to test parts of our structures without wasting Delrin.

Test Pieces:

Notches:  We made small pieces to test that the notches at the top would be a tight fit.  We made the widths of the tabs equal the first time, but this was too loose.  We changed the dimensions so that the tabs were wider, and the part the tab goes into was smaller.  This created too tight of a fit, even with some filing down, so we made a third test piece to find a fit between the two.


Slots:

We needed the supports to have a tight fit with the sides because they needed to hold the sides together and prevent them from collapsing.  We made a small piece with different size slots.  When we to print, however, we decided to use 1/8" Delrin when we had based measurements off of 3/16" Delrin.  This mattered because the width of the Delrin was the width of the slots.  We made a second test piece with corrected measurements.  We made a short version of the support piece, so we could test its fit with the slots.


Bushings:
We wanted these rectangular bushings to be as tight as possible.  The first test bushings were too loose.  We think it was because it was they were too thin on the outside,  so the material was bending as we put it around the support.  The second test, the bushings made a very tight fit with the support bars.

Holes/Circles:
We wanted the hole for the crank to be tight so the crank and rod would move together, not separately. We found that our test piece holes were not tight enough, so decided to just change this on our final piece (we made it slightly more small, so we could just sand it if needed).   
Also, the sides needed a loose hole for the rod to rotate in.  Maggie had a great idea to make it an ellipse instead of a circle. This way the rod wouldn't slide or move from side to side at the windlass was cranked, but the rod would be able to lie at an angle because the ellipse is tall in the vertical position.  We made a quick test piece to make sure it would fit right.  


After our dimensions were decided from testing parts, we sketched our final parts in SolidWorks.  We made drawings from the parts to print on the laser cutter.

We thought we were done when we printed our final pieces, but we had a few problems.  On CorelDraw, we couldn't make the ovals on the side piece hairline width, and when we hand-drew the oval in CorelDraw (and made it hairline) and ran it again, the Delrin had shifted or warped and made the holes in a slightly different place.  We had to reprint the sides a second time, and this time were very careful about placement.  Also, we had a lot of trouble heat staking the rod and the crank, and when we turned the crank it kept breaking off the rod.  We printed it a second time with a much smaller hole to try and fix this problem.

Final Product and Testing

Finally, we finished our well windlass!  We had time to test it once before the demo and it worked.  The class demonstrated our windlasses on Friday.  Ours worked and was able to lift the bottle 10 cm above the table without too much trouble and with only one hand doing the cranking.  Using one hand on the crank was doable, but a bit difficult.  Our structure was very strong though, and showed no signs of strain or breaking.  

It was also great to see the rest of the class's amazing designs!  It was a big step up from all of our one piece bottle openers.  As we watched the other demos, I took mental notes of certain design elements that were interesting, that worked well, and some that everyone struggled with.  Many designs had a spool for the string to wrap around when winding, larger than a single rod, making it faster to bring the bottle up the well, which I thought was a great design element.  Also, it was interesting to see some of the arched or rounded designs, which were aesthetically pleasing, and to see those that enclosed or supported the rod completely with Delrin parts so it wouldn't bend.  Some projects had multiple rods for their axle that were connected to the crank, and this not only made their axle strong, but also helped the crank move with the axle and not slip.  One thing that most designs struggled with was the crank.  Many were difficult, as with ours, to crank with one hand.  There are definitely improvement opportunities for the cranks.

Accounting of materials

Our limit for materials was 500 cm^2 of Delrin and 50 cm Delrin rod

Sides:        22 cm  x  9 cm = 198 cm^2 (x2) = 396.0 cm^2
Supports:   20 cm  x  1 cm =  20 cm^2 (x2) =   40.0 cm^2
Crank:       pi(2)^2 cm + (4x2) cm               =   20.6 cm^2
Bushings:   9.15 x 15.8 = 144.6 mm^2 (x4)  =    5.8 cm^2

Total:  463 cm^2 Delrin, 25 cm Delrin rod

Reflection

Overall, I'm proud of our design for working and meeting all of the requirements.  Our design was simple, and we were able to incorporate different ways of attaching Delrin together.  We also learned how to use many new power tools in the Engineering Lab and improved our skills with SolidWorks parts, assemblies, and drawings.

