Mechanisms: Multiple Straight Line Drive

There are many different ways to convert rotational motion to linear motion. As part of class this week our assignment was to explore these mechanisms and report on one that we found interesting.

One that I enjoyed learning about was straight line drives. I was drawn to this mechanism by both the smoothness of its motion, and the simplicity of its design. 

 

The video seen above is an example of a hypocycloid gear straight line mechanism. In this example, the smaller gear of the mechanism is driven along the larger gear by the piston rod which moves in a linear fashion.

Hypocycloid gear straight line mechanisms work by having a small gear whose diameter is half  that of the large gear. This creates a point on the circumference for the small gear to trace when moved up and down in a linear fashion. Having the smaller circle move up and down in an exact straight line adds a simplicity to this mechanism that I think helps in its design and application.

A notable use of this mechanism is in old trains, as well as in the application of other mechanisms such as planetary gears.

Well Windlass

This past week I've been working with Brooke Fieldman to design and build a model of a well windlass. For this project, the model had to be capable of pulling up a 1 liter bottle at least 10cm above the table without buckling or breaking. The model also had to span a gap of at least 12 cm and use at most 500 cm² Delrin sheet and 50cm of Delrin rod. 

In the post below I have documented our design process, an analysis of our product's function, and an account of the materials used.

Design Process: A Reflection and Summary

After brainstorming a couple designs, we settled for using two parallel arches as our bridging component. As per design requirements we also put the windlass handle on the bottom of the two arches. This meant we had to and an additional pulley at the top of the arches to run the string over. To test this design we built a foam core model (seen below)

Our foam core model appeared to be structurally sound, so we continued on to SolidWorks to draw up the parts, and refine our design. For the first test run, our model consisted of all the parts seen below; two arches to be put in parallel (3 cm wide), holes along the arch to put Delrin rod through, triangular supports for the base of the arches, circular bushings for the 4 rods that would go through the handle, and finally the two rectangular parts that would make up the handle. All these pieces were arranged to fit inside a rectangular area of 500cm².

Before printing out the whole design, however, we made test pieces. Below are all the test pieces used over the course of the project.

For our first design we had wanted to secure the two arches together by heat staking Delrin rods through each side. After doing this, however we realized that the rods were secured in the first arch at an angle. This caused the other arch to be lifted off the table, which was not good for the windlass's stability

Though this first iteration of our design was not a complete success, we were able to see some ways we could improve our design and make use of the remaining material allowance. Firstly, we noticed that the arches, while joined together by the rods, still shifted around the table. To help prevent this we added a base into which both arches and their triangular supports fit into. 

We also moved away from using heat staking our parts together to using primarily press fitting and bushings. This meant that the rods alone the arches would no longer be used to keep them in place, but rather to just thread the string over. To provide the support the rods were no longer providing, 3 pieces of Delrin were fit along the arcs. 

These changes greatly improved the stability of our design. The only thing that wasn't altered was the design of our handle; 4 Delrin rods held in place by large circular bushings, and heat staked to the handle.

In the end we were able to fit all our parts within a 25x20cm box and print them out using the laser cutter using a 3/16" sheet of Delrin:

In this version everything fit together well, and best of all it worked!

We tested this model multiple times, and it was constantly able to wind up the bottle without showing structural weakness. The only issues we had were that it slid along the table because it wasn't bolted down and that one had to keep a firm grasp on the handle at all times to prevent it from unwinding from the weight of the bottle. While the overall structure was very good, I feel that given more time we could design a model that uses less material. It would also be nice to design a handle that does not need to be grasped as tightly as the one we used.  

Analysis of Function:

When designing our windlass we put a lot of thought into how the structure and materials used would be affected by the downward force of the bottle. We chose to use arches as our bridging component because they would be able to dissipate force from the top of the arch, where the bottle would be drawn up, outward to each side. This would greatly reduce the strain placed on any one area off the windlass.

We also needed to consider the strength of the materials we were using. In testing the strength of the materials we noticed that both the Delrin rods and sheet would bend if put under enough pressure. This was due to the physics of beam bending, which takes into account the Moment of Inertia (I), Young's Modulus (E), the weight of the load, and the length of the rod (d). Our thinner material, the rods were especially prone to bending. We wanted to ensure this didn’t happen though, because we wanted to use them as beams to wind the string over. While we couldn’t change the material we were using, we did two things to minimize the amount that our beams bended. Firstly we shortened the length of the rods, supporting them on either side by one of the arches. This helped to ensure that the stronger Delrin sheet material would take a lot of the force. Secondly, we used multiple beams in tandem so that the individual force experienced by each beam was lower. In the event that the rods between the two arches did bend, we did not want the two arches to bow together. To prevent this we added thicker beams made of 3/16” Delrin. 

This concept is the same reason we used bushings with four holes to secure the rods that make up the winding mechanism. The stronger sheet material was able to provide structural support for the rods to prevent them from bending together under the force of the winding string.

