Mechanisms

For our next project, we will be using a direct current (DC) motor to build a lego car. However, before we start putting together, we must understand the mechanisms first. As our motor produces a rotary motion, but our car will be moving in a linear motion, we want to understand different ways of converting rotational motion to linear motion.

First I would like to talk about the importance of this conversion. Even beyond our project, DC motors are used commonly. Most DC motors produce rotary motions. Typically, DC motor works like this: when the current goes through the coil around the rotor, it creates a magnetic field the same as the permanent magnet stator around it. As a result, the two magnetic fields repels each other, causing the rotor to turn to the other side. The rotor will keep turning either due to inertia, or due to the fact the the electromagnetic field has changed its direction with respect to the motor and thus repels the magnet stator, causing the rotor to keep turning.

"Electric motor cycle 2". Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Electric_motor_cycle_2.png#mediaviewer/File:Electric_motor_cycle_2.png

While many DC motors produce rotary motion, that is not always what we need. For example, in our scenario, we would like a linear motion as our final output. Therefore, it is clear that we must study different ways of conversion from rotational motion to linear motion.

In particular, I was intrigued by Model 037 Straight Line Drive from the Kinematic Models for Design Digital Models from Cornell.

http://kmoddl.library.cornell.edu/model.php?m=467

This structure consists a flywheel merged with a small gear, two bigger gears (one of which connects with the small gear), rank arms, and a piston. The flywheel is used here since it has a great moment of inertia and is more reluctant to change its rotational velocity. It is particularly useful when the source of energy is not continuous. However, if the center small gear is directly connected to a sustainable energy source, the fly wheel can be eliminated as it actually takes up energy to move.

The connection between the small gear and the bigger gear has a great gear reduction. Although it is not clear what the number is, we can tell by our own eyes that the big gear has much more teeth than the small one. This connection reduces the rotational velocity of the big gear, but increase the torque significantly.

The two bigger gears have the same rotational velocity and are attached to the piston with two crank arms. The tangent force applied to the crank arms at the connection on the perimeter of the gear can be split into forces in the horizontal direction and in the vertical direction. The force in the horizontal direction causes and crank arms to move from the left to the right and vice versa, but the vertical force is transferred to the piston. Since there are two crank arms connected to two gears, this puts double amount of force to the piston, making the mechanism very powerful. 

In general, this model transforms high velocity to greater torque.

Well Windlass

Brainstorm

Another challenging brainstorm process! A windlass is not something that I see all the time in my life, but I definitely have an idea of what a windlass should look like in my mind. However, to my surprise, we came up with quite a few ideas that covered a whole foam core!

Similar with the bottle opener brainstorm, I realized that our structures are different combinations of variations of certain parts of the structure, i.e. we can summarize these different ideas by parts. There are four different categories: bottom supports, side supports, supporting rods, and handles.

The bottom parts are in shapes of a circle, a rectangle, two stripes across, and two feet. 

The side supports vary from simple walls, triangular structures, and boxes. The side support is very crucial to the stability of the model.

The supporting rods started with one simple rod, but then was increased to a combination of multiple rods, since the Delrin rod provided is not stiff enough. The most important part is that we want multiple rods to distribute pressure.
The handle also had a couple different structures, including both Delrin sheets and Delrin rods. We want to ensure the ease of usage but also strength of the handle, since it will take on a lot of pressure from the spinning.

The two key focuses of the windlass is stability and easy usage, but we also have to be aware of the limit on the material that we were allowed to use. As a result, for the bottom, we decided on a rectangular bottom which will hold the wall very firmly with multiple slots for press fit. For side supports, we initially chose rectangular side walls with a rectangular top, which is much easier to make than the other more complicated structures. For rods, we decided to go with three rods held apart by Delrin disks for its obvious strength. For the handle, we chose to add a rectangle to the Delrin disk holding the rods at the end to make sure that it would not break off as easily when turned.

Drawing of parts of our first iteration

However, this idea did not last long when we started thinking about the measurement--it uses way too much material. We very soon realized that certain parts of the structure do not contribute much to the stability of the structure or does not contribute enough compared to how much material it used. We first changed the bottom, since most of the part resting on the table with many notches for press fit does not contribute much to stability. Thus we changed it to the two-stripe design. The top also uses a lot of material. We decided to make it into a cross, which in fact provides more stability than a rectangle, since it is a triangular shape in the direction perpendicular to the gap (we believed that if our structure is not stable, it is most likely that it will shake in this direction. If structure shakes in the direction parallel to the gap, it would slide along the table, as opposed to collapsing.)

