Shigley Hauler EME 150B Final Report Team Castor March 20, 2014 Sean Raley Josh Aguilar Rocco Hollaway Zachary March Bryce Yee 1
Table of Contents 1. Introduction... 3 2. Analysis... 4 3. Figures... 6 4. Results... 10 5. Discussion... 10 6. Conclusion... 11 7. Poem... 12 2
1. Introduction The purpose of the project was to exercise our mechanical ingenuity through design, analysis, fabrication, and testing of a gearbox. Competing teams were provided with two sets of nylon spur gears and a Mabuchi RE280 DC electric motor to be powered by two AA batteries. Teams were then tasked with designing and manufacturing a gear train system utilizing the provided materials. However, the provided materials alone would not be enough requiring the acquisition of other resources to bring the design to fruition. The Shigley Hauler, as it s called, was to haul a number of Shigley s Mechanical Engineering Design books up various pre-determined inclined slopes. To accomplish this, a gearbox and accompanying cart were designed and built in the EFL throughout the quarter. A competition was held at the end of the quarter where the teams competed to move the Shigley s up the incline provided. The hauler had to move a number of books up a predetermined angled incline from a stand-still. Required runs included a single book at 20 degrees, two books at 30 degrees, four books at 40 degrees, and five books at 60 degrees! Teams were scored based on the time to complete each run. Of the nine teams that completed all four events, team Castor placed seventh. To exercise and demonstrate the well-roundedness of the engineers produced at UC Davis, a great poetry recitation was held the day before the official Shigley Hauler competition. This not only displayed the great talents of the engineers, but provided theme songs and an increase in morale for the competition the following day. Team Castor pulled out all of the stops by performing a Mission Impossible theme song trombone and tuba duet accompanied by matching poem, presented later in this report. 3
2. Analysis For this project, 10 nylon spur gears were provided. Two gears each of ten, twenty, thirty, forty and fifty tooth gears. After some quick calculations, it became apparent that the highest gear reduction that could be achieved with the provided set of gears was 100:1. This final ratio could be achieved several ways, but after additional calculations that can be seen in Figure 3, the team decided to run a series of compound ratios with three output shafts at three different ratios. The first ratio was a 5:1 using a 10 tooth gear attached to the motor output shaft driving a 50 tooth on an idler shaft. The 50 tooth gear was attached to the same shaft as another 10 tooth gear which drove another 50 tooth gear on the first output shaft at the same 5:1 ratio. The first output shaft thus had a 5 * 5 = 25:1 reduction ratio with respect to the motor output. Continuing on, a 20 tooth gear on the first output shaft meshed with a 40 tooth gear on the second output shaft resulting in a 2 * 25 = 50:1 reduction ratio. And last but not least, the same 2:1 ratio was used between the second and third output shafts with 20 and 40 tooth gears again to achieve the final output of 100:1. This sequencing of the gears was strategically chosen so that the small 10 tooth gears were placed at the beginning of the compound sequence where the forces between the gears were the least. This also allowed the use of a smaller diameter shaft size where the 10 tooth gear was positioned, reducing the amount of boring necessary to mount the gear while still maintaining structural integrity of the gear. Team Castor s hauler was designed to be small, light, cheap and easy to manufacture. To obtain these goals, the typical aluminum plating was scrapped in favor of lighter and cheaper plastic plating. Taking into account the weight of the required Shigley book loads, it became apparent that a strong plastic that was also machineable able was necessary. Through some online research, the team ultimately decided on Delrin as the material of choice. Not only was Delrin very strong and easily machinable, but it was also cheap and had a sleek shiny black finish. An added bonus with using Delrin was its low coefficient of friction properties, allowing it to be used as bearing material. Since Team Castor was operating on the mantra keep it simple, we opted to not use any roller bearings. Instead we rolled on plastic plating. One square foot of ⅛ Delrin Sheet was purchased for this project. This was more than adequate material intended to provided extra in case of manufacture errors. Three 4 squares were cut out to form the main structure and set up parallel to form a 4 x4 x4 cube. The gear train was placed between the middle and left plates, and between the right and middle plates were the replaceable delrin spools. The delrin cube was then attached to a slightly larger aluminum plate using aluminum angle pieces. Although the aluminum plate was light weight, it was available for free and formed a strong and stable platform which was used to mount the hauler to the top of the incline. One major strength concerns for the hauler was the bending of the shafts. The sideplates experienced no bending loads, which typically produce the largest stresses. This is why a side plate that was thinner and weaker than many common metals used by the competition was chosen. The supporting shafts for the gears and spools experience both bending and torsional stresses. To withstand these stresses we chose steel for our shafts. The size of our shafts were constrained by the size of the gears riding on them, specifically the ten toothed gear. The first shaft was downsized to ⅛ because it was only a 5 fold increase over the motors torque and had 4
