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| Dyan Pratt (left) and Travis Wuertz align the transmissions on a tractor that UA students will enter in a June 2 tractor pull competition in East Moline, Ill. |
"They give you a 16-horsepower Briggs & Stratton engine and a set of tires," said Wuertz, a mechanical engineering senior. "Then you build everything else from scratch."
He paused, reflected for a moment and added, "Of course, our scratch is a little bit different from other people's scratch."
Which is a bit of an understatement.
Most of the teams that enter this year's American Society of Agricultural Engineers' "¼-Scale Tractor Student Design Competition" will use off-the-shelf hydraulic drive systems. This allows them to run the engine to a hydraulic pump and to send power to the wheels via hydraulic hoses. This system sidesteps aligning drive shafts, dealing with mechanical transmissions, or fabricating lots of components.
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A few teams also will use CV (continuously variable) transmissions, which make design and shifting easier. They work basically like an automatic transmission with a nearly infinite number of gear ratios.
UA's team is not taking the easy road because they want a tractor that offers maximum efficiency and a wide range of speeds, which are handy to have during competition. "We found that when you're hurting for traction, when you have more power than you have traction, speed makes a huge difference," Wuertz said.
Tractor pulls involve dragging a weighted sled that creates more and more drag the farther it's pulled. The tractor that can pull the sled with the greatest drag the farthest distance wins.
UA's design features a totally mechanical drive system using two transmissions and four-wheel drive to produce a tractor that can crawl at 0.75 mph or fly at 26 mph. "If we took the governor off, it would go faster — probably faster than we should go," Wuertz mused.
Wuertz estimates that UA's tractor has at least 200 hours of machine shop work in it so far. Some components are one-of-a-kind. Others are highly modified.
Take the 1985 Celica transmission, for instance. "We pulled the tranny and crankshaft from the car in about half an hour," Wuertz said. "Then we had to adapt the clutch because it has to bolt to our engine. So we took a piece of the crankshaft and welded on an extension. Then we turned it down to one-and-a-quarter inches and it will press fit into this bearing."
 Travis Wuertz test fits a 1985 Celica transmission in UA's ¼-scale tractor. |
The Celica transmission feeds power to a Geo Metro transmission. The result is a two-transmission drivetrain that can be geared down to about a 100-to-1 gear ratio.
Wuertz, who started working in the family-farm machine shop about the same time that he learned to walk, also has TIG welded the tractor's aluminum frame, narrowed the front differential that came from an S-10 Blazer by about 16 inches and shortened the axles.
How does the team do all the sophisticated design work for building a one-off tractor from highly modified components?
"I have a real fancy design program," Wuertz says, as he picks up a piece of chalk from the bench and kneels on the floor. "Except for sketches in class, it's mostly chalk on the floor. I don't do computers very well, and we didn't have anyone draw this up until later."
"Travis is a brilliant artist," said the team's advisor, Don Slack, head of Agricultural and Biosystems Engineering (ABE). "He builds guitars and has been making custom knives for about ten years. He does blacksmithing and wrought iron work. He's in engineering because he wants to know more about technology and materials so that he can apply them to art."
Or four-wheel-drive hotrods.
 Dyan Pratt works on the locker that will be installed in the front differential on UA's four-wheel-drive tractor. |
Dyan Pratt, who just graduated in ABE, also is a mainstay of the team. She grew up on a family farm near Maricopa, Ariz. and is an ace tractor driver, as well as a good mechanic. Last week, for instance, she was putting together the locker system for the tractor's front differential.
Building the tractor has consumed thousands of hours and now is a round-the-clock operation as the students rush to meet the contest deadline. While this would be daunting for many of us, it's not too complicated for Pratt. "When you grow up on a farm, you learn how to work — and how to drive a tractor," she observed.
This is the fourth year that UA has entered the competition. "B&G Auto Salvage in Casa Grande has given us every drivetrain part we've used for the past three years," Wuertz said, which has made it possible for the team to compete on a tight budget.
The completed tractor can be no longer than 96 inches from the rear axle to the tractor's nose and must weigh no more than 850 pounds. Last week, the team was holding off on making the driveshaft until they were sure that the tractor wouldn't be too long. "We may have to cut an inch out of the frame," Wuertz said. "That's no big deal — four cuts and four welds."
While some teams would send this kind of job out to a welding shop and their machine work out to a machinist, everything on the UA tractor is being done in the ABE machine shop at UA's Campbell Avenue Farm.
This year's competition runs from June 2 to June 5. Last year, the UA team placed fifth in the tractor pull and 12th overall. The overall competition factors in points from a written report, design review, oral presentation, and other judging.
"I would not be surprised this year if our team won the pull," Slack said.
But winning isn't the motivating factor for Slack. "They're a fun group to work with," he said. "They're just so dedicated and so full of ideas, enthusiasm and talent. This is the reason I work at the university — because of kids like this. They just really know what they're doing."
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