Some improvements I could think of for our design would be for the design to be a little more appealing and use less material.  For example, we could probably create the triangle shape without using so much material for the sides. Our overall design could be scaled down about ten percent because we had plenty of wiggle room in the 12 cm width as well as the 10 cm bottle height requirement.  Also, our crank and axle can definitely be improved to make it easier to rotate with one hand, maybe creating a longer handle or a different design.  Creating a spool for the string to wrap around would also be a good idea because we had trouble with the string wrapping all around the rod and not being able to turn it any more because the string hit the sides.

Also, Maggie and I worked really well together and communicated well.  We both made great design contributions to our windlass and we were able to combine and improve each others ideas.  (Fun fact: we figured out we took PHYS 107 together last semester, but we didn't really know each other at the time!)

We're now starting a new project, with a new partner (I enjoy changing partners and getting to know everyone in the class through projects).  Kasirha and I are working together for the next project, building a Lego racer.  When we started on Friday, we got super excited about gears and Legos and are eager to work on building our car!

Fastening & Attaching

The material we use for most of our projects is Delrin (polyoxymethylene).  This plastic has a high stiffness and low friction, and glue doesn't hold it together.  So during class on Tuesday, we learned different ways of fastening and attaching two pieces of Delrin together.

Types of Fastenings and Attachments

Heat Staking

Heat staking uses a hole in one piece and a notch in the other, so that when the notch is melted down and then cooled off using air, it forms a small bump that is attached to the other piece.  The benefits of heat staking are that the two pieces are permanently fastened together and they will not slip or move.  This permanence can also be a drawback though because the pieces cannot be disassembled.  Also, if the heat staking is done incorrectly, the piece will be ruined and not be reusable. For example, if we wanted heat stake an axle and wheel together, we couldn't separate them afterward if we needed to fix something or redesign a part.  However, if the car were definitely going to stay permanent, heat staking would ensure the wheel was firmly attached to the axle.

Piano Wire

Using piano wire involves the drill press, drilling a hole through your pieces, and threading the piano wire though both.  We learned how to put in the drill bits, align our pieces, and carefully drill a hole through where we want the piano wire.  The benefits of piano wire can be if you want a hinge in your structure or want to connect pieces in a line using the wire.  With the two different widths of wire, we can make tight fits so the pieces don't move and loose fits so the pieces make a hinge.  Drawbacks of using piano wire are that it takes practice to drill accurate holes.  During class, most of us weren't able to get it perfect.  If you wanted to make a hinge, piano wire would be perfect, but if you need the pieces to permanently stay together, heat staking might be a better choice.

Notches/Pegs

Notches and pegs have the advantage of dis-assembly, so nothing in your structure is permanent.  This way of attachment can be a tight fit loose fit depending on your measurements in SolidWorks.  However, a disadvantage is that it's hard to get your measurements perfect, especially since the laser cutter often won't cut the exact dimensions you specified.  For example, on our next project (constructing a mini well windlass), we are deciding to use notches more instead of heat staking because we want to be able to disassemble the parts, but we have to cut small test pieces to check measurements before we cut our final large pieces.

Bushing Tolerances

Bushings are small circles or rectangles that attach around another small piece.  They can be tight if you do not want another piece beside it to fall off or they can be loose if you want something to spin or slide.  A tight bushing would have a similar purpose of heat staking, except that it wouldn't be completely permanent.  A drawback of bushings is that you must get the measurements exactly right, and so many test pieces have to be made.  Let's go back to the wheel and axle example.  If we wanted to attach the wheel to the axle without it sliding off, we could use two very tight bushings on either side.  This way we could take it apart if we need to (unlike heat staking).  An example of using a loose bushing would be if you wanted to stabilize a part, but wanted it to spin.

In class, we measured the inside diameter of three different bushings and compared those with the diameter of a Delrin rod. We measured each one three times.