a small gear on it. The following three shafts the 25:1, 50:1, and 100:1 gear reductions were chosen to be 3/16. This shaft diameter was chosen because it fit in all of the gears with only a slight bore to the gears necessary, and would only experience a slight deflection under high loads. The calculation is shown in Figure 2. A dynamic analysis was used to select spool sizes. A design with removable spools was chosen so that the design could be perfected in real world conditions. Although the output shaft speeds were limited to fixed numbers, a variable spool size on the driven shafts allowed for a range of possible final torque multiplication. The excel spreadsheet (Figure 3) below shows analysis with what we believed, before testing, to be the best spool sizes. A desirable spool would be large enough to provide a decent line speed, yet small enough that enough torque would be provided to achieve that line speed. Both friction and the time that it would take the motor to achieve the predicted steady-state speed were neglected in this analysis. Although we expected times slightly slower than the predicted times, we were prepared for the unexpected. Our strategy for this was to machine a variety of spool sizes that would be easily replaceable on any of the final three shafts. 5
3. Figures Figure 1: Calculations for gear spacing to ensure the gears are properly placed relative to one another in gear train. 6
Figure 2: Simple strength analysis calculation to determine the approximate deflection of the 3/16 diameter A36 Steel shafts. The analysis was performed with more than the maximum possible load that would encountered during operation. 7
Figure 3: Excel spreadsheet showing ideal and final gear train calculations. It was formulated to allow for adjustability of the spools sizes at the various runs that would be required in the competition. 8
Figure 4: A Solidworks model of the Shigley Hauler full assembly. Figure 5: A Solidworks model of the Shigley Hauler cart and Shigley books. 9
4. Results Middle of the pack! Woohoo! Overall, team Castor placed 7th out of the 9 teams that completed all four events based on combined total time. There were 15 teams total in the competition. 5. Discussion The overarching lesson we learned from this project was: don t design to constraints that don t exist. We did a lot of things very well, and some of them worked out for us; but some other decisions were not necessarily the best for reaching our performance goals in the Hauler Competition. For instance, one decision we made early on was to build the gearbox as compact and lightweight as possible. While this would be desirable in many applications (such as a lightweight vehicle, especially an airborne vehicle), it was completely unnecessary for us. To achieve this goal, we used thin Delrin plates to support the gear shafts. Though the material did allow for quick machining, it posed other problems we did not anticipate. The plating flexed in 10
the vice during machining, which made drilling precision holes a challenge. Yield strength was a major source of concern as well, but the Delrin held up very well during competition. Because we used Delrin for the plates (a known low load bearing material), we also decided early on not to use bearings. We were well aware that this was a bold move since the use of bearings seems to be the standard throughout the years of Shigley Haulers. However, Delrin is a bearing material, so we ran with it. Unfortunately, in the week before the event, our Hauler was not performing even close to the level we desired. The only plausible reason we could come up with for the huge disparity between theoretical performance and actual results was friction between the Delrin plates and steel shafts. A potential solution to this would have been polishing the holes in the plates as well as the shafts at the points of contact. In fact, we probably should have done that in the initial round of machining. This solution did not occur to us, so we panicked and installed bearings on the shaft that would undergo the highest load (5 Shigley s at 60 degrees). As it turned out, our best run was when the end bearing slipped out of the plate, causing the spool to become a cantilever! Luckily, our shafts had a high factor of safety, and a disaster was averted. Clearly, our bearings were not installed as well as they should have been. This brings us to another lesson: never rush a machining job!! In the interest of time, we attempted to perform the hole expansions for the bearings using a drill press. One of the plates came out fine, but on the other one the bearing hole ended up about a quarter inch off. Because of this, we had to completely remake that plate. When making the original plates, the entire process was carried out with all three plates clamped together, allowing very precise hole placement. Making a new plate separately proved very difficult, and though all the shafts ended up fitting through the holes, the precision was not nearly as high as it had been before, which definitely caused some friction on the shafts. A lot of time was wasted when we could have taken a little extra time to find a way to machine the part on the mill without it flexing or flying out of the vice. 6. Conclusion Despite some risks and missteps, a few last-minute tweaks in combination with some good initial design decisions allowed our Hauler to rise to the occasion on competition day (pun intended). Our JB Weld hubs held together, our gears aligned properly, and the Hauler looked pretty cool while doing its job! Given the chance, there are definitely some things we would do differently; but as it stands, we are all proud of our compact accomplishment. 11
7. Poem A tuba and trombone duet to the tune of the Mission Impossible theme song. Shigley hauler, full of gears. Shigley hauler, full of fears. Will it bend? Will it break? Undoing all we ve worked to make? Not content with block aluminum, We ve pushed materials to the minimum. Hoping that the plastic plating Won t cause lockups, shearing, grating. Don t be stupid, keep it simple! Every detail, to the spindle. Who needs bearings? Not team Castor! Who needs gear hubs? Still not Castor! Se we put all our faith in this little black cube, And in Farouki, who says Don t use lube! We tie down our Shigley s with fishing line tether Then all cross our fingers, and hope it stays together! 12