Drawing of parts of our second iteration

We were again restrained by the amount of materials that we have and decided that we must reduce the wall as well. The wall then became a trapezoid and thus changing the top into one stripe across, saving more material. The trapezoid wall, in fact, not only reduces the amount of material that we use, but also provide better support in the direction perpendicular to the gap, since it is harder to bend a triangle-like shape than a rectangle. The structure also becomes much easier to measure and to make, both as a foam model and in SolidWork.

Drawing of parts of our final iteration. The circular disks can be moved around and we can thus fit many more of them on our sheet of Delrin. There are also two holes on the wall for the Delrin rods to turn.

SolidWork drawing of the final version parts, before testing.

Engineering Analysis

SolidWork model, without securing circular disks

Foam Core model without handle

The overall stability of the structure is explained above during the brainstorming process. The problem of structure shaking in the direction perpendicular to the gap is solved by triangle-like walls. The problem of two walls falling to each other due to the potential bend of the winding structure is avoided by the top beam. In fact, there are are three pieces in the model that may bend under the pressure of the water bottle: the two bottom beams and the rods supporting and winding the rope. The risk of the bottom beams bending is avoided by putting stress on the walls which transfer the pressure to the table. The pressure on the winding rods are distributed to three rods so that two rods will always be bearing the pressure as the winding structure spins. It also ensures that no rod takes the full pressure during the whole time when the bottle is lifted, reducing the risk of bending and breaking.

The structure also saves energy and time for winding up the water bottle by using an axle-wheel structure. The axle-wheel structure is in fact also a form of leverage, following the formula:
F*R=f*r, where, in this case, F is the force applied by hand, R is the radius from finger tip to the center of the circular plate, f is the force applied by the water bottle, and r is the radius of the circle formed by the three rods.
Clearly, R is greater than r, then F is smaller than f. The three separated rod also allows the bottle to rise faster than it would have on just one rod. However, if we make r bigger, which allows the bottle to rise faster, we inevitable will increase F. Here we have to do a trade-off.

The string will be attached to one rod then wrapped around the three rods. This ensures that the rope will wind around the winding structure of three rods, as opposed to having the rods spin freely in the hoop made by the rope.

SolidWork and Testing

To put everything in SolidWork is in fact much easier for us this time, since the measurement is already settled beforehand and we are more familiar with the program. The biggest challenge came when we starting our testing process.

We decided to press fit the bottom beams to the side walls and heat stake the top. The winding structure will be joined by tight fitting rods through the plate, but we want to make sure that it can still slide when we apply enough force so that we can move each plate to the right place.

The press fitting, fortunately, did not take too many tries. We had to hammer the pegs into the notches, but it did no damage to the Delrin sheet and would not affect the strength and sturdiness of the structure. In fact, a good press fit ensures that the whole structure would wobble very minimally in the direction perpendicular to the gap.

Successful test piece for press fit 

The fitting between the circle plates and the rods took very little time as well, since we are not very strict about the tolerance of the fit. We simply want to make sure that the plates do not slide along the rods freely without much force applied to it. The plates will endure force caused by the bottle transferred through the rods. However, these forces will be perpendicular to the rods as opposed to parallel, thus having little effect on moving the circle plates around.

The heat staking, which we initially thought would not be as troublesome as press fitting, took five tries before it worked. At first, the peg of the initial design was too wide to fit under the heat staking machine. We then changed it into two pegs, but when we cut out the test piece, it became a press fit and the peg cannot go pass the notch and then be melted onto the top beam. The third try had a peg that was too long. The fourth try had a peg that was too short so there was not enough Delrin to form a heat stake. Eventually, the fifth try was successful. However, this was not the end! After we printed out the whole structure and when I was trying to heat stake the top, it did not heat stake as it did on the test piece! I consulted Professor Banzaert and learned that this may due to a lack of length in Delrin, the time of heat staking being too short, or the temperature being too low. I also learned that the best way to determine whether the staking is done is to look for curled up edge. Since the air tube was leaking, it was blowing more air to the heat staker than usual and thus decreasing the temperature. I solved this problem by moving away the air tube during the heating process and by checking the edge before I stop heating.