Rod: 0.2500, 0.2495, 0.2500 (mm)
Bushings:
    Loose: 0.2620, 0.2600, 0.2620 (mm).  [About a .01 mm bigger than rod]
    Tight: 0.2510, 0.2515, 0.2500 (mm).   [Right around diameter of rod]
    Very Tight: 0.2495, 0.2520, 0.2465 (mm).  [On average, slightly smaller than the rod]

Notches/Pegs Tolerances

We also used a sheet of different notches to see the differences between loose fits and tight fits.  We found a notch that was tight with our peg and one that was loose with our peg, then measured them three times each.

Peg thickness: 3.25, 3.20, 3.21 mm
Tight Slot:
    Width: 12.63, 12.71, 12.64 mm
    Height:  3.15,   3.10,   3.11 mm
Loose Slot:
    Width: 13.43, 13.61, 13.57 mm
    Height:  4.11,   4.29,   4.20 mm

We saw that the height of the tight slot was smaller than the width of the peg and the height of the loose fit was larger than the thickness of the peg.  We didn't measure the width of the peg, but it's clear that the width of the tight slot is about 1 mm smaller than the loose slot.

Discrepancies between SolidWorks and Actual Measurements

Another measurement we wanted to test was whether or not the laser cutter cut the exact dimensions specified in SolidWorks.  We had a small sheet with three different widths of slots: 0.135, 0.125, and 0.115 inches.  When we measured them we found some discrepancies between what they actually measured and what they were supposed to be.

SolidWorks:  Actual measurements
0.135 in:   0.1435, 0.1425, 0.1425 in
0.125 in:   0.1365, 0.1350, 0.1375 in
0.115 in:   0.1235, 0.1150, 0.1190 in

As you can see, all of the slots turned out to be a little more than 1/100 mm larger than the specified dimension in SolidWorks.  This is a result of the laser because even though we set the laser to "hairline width" it still affects the measurements.


We will be applying some of these techniques to our well windlass project.  One thing we need to keep in mind are that SolidWorks measurements aren't always exact when printed out, so we need to print some test pieces to check the right dimensions.  We also need to think about how tight or loose of a fit we want for our notches and/or bushings, and use the examples we did in class to guide our thinking for those measurements.

My partner, Maggie, and I have been working hard on our windlass over the long (and snowy) weekend and can't wait to share our design!

Bottle Opener

The class started our first engineering project on the first day of class (well, technically the second class due to the snow days!)  Our task was to create a working 2D bottle opener with a single piece of Delrin (1/8", 3/16", or 1/4") no more than 6" in any dimensions using the laser cutter.

• Brainstorming Process
• Choosing the Design
• Engineering Analysis
• SolidWorks and Testing
• Reflection

Brainstorming Process

Our goal for brainstorming was to spend about ten minutes coming up with as many ideas as possible, amazing or silly.  The idea behind this is that the more ideas you produce, you will eventually come up with some fantastic ideas.  We drew a simple sketch, wrote a short description, shared the idea with each other, and kept going.  When we got stuck, some strategies were to combine different ideas, adapt the part to something else, or modify each other's designs.  For example, we thought a lot about the part that actually opened the bottle, but later thought about the part the user holds.  We ended up with about twenty pieces of paper spread across the table, some with a couple of ideas on the same paper.

Here are our many brainstorming ideas. 

Choosing the Design

After we mass-produced our initial bottle opener ideas, we grouped all of them into categories, such as rectangular, rounded, etc.  We combined, modified, and eliminated ideas until we narrowed it down to only 10 design ideas:

These four ideas all included an outer shape with an inner shape cut out.  For the half-circle shapes, the straight edge would go under the cap and the round edge on top.  The oval idea would work by putting the large end over the bottle, sliding it to the small end and pulling up on the handle.

These three ideas all had one edge longer than the other. The second two designs are modifications of the first: one with flat edges and the other with teeth.  The bottle would be opened by pulling the bottle cap from beneath using mostly the longer edge.

These three designs all have a rectangular shape.  The first had a smooth rounded end that would fit around the bottle cap.  For the second, we thought adding teeth might help it get under the cap.  The third incorporated an idea to make the handle more comfortable for the user.  On all three, the handle would provide leverage to pop open the bottle cap.