To make sure our structure was as sturdy as possible, we also added supports at the bas of the arches to prevent them from wobbling from side to side. In this case the triangular supports prevented the arches from tilting to one side or the other, while the base prevented them from spreading apart.

All in all this made for a very sturdy structure. 

Account of Materials Used:

In the end we were able to fit all our parts within a 25x20cm box before printing them out, this ensured that the area of Delrin used was within the 500cm^2 limit. As for the Delrin rod, we used 5cm worth for each of the top 3 beams, and then 7cm worth for each of the rods used in the handle. This totaled up to 43 cm worth of rod used. 

Thanks for stopping by, tune in next week for Lego race cars!

Fastening & Attaching

When working with Delrin, there are many ways to fasten it together. Today we learned three methods that can be employed on our new project of building a windlass; using piano wire, heat staking, and notches/pegs.

Each method is described below.

Piano Wire

Piano Wire is often used to create a hinge between two pieces. While it is a good method to create a point of movement, it is not the best method to keep two objects bound tightly in place.

When using this method, you must first construct two pieces with overlapping pegs in SolidWorks. In practice, a hole will be drilled through these overlapping pegs to create a path through witch to thread the wire. For example, if you were joining two pieces of Delrin, each with two pegs, you would first drill through the top three pegs to create “clearance” holes. These “clearance” holes are to be made with a drill bit that has a diameter slightly large than that of the wire you are using. This is to make sure that the wire can fit through the pieces without bending and still allow for movement. For the last peg, you must switch to using a drill bit that has the same diameter as your wire. This is to create hole that will keep the wire snugly in place. After threading the wire through the first three pegs, it is press fit into this final hole to secure it in place.

When setting up the drill for use, there are several things you must consider. Firstly, you must make sure that the two pieces of Delrin are far enough apart that they do not get in the way of each other’s movement. You must also make sure that the drill bit is perpendicular to the cutting surface when it is put into the drill chick, and then completely secured with the drill chuck. When actually drilling your pieces, it is often wise to employ a “pecking” technique (drilling a little portion and then pulling the drill out in order to clear the charts), to prevent the drill from getting stuck in the material you are working with.

Heat Staking

Heat staking is a method that connects two pieces of Delrin together by melting a peg that protrudes from one component into a notch in the second component. This method of joining is extremely strong, and useful in creating secure structural connections. It is, however, a permanent method of joining, so one must be sure of their design before employing it.

Before heat staking two Delrin pieces, one must first ensure in SolidWorks that the dimensions of the first piece’s peg matched that of the other piece’s notch.  While it does not have to be an exact fit, it can only help the structure’s stability to have no leftover spaces between the two pieces.

Notches/Pegs and Bushings

This last method of joining relies on design, rather than machinery to hold two separate pieces together. In the notch/peg method, the dimensions of one piece’s notch should match those of another piece’s peg closely enough that they can be joined in a snug and secure fashion.

The benefits of this method are that the two pieces can be joined and unjoined as much as needed. This allows for alteration and substitutions to occur in the overall structure of your object.

The major issue with this method, however, is that it can be challenging to get the dimensions of the pieces close enough to create a “press fit” between them. In the trials we did in class regarding this, we found that even less than a millimeter difference in size could mean the difference between fitting, or falling out/not going in.

When working with Delrin rods, rather than sheets, the notch/peg method is a little bit different. In this sub-method, hollow circular pieces of Delrin called bushings are cut from Delrin sheets to fit around the rod. Unlike the notch/peg method described above, bushings can be either loose or tight. Tight bushings are used to keep rods in place, while loose ones allow them to rotate or slide back and forth freely. Thus, this method can be used to create both mobile and stationary objects.

Our in-class trials with this joining method were much like those for the notch/peg method. Using a caliper to get exact measurements, we observed that even the slightest difference in diameter could mean the difference between tight and loose bushings. In our trials we measure the diameter of the Delrin rod to be 6.35mm, the diameter of the tight fitting bushing to be 6.41mm, and the diameter of loose bushing to be 6.53mm diameter. The observed 0.12mm difference between tight and loose fit bushings just shows how exact design dimensions need to be. 

This brings us to the issue of discrepancy between the dimensions of objects drawn in SolidWorks, and their printed counterparts. Even if one is careful in setting dimensions in Solidworks, the printed outcome is not always the same. This is due to several reasons. For one, one must take into account the thickness of the laser. Although set to the thinnest setting (hairline), as seen in our in class trials, even this tiny thickness can throw off the fit of the knotches/pegs. One must also take into account the material being used. While considered the same material, Delrin sheets may vary slightly in temper. For this reason, it is often wise to use the same sheet for one project.

Moving forward this discrepancy means that certain precautions should be taken. Firstly, one should always test the tightness of there notch/peg dimensions using their selected material before printing out their whole design. It would also be wise to employ het staking when possible.