Failed test pieces. Top row: 1, 2 Bottom row: 3, 4

Successful test piece

Reflection

 

Our design ended up saving much more material than we expected.
The total area = side*2+bottom*2+top+circle*6+handle
(14+1)*15*.5*2+2*14*2+2.4*14+pi*1.3^2*6+3*1=349.5cm^2
Everything fits in a rectangle of 500cm^2 comfortably. In fact, we did not even need six plates. While the rods do twist when there were only four plates (two inside the frame and two outside), it was only when the center two plates are far from each other. When they are placed closer to each other, the twisting does not happen severely. We still added two more plates to the structure, because we had a lot of extra room in our sheet, and we wanted to reduce the twisting to an extent that it was not visible.

One thing I would have changed was to press fit a small piece of rod on the handle, so that when winding the machine, the operator can hold onto a bar parallel to themselves, which makes it easier to operate with one hand. However, our design did end up using all of the rod provided. Thus such a design may require more material. We could also press fit another piece of Delrin sheet.

In terms of saving materials, we had a satisfying balance between sturdiness and reduction of material use. However, I truly admire the windlass designed by Olivia and Brooke (need to double-check) which is an arc and uses a multi-rod winding structure but much shorter. Their design allows the winding structure to sit parallel to the gap as opposed to across it, significantly reducing the length of the whole structure, providing more material to distribute pressure on the winding structure.

To me, it is challenging and interesting to make something I already know, since I find it a good practice for me to think outside of the box. In the past two projects, I feel like my brain was still rather restricted by the stereotypes that I already had in the back of my mind. My windlass, though not the exact same as the what I normally think of a windlass, takes on a very similar structure. It is hard to notice the flaws or restrictions of things that have existed for so long and have been used so widely. However, I truly appreciate watching different projects made by different people, since it provides me with examples on how to break boundaries.

I also learned a lot by analyzing how my brain process information. It turned out that my brain is very good at comparing things and organizing differences very well. Such way to organizing information is indeed helpful, since it breaks down the process of creating and innovating. I do not have to pull out a random design out of thin air. Instead, when I run out of ideas, I can alter the ones I have parts by parts, and who knows what will come out eventually!

Last but not least, I would like to thank my hardworking and talented co-worker Julie Chase, who has devoted much of her time to our project even after she decided to drop the class. We will miss you and the knowledge you bring to us a lot!

Fastening & Attaching

Heat Staking

Heat Staking refers to the method where pegs are inserted into a slot and then melted to join the peg and the slot permanently.

Advantages:

• Heat staking is permanent. This means that the product cannot be altered unless destroyed.
• As a result, it is very strong.

Disadvantages:

• Heat staking creates a round top, which makes it inconvenient to stabilize on a flat surface.
• Since it is permanent, the product cannot be adjusted if any problem occurs.

Possible uses:

Heat staking seems to be useful in many situations. For example, the permanence of the structure would be helpful in children's toys so that little parts that are swallowable would not come off and become potential danger to children. The permanence is also helpful when a product requires a lot of strength as opposed to the ability to be altered.

Piano Wire

Piano wire can be used to join two pieces of Delrin together so that they can open and close like book pages around the joint edge.

Advantages:

• The piano wire makes sure that the two pieces will join firmly together (unless the wire breaks).
• The piano wire gives mobility to the structure.

Disadvantages:

• The piano wire does not provide a still structure, i.e. it cannot hold pieces together at a specific angle, since it will spin.
• The piano wire requires a lot of precision.
• The piano wire cannot be used on pieces that are too thin. The drill hole makes the material thinner and more vulnerable to break.

Possible uses:

The piano wire, as mentioned above, is very good for structures requiring mobility. It can be used with structures like doors.

Pegs and Notches (Press fit)

Pegs and notches is a way to join Delrin together by friction caused by pressure. One cuts out pegs on one piece of Delrin and notches on the other piece. Then the pegs are inserted into the notches. If the size is correct, the two pieces will be held together very well due to the friction caused by the pressure.

Advantages:

• Press fit is a very sturdy structure.
• However, it is still reversible, unlike heat staking.

Disadvantages:

• It takes many tries to find the right size.
• As a result, making multiple test pieces wastes a lot of material.

Possible uses:

• Press fit is perfect for sturdy structures that require smooth surfaces.
• It is also good for structures that may require dissembling, e.g. when adjustments are needed.

Tolerance and measurements:

For a peg that is of size 3.15 mm * 12.63 mm, the size of the slots of different fits are:
Tight fit = 3.16 mm * 12.65 mm
Looser fit = 3.44 mm * 12.93 mm

Clearly, the slot has to be bigger than the peg, but not by too much. The slot has to be almost the same size of the peg, otherwise the structure is very unstable.