After reviewing these ten sketches, we had to narrow it down to one idea.  We eliminated any ideas with teeth because we thought it would make the bottle harder to open.  We also eliminated the hand-shaped idea on the first image because of the complexity, the donut idea because we simply thought it would not work, and the oval idea because it would involve detailed measurements.  

At this point, we needed to decide on the overall shape:  rectangular or rounded.  We came to the conclusion that the longer, rectangular shapes might not be as sturdy as a smaller shape, and that they would bend under the force.  Therefore, we ended up choosing the simple design of a circle with a smaller half-circle cut out on the inside. (which we promptly named the "smiley face" one.  So scientific.) 

Engineering Analysis

We decided on this design because the straight edge lifts up underneath the cap and the rounded edge would be on top of the cap, and this would provide nice leverage.  This design would also be strong and easy to hold in your palm.  Our bottle openers also act as cantilevers, so we considered the equation for deflection:

         deflection = FL^3/3EI

         where F is force, L is length, E is Young's Modulus (material stiffness), and I is area moment of inertia (stiffness of cross-sectional area).

The factors we could change in this situation were L, the length, and I, the moment of inertia (stiffness of cross-sectional area).  We don't want the deflection to be too large (or else the bottle opener would bend too much), so we had to incorporate the two variables we could change into our decision.  Our decision to use the circle shape instead of a rectangle was to minimize the length L.

We took measurements of the bottle cap, sketched to scale model on foam core, and cut it out.  The outer circle was to be roughly the size of someone's palm, so it would feel comfortable when you hold it.  And we wanted the half-circle to be slightly larger than the bottle cap size.


Measurements
Radius of outer circle: 30 mm
Radius of inner half-circle: 20 mm

SolidWorks and Testing

Our machinist, Larry, helped us with sketching our part in SolidWorks and taught us how to use the laser cut, so we could cut out our design.  We decided to use 3/16" Delrin because we didn't want it to be too thick to fit under the cap, but we thought that the 1/8" might be too thin.  Here is our final cut piece:

Then came the moment of truth!  We tested our opener on a soda bottle and ... it didn't work right away.  It took several times to get the hang of it, and after a while we were both able to get the bottle open in about two or three tries.  The next class, since we decided not to do another iteration, we spent time practicing opening the bottle.  One thing that worried us was that the plastic on the straight edge chipped each time we opened the bottle.  If this became a real bottle opener, though, it would probably be made of metal and prevent this problem. 

When we presented our project in class, our bottle opener worked fantastically!  We were able to open the soda bottle in two tries and enjoy our reward.  Also, later on in class, another classmate tried using our opener and opened a bottle very easily.

Reflection

Overall, I really enjoyed having this as a first engineering project.  This project taught us how to brainstorm, and was my first experience using SolidWorks and the laser cutter.  Our design definitely has some improvement opportunities, such as adjusting the size of the inner half-circle to make the bottle easier to open or using the 1/8" Delrin to make sure the straight edge is able to go under the cap easier (however we don't have years to perfect it!)  Sonja and I worked well and efficiently together and we are definitely happy with our working result.

The next project, which my new partner and I have already started, is constructing a well windlass, so stay tuned!

About Me

Welcome to my blog!  My name is Sarah Barden and I am a first-year at Wellesley taking ENGR 160. This blog is where I'll be writing mostly about our projects in class and documenting my journey throughout the semester!  As of now, I am going to be a Mathematics major at Wellesley and I'm currently taking Linear Algebra.  I also plan to pursue the Wellesley-Olin 4+1 Program to earn a degree in engineering from Olin.

Engineering is a field that's always interested me.  I've always loved creating things, doing hands-on projects, and learning how things work.  I only have a little experience with engineering, so my goals for this course are to learn more about the engineering process and figure out which field of engineering I might want to go into.  (And to have fun!)

An interesting fact about myself is that I've been playing the flute for 10 years (I'm in Chamber Music Society at Wellesley).  I find it fascinating that I'm good at music, which is associated with the more creative "right brain" and also math, which is associated with the more analytical "left brain" (that's something I learned from PSYC 101 this semester...did you also know that there is a field called Engineering Psychology?  How cool is that?!).  Overall, I am excited to be taking ENGR 160 this semester!