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How engineers developed the rubber-belted tractor

Part One: In the late 1970s Caterpillar was looking for a way to 
break into the ag tractor market. It had two options

This year even a casual stroll through a farm machinery show will reveal the obvious trend toward increased use of rubber-belted track systems. And manufacturers have recently introduced even more options for those who want the benefits that technology offers.

In less than three decades belts have gone from the fringe to the mainstream, so we at Grainews decided it was time to take a look back at how the technology was created. In 2007 I had a chance to talk to the two engineers who collaborated and developed the first belted tractor and hear from them how the process evolved. In the mid 1970s Caterpillar had been selling farmers a relatively small number of steel tracked SA (Special Application) crawlers suited for field work, particularly in some regions of California. But the SA crawlers were gradually losing market share to the increasing number of high-horsepower, four-wheel drive tractors pouring into the segment from all the major manufacturers. With weakened demand for equipment in the construction sector, executives at Cat decided to develop their own four-wheel drive tractor to break into that lucrative market, which offered the potential to boost the company’s flagging sales numbers. In-house development soon began at Cat to create the wheeled ag tractor executives wanted.

“In 1978 I was sitting having lunch with three of my colleagues at Caterpillar’s Peoria Proving Grounds in Illinois, and we were lamenting how Cat was pursuing a ‘me too’ (wheeled tractor) design while our name and reputation were in building the best crawlers in the world,” says Dave Janzen a now-retired Cat engineer and a key force in the eventual development of the belted Challenger tractor. Janzen had been working on making the steel-tracked SA crawlers more attractive to farmers. He believed it was essential to increase their notoriously slow working speeds. To do that, he would need to stuff more power under the hoods. An initial experiment with a modified D5SA fitted with a 225 horsepower engine proved it could be done. On seeing the results of Janzen’s work, management at Cat approved further development aimed at creating a high-horsepower crawler with a forward weight bias, which made it better suited for drawbar pulling.

The project begins

In 1979 the project began in ernest. “Our calculations showed that if we fitted a six-cylinder Cat 3306 engine of 240 horsepower into a smaller, lighter D4, that would give us the 100 pounds per horsepower that we sought to match the wheeled tractors,” says Janzen. Eventually, to evaluate Janzen’s R&D progress and make a decision on the future of the ag crawler project, managers at Cat wanted to see for themselves what the D4 on steroids was really capable of. So another field demonstration was arranged.

cardboard model of a belted tractor

This cardboard model shows the initial design objective for the Caterpillar wheeled tractor project early on in the development phase.
photo: AGCO

The company’s executive vice president and his staff came out to see the D4 be pitted in a head-to-head competition with a much larger, 350 horsepower, four-wheel drive tractor. Both machines were hitched to identical nine-bottom ploughs. As they set off side-by-side down the length of a half-mile field, the executives followed along watching while they sat on a row of hay bales on a wagon pulled by a Farmall M tractor.

Even with a pretty significant 110 horsepower advantage, the bigger wheeled tractor couldn’t outrun its smaller, tracked rival.

The executives were suitably impressed, and they decided to continue funding the ag crawler development. Janzen had proven that the crawler no longer needed to be slow, and moving the chassis forward in relation to the tracks improved its ability to pull. But there were still problems to overcome.

The experimental tractor still used the standard clutch and brake or “jerk” steering common to crawlers of the day, which meant power only flowed to one track during turns. And there was still the mobility restriction due to the steel tracks. Most rural roads around the Proving Ground in Illinois were paved, so the D4 couldn’t travel on them. Wheeled tractors still offered farmers a big advantage when it came to moving from field to field.

But there were potential solutions to those drawbacks, and they were within Cat’s grasp. In the 1960s Cat created its own rubber products division to manufacture steel reinforced hydraulic hose, which had a design created by one of its own engineers. The hose was for use in the manufacture of construction machines. One of the other products that division eventually created was the beadless tire for large off-road machines, which had a removable tire tread. It allowed equipment owners to just replace the tread rather than a whole tire. Even though Cat sold its rubber products division to Goodyear a few years later, it still retained the capability to continue making some rubber products, including the removable tread for the beadless tire.

At the same time Janzen was working on the ag crawler project, company executives had also decided to have another engineer, Ron Satzler, look at potential uses for the removable tire tread, which was about the same size as a steel track.

However, there was still Cat’s own articulated ag tractor project, which was much further advanced than Janzen’s ag crawler concept that was still running on steel tracks. Given the very difficult financial position Cat found itself in at the time, only one ag tractor project was likely to get funding through to completion.

We’ll continue with part two of the story in the next issue of Grainews.

About the author

Contributor

Scott Garvey

Scott Garvey is a freelance writer and video producer. He is also the former machinery editor at Grainews.

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