Discrepancy:

For a notch with a nominal width of .115 inch, the three measurements we took are .126 inch, .117 inch, and .117 inch.
For a notch with a nominal width of .125 inch, the three measurements we took are .135 inch, .136 inch, and .135 inch.

Clearly, the laser cutter makes the notch bigger than it is intended to be, possibly because it cuts by burning the plastic and thus consume parts of it, i.e., it does not cut through a line with a negligible width.

Bushings

A bushing refers to a circle cut out on a Delrin sheet that fits over a Delrin rod. It is normally used to fasten a rod to another piece of Delrin.

Advantages:

• Makes sure that the rod stays in place, perhaps against a sheet of plastic
• Can be also made loose to separate pieces on a rod.

Disadvantages:

• Just like press fit, it is very hard to find the correct measurement.
• Bushing does not guarantee that the rod would not move at all. There is a limit to the force that it can stand.

Possible Uses:

• A good use of bushing would be our project, as we would like to fix the rods onto a wall or a plate that spins. A tight fit bushing would work very well.
• A loose bushing may be used to separate pieces on the rod.

Tolerance:

For a rod with a diameter of 6.35 mm, the diameter of bushings of different fit are:

Tight fit = 6.40 mm

Medium fit  = 6.52 mm

Loose fit = 6.61 mm

Just like press fit, the bushings must be slightly bigger than the rod, but not too much. While we have no data for discrepancy in bushing, I can imagine that the actual measurement may be slightly bigger than the nominal measurement. Another observation we made was that sometimes the laser cutter does not cut perfectly vertically, making it tighter to push in from one side than the other.

Bottle Opener

Brainstorm

When we first received the task of designing a bottle opener, I had mixed feelings. The good news is that we have a general idea of what a bottle opener should look like and how it works. On the other hand, since it is such a common object, I thought it would be hard to brainstorm on different ideas! In about ten to twenty minutes, we tried altering different ways of opening the bottle, considering people with different strength and perhaps disability, and altering shapes of certain parts of the bottle opener. We certainly felt silly at times, but eventually, we were able to narrow down to ten designs we liked or thought interesting!

Img 1 Final ten brainstorms (not to scale)

Theses designs most certainly have overlapping qualities (or some of them came out of no where). I (or I shall say my brain automatically) summarized the ideas based on a few categories.

First is the way to open a cap. The pressure could be applied in a downward motion (3, 5, 6), pulling the cap up from the top.  The pressure could also be applied in an upward motion (1, 2, 5, 6, 8), lifting the cap from the bottom. We also came up with more innovative ways to open the bottle, including twisting it (hoping that it will be displaced and then come off, 4, 7, 10) and poking a hole through it (9).

Now it is rather obvious that we want to stay with the first two ways to open the cap, and there are two basic shapes.

Img 2 A hooked design (left) and a flat ring design (right)

We narrowed down the design to the second flat ring design which lies horizontally, because it seems easier to measure. Then we had three different ideas for the edges: a straight line, a curve, and a line of teeth. We decided that we will make the top edge (which we wanted on top of the cap) tooth-like, because we wanted a little more friction to keep the top stable. On the other hand, we made the bottom edge (which goes below the cap) an arc, so that there will be more surface contact between the opener and the cap.

[Aesthetics] As we were sketching our final design, making adjustments here and there, we most certainly saw a chubby monster face, and thus we added eyes and ears to the design, changing the handle into a chubby body. And voila! Our baby monster is now born. We (or at least I) had much fun personifying the monster, imagining the color of its fur and silly things it would do trying to be scary.

Engineering Analysis

Our bottle opener resembles a classic cantilever as shown in the picture.

It follows the following equation:

deflection = FL^3/3EI,

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

With the conditions provided by the assignment, we can only control the L and the I in the equation. In general, we want a bigger leverage for our hand, but a smaller leverage for the bottle cap. A longer handle will certainly provide more leverage, but the Delrin that we use is much more flimsy than regular metal, so we want to make sure that the handle is not too thin, otherwise it will bend instead of transferring the force to the cap.

Measurement and Foam Core Making

We took a rough measurement of the radius of the cap, since we did not have to be extremely precise. As mentioned before, we initially did not want a big opening so that we can reduce the leverage that the cap has on the cantilever. As the picture suggests, we made the vertical opening to be same as the radius of the cap, 1.3 cm.

