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The small but avid group of students in the Springfield, Missouri college’s manufacturing technology program learned of the annual event at NASA’s Marshall Flight Center in Huntsville, AL, where the Lunar Roving Vehicle was designed and developed. With the encouragement of machining instructor Scott Beshears and drafting instructor Richard Faber, OTC students and project team leader Pat Clawson sent for information and entry materials. Confirmation and vehicle specs arrived just three weeks before the April 19th competition!
Some entrance requirements were: the vehicle must be solely human powered by one male and one female student; maximum allowable width is 4’, including wheels; vehicle must be collapsible so that all of the moon buggy components fit into a 4’x4’x4’ box. Most entries came from engineering colleges like Arizona State and the University of Florida, with the exception of the confident, eager team from OTC.
The team planned on a 4’ long triangular truss chassis of welded aluminum tubing for central support of a 10-speed bicycle gear train, four double-arm wheel assemblies, laterally extending tube truss seat and foot supports, the steering mechanism, and the simulated instrumentation and equipment NASA required. Pat Clawson began to draw the basic design in AutoCAD and transferred the geometry files directly into Mastercam to generate the toolpaths and NC-code to machine the parts. Mr. Beshears drew up the bare-bones budget for this unexpected project: $1,223.76 for the bicycle, four mountain bike wheels, aluminum tubing and stock, and the hardware to hold it all together. And the search began for qualified aluminum welding students.
The wheel suspension system was the most complex mechanism on the moon buggy. Not unlike a 4WD motorized vehicle, the design had to include basic support, independent suspension, power transmission, and, for the front wheels, a steering mechanism. Pat designed “C” shaped spindle housings, a pair of machined aluminum brackets welded together with concentric vertical bushings to allow the wheel hub spindles to pivot. Horizontal bushings through either end join the wheel hub assembly to the top and bottom A-frames. As the wheels rise and fall with the terrain, the A-frame/spindle housing parallelogram geometry keeps them vertical. Mechanical stops limit up-and-down travel. To machine the eight “C” brackets Pat imported his drawings from DXF format cleanly into Mastercam, which can also accept nearly any other format. Using contour, drill, and pocket functions in Mastercam, he quickly generated a toolpath for the parts for one side of the assemblies.
Checked with solid model verification, the results were perfect. “We virtually never have to do a modification of a toolpath that comes out of Mastercam,” Mr. Beshears notes. “This type of simulated verification helps us find errors and fix them before we cut parts. This increase in productivity allows us the chance to finish projects under tight time restraints.” Beshears also comments about the fact that they own 47 seats of Mastercam. “This tells you how valuable we find Mastercam for real-world training.” Using the program’s “mirror” feature, Pat created the toolpath for the four parts. “Every step of the way,” Beshears notes, “the Mastercam people were quick to respond with technical support, as usual.”
The spindle assemblies, chassis, and tubing sections were ready for welding. Most of the tubing joints were awkward; not the flat-stock stuff of beginning class projects. The welder, student Brian Clift, would have to deal with “fishmouth” joints, the sides of which would vary in heat dispersal and joining spaces. Brian worked closely with Pat as frequently as class load and sleep requirements allowed, given the fleeting time until the competition. Though other students joined in from time to time, Pat carried most of the weight of the job on his shoulders.
The drive train linked the dual pedals with a bicycle chain to a geared jack shaft, which drove another chain to the front and real axle drive gears. At each wheel hub a short axle joined a fixed socket to the transverse axle through a rubber-damped 5/8” deep well socket. The wheel axles, just long enough to remain in the sockets when the A-frames were fully lowered to their stops, slid back and forth to make up the difference in length as the A-frame assemblies adjusted to the terrain.
The team knew that from hub to hub, the moon buggy cleared NASA’s 4’ specification. So did the height, including the plastic seats from classroom chairs, bolted to supports with 1/2”-spaced holes to adjust to the riders. But the length, already 4’, would exceed specs once the wheels were attached. The strategy: Make the wheel hubs the “pre-lunar excursion” assembly points. Pat had designed and machined solid aluminum quick-change hubs in Mastercam and the whole package, with the wheels tucked into the seats, made the four-foot cube as planned.
The team used a novel approach to counter the weight variations between the male and female drivers. Bungee cords of differing lengths, and thus different elasticity, were run between the lower A-frames and the center of the chassis frame, and interchanged as needed.
The drivers were Samantha Allspaw and David Snick. During the last minute testing, David exerted so much pedal force that the torque bent some of the drive axles. A hectic night-before-the-race increase in axle size seemed to solve the problem.
About a quarter of the way through their run at Huntsville, David’s shaft broke so he and Sam switched places. She took over the steering lever, while David provided the only motive power. Occasionally each would grab a tire and help a wheel over a rough spot. They figured out the 4’ wheelbase was too short for the job and decided to make subsequent entries longer. David and Samantha were 18th in the lineup and scored 30.8 seconds for setup and 52 minutes completing the course. The combined time won them 8th place in a field of 22 entries, most with more experience and greater time for design, construction, and testing.
Mr. Beshears feels strongly that the students’ participation in the moon buggy project prepared them for thinking on their feet and putting their skills to work under pressure.
What’s next for OTC? They are already redesigning their entry for next year. Mr. Beshears says that with over 6 programs to choose from, over half of his students choose Mastercam as a package to use because it’s easier to draw in, edits easily, and eliminates file transfers. |