Mechanisms — Slider crank

In my explorations of mechanisms, I found the slider crank to be the most compelling because it plays a central role in our daily lives and it is a very practical yet fascinating mechanism.

How it works
The slider crank turns circular motion into linear or other motion through the use of linkages and constraints, and of course a crank. One sets the linkage within a specific constraint which might be fixed horizontal and vertical tracks or one sliding track, among other variations. When one turns the crank, the linkage slides within its constraints to create a completely new type of motion.

Variations

There have been many variations on the classing slider crank, including the double slider crank, the swinging slider crank and a slider crank with reduced centrodes, each shown below respectively.

Uses

The main reason I am interested in the slider crank is because I was surprised to find out all the relevant uses it has. One main advantage of turning circular motion into a type of linear motion is that it can serve as a piston. Pistons are used in most internal combustion engines to derive motion. Further, a slider crank with centrodes, as shown in the above middle photo, is used in the design of prosthetics for joint replacement. These are just some of the uses for such a simple yet effective mechanism such as the slider crank.

Personal thought

During my research of the slider crank, many descriptions were enthusiastic about its ability to trace ellipses. I don't see how this feature is particularly useful but I find it interesting nonetheless. In fact, Leonardo da Vinci designed a mechanism called the Ellipsograph which specifically traces ellipses.

All media from http://kmoddl.library.cornell.edu

Well Windlass

My second project consists of designing and creating a well windlass made out of Delrin, which spans a 12 cm gap. My partner and I went through several designs before coming up with one we thought was effective with only 500 cm^2 of Delrin. First, we thought of using pulleys in a box-shaped frame:

However, we realized that with the pulleys in the center of the box, the weight would not be able to be pulled up ten centimeters above the table, which was one of the contraints. Also, the hole at the bottom of the box would not be big enough to fit the weight through it. Next, we thought of a triangular shaped frame so that we would not have to worry about our piece collapsing with the weight of the bottle we were trying to pull up.

However, this also didn't satisfy the 10 cm constraint and we decided it used a lot of unnecessary Delrin. Finally, we decided on a design using a notched wheel that would span the 12 cm and contained a crank to wind up the string:

 

Our piece consists of five unique parts, bonded together using press fit or piano wire. My partner and I had to make several different test pieces to get the right level of tightness for our press fit pieces, and it took a lot of measuring and re-measuring, due to the discrepancy between SolidWorks and real life measurements.

However, we finally got the right fit and were able to print out our pieces. While originally, our windlass featured two semi-circular grooves which would hold the spinning rod, we decided to instead make loose-fit holes to put the rod in so that there was less of a chance of it slipping out. Our final iteration also differed from our foam core model in that we used piano wire to secure the Delrin rod to the casing around it. In our foam model, the rod was a tight fit inside the casing and the wheel spun with the rod when the crank was turned. However in practice, with the weight of the bottle, the rod ended up spinning in place without turning the wheel. Another issue we encountered was that the two base pieces kept sliding in opposite direction because of the loose fit hole. We then secured the two pieces with a long piece of piano wire and the problem was solved. Here is our final product:

We made the crank long relative to the rest of the device in order to reduce the force one needs to wind up the string. Here is a video of the string winding up with no weight attached to it to show the general function of the windlass.

All in all, I am very pleased with the design we came up with and the effort we put in to make a successful final windlass, staying under the limit of 500 cm^2 of material.

Fastenings and Attachments

Joining methods: I will discuss the different methods I have used to join pieces of Delrin and their respective benefits and drawbacks.

• Heat Staking: This method is probably the most effective just in terms of joining two pieces together. The pieces are fused together and are inseparable afterwards. However, this could be problematic if you need to later take it apart or unseal the pieces for any reason. Also, it can be difficult to heat stake oddly shaped or very large pieces.
• Hinge: Using a drill press to make a hinge is also an effective way to join pieces if you want the pieces to rotate about an axis. However, this method requires you to be extremely precise in order to ensure you make the hole big enough, make the piano wire long enough, get the correct alignment, etc.
• Press fit: This method is the trickiest to achieve perfectly, because it will probably take a few attempts to get the correct fit. However it may also be the most sensible option for attachment because it can be easily taken out for disassembly.

Bushings: There can be loose or tight fitting bushings, which can both be useful for different purposes. For example, a loose fitting bushing may be useful for a pulley, where the center must spin around an axis. On the other hand, if you want to keep a certain piece locked onto a rod, you could press a tight fitting bushing onto the rod and against the piece you want.

Tolerances: Below are the tolerances my partner and I measured for different bushings and press fit holes.

• Bushings: For a rod that is 6.35 mm in diameter, we found a good tight fit bushing to have an inner diameter of 6.38 mm and a good loose fit bushing to have an inner diameter of 6.55 +/- 0.05 mm. 
•  Press fit holes: For a tab measuring .1221 inches in width, we found a good tight fit hole to be .1225 in and a loose fit to be .1317 in.

We also noticed a significant discrepancy between SolidWorks measurement of hole width and reality. For a sheet of thickness 3.19 mm SolidWorks proposed three different hole widths: .135 in, .125 in, and .115in. In reality, we measured the widths to be .1425 in, .1350 in, and .1260 in respectively. This tells us that SolidWorks measurements are not 100% accurate. Moreover, the discrepancy will differ depending on the thickness and type of material due to the level of accuracy of the laser cutter. It is important to note that you must make test models of press fit holes to get the right fit before cutting out your entire piece so as to not waste material and time.

Bottle opener

Making a bottle opener is my first project in the course/in engineering. First, my partner and I did some research to learn about the different types of bottle openers (horizontal or vertical). Then, we both did a series of sketches to get our ideas flowing and to select different design features that we both liked. One of my sketch pages is below:

I circled the 5 designs I liked best

Taking elements from my partners sketches and my own, we decided we wanted a vertical bottle opener that referenced a cute object or animal. One of my partner's designs featured many cutouts that I found to be interesting so we decided to incorporate those as well. Below is the final design we decided on:

Then, taking measurements of the height and width of an actual bottle cap we roughly scaled out the size of the bottle opener and the ratio of its parts. The length of the bottle opener would be approximately four inches long to accommodate three fingers on the underside, which would fit into the grooves. After making a foam core model, we sketched (many times) a design of our bottle opener on SolidWorks and, with much trial and error, were able to print it out on Delrin with a laser cutter. Below is the final product:

Now what should we name him?

Update:

Upon testing this model of the bottle opener, we saw that the bottom piece that catches under the cap snaps under the pressure and the rounded head did not hit exactly in the middle of the cap. We then went back to SolidWorks and make the necessary adjustments to lengthen the head and thicken the bottom piece. In addition, we rounded out some of the top spikes to that it wouldn't cut into anyone's finger while using it. Below are all three versions (foam, first, and final) of our bottle opener:

As you can see, the final model is larger than the first and we added a keychain hole at the tail. We also removed the inner eye for aesthetic purposes. And, when we re-tested the bottle opener, it was a success! Sprite never tasted to so good (maybe having to due with the fact it was made in Mexico).

Introduction

Hello! My name is Helena, I'm a sophomore and I'm a Math/French double major. Something interesting about me is that I live to travel. I lived in Milan, Italy with my family for three years (as well as multiple US cities) and my goal is to visit every continent. I've always been really interested in design and products but have just recently become interested in engineering. I'm taking this course to learn more about the subject of engineering and to see if it is something I might want to pursue after college. I also love blogging so I'm very excited about this portion of the course :)