Pic. 4 Here is a rough sketch of the "mouth"of our Monster Cutie 1.0

However, the first foam core model made us think twice about the vertical width of the mouth. The foam core was about 1/4", and when we were testing the model on a bottle to see its fit, we realized that we may not be able to fit the bottle opener below the bottle cap. We then found the Delrin sheets available to us, and decided that we would use the 1/8" sheet, as it is the most likely to fit beneath the cap. At this point, we had two thoughts. One is that we should perhaps change the design completely, and use the vertical hook shape as shown in Pic. 2. However, we realized that the measuring may be too complicated and the tip may break easily, as it takes most of the pressure from the cap. Besides, we grew much attached to our baby monster, so we decided to amend the original design, as opposed to start a new one.

Pic. 5 The top of the bottom edge of the bottle opener barely touches the edge of the cap

After some testing with a 1/8" Delrin sheet, we figured that we may want a larger mouth to help the bottom edge stay below the cap and put enough pressure on the cap. We thus extended the vertical measurement to 2cm. We also adjusted the radius of the head and the body of the monster accordingly, to provide a bigger cross-sectional area and make the shape more aesthetically pleasing. The second model seems to have a good fit to the bottle, and we thus moved onto Solid work.

Pic. 6 The second model and its sketch on paper

Solid Work and Testings

A monster is most certainly not the easiest thing to draw on solid works. Certain parts require combinations of multiple circles and arcs, and we had to trim off many reference lines and solid lines. The measurements from paper also do not completely match up with what we were drawing in Solid Work, since many curves and arcs are random free-hand drawings (since they do not have to be precise to function).

Pic. 7 Half of the monster cutie

With the help of Larry, we only had to draw half of the graph and we could mirror the image. Although we finished the first drawing, feeling the triumph, our first plastic iteration was a failure. Somewhere along the process of doing the complicated drawing, the opening of the mouth became 3 cm wide as opposed to 2 cm wide, and thus the bottle went straight through the mouth of our monster. In addition, the way of saving the Solid Work into a dxf did not quite work for our design. There were a few edges that appeared thicker than normal on the graph, and the laser cutter ended up going through these lines about 30 times, causing the back of our monster to burn relatively badly.

It was undoubtedly disappointing, but we went back to the computer and tried to fix it. It was certainly not a quick task to do, since creating the new edge requires quite a few reference lines and arcs. The second model was not perfect when it came out, but it certainly worked! We did a few adjustments. We had to file down the bottom edge a little for it to fit underneath the cap, and we had to hold the head in our palm for it to work as opposed to using the handle.

Pic. 8 The first iteration in plastic (left) and the final version (right)

Final Comments

Overall, we are happy with our monster cutie, since it successfully opened the soda bottle in class. However, here are two things that I would have changed if we had more time. 

I would have made the gap smaller than 2 cm, since we could have filed down the bottom edge to have it fit under the cap, though I would make it bigger than the 1.3 cm that we had for the first foam core, because it only caught a small part of the cap. We would have needed some more experimenting with foam core.

I would also get rid of the long body and have a bigger head, since it turned out that we only used the head part. Keeping the part that takes the pressure in our palm definitely helped compensate for the relatively weaker stiffness of the Delrin compared to metal. Aesthetically, this change also makes the monster even cuter.

All above being said, the biggest lesson we learned is always to give ourselves more time. We would have made the changes had we had enough time.

Last but not least, I would like to thank my partners, Maggie and Sebiha, who are extremely brilliant and fun to work with. I hope that we will get to work together some time in the future outside of this class!

Yes That IS My Full Name!

Hello everyone! My name is Xi Xi and as the title suggests, this is my full name (and I would really appreciate the space between the two "Xi"s). And yes, my first and last names are the same in English (but different in written Chinese)! I know it is a very confusing name to pronounce, but to make it simple, pronouncing it as "CC" is fine. (If you would like to know how it's actually pronounced, feel free to ask me!)

I came from China to the States when I was sixteen, and I'm now majoring in Mathematics. However, I'm also interested in Engineering, especially with a focus on sustainability. I have only decided that I am interested in the certificate/degree program at Olin last semester, and I hope that I'm not too late to be on board!

Some fun facts about me: I am in Yanvalou, an ensemble studying and performing music and dances (and the historical and cultural context behind them) from the African Diaspora. I also work at El Table, which is a lovely student-run cafe on campus. Another favorite part of my Wellesley life is being an RA in Shafer Hall.

By taking ENGR 160, I hope that I will have a better understanding of what it means to study engineering and to make sure that engineering is something I want to learn more about. In addition, I really hope to know more intelligent and creative Wellesley students!