Machine Design

Digital prototypes deliver four-wheel steering

Moving 3D computer models helped perfect four-wheel steering for a lawn tractor, while fatigue-analysis software zeroed in on areas most prone to failure.

ITI-Manta's design team fashioned four-wheel steering for the Murray lawn tractor. The project finished in only six months and hit its design goals.

Vertical rearwheel forces on the inner pivot bracket were measured during mowing operations. The integral structural components were later analyzed for stress and fatigue.

The steering system was devised for tractors from Murray Inc., Brentwood, Tenn. It uses no power assist yet turns almost as easily as models with two-wheel steering. The mechanism, designed by ITI-Manta, Milford, Ohio (, is less expensive than a previous version and went from initial brainstorming to production in six months.

The tractor maker had fourwheel steering on an older model, but newer competitive tractors were easier to steer. The company analyzed competing designs and decided that a less-expensive unit would be an advantage. Consequently, power adders used on competitive tractors were forbidden.

"A first task was to correlate an existing steering-mechanism model with load and strain data," says Brian Lewis, ITI team leader. "Then we spent a lot of time minimizing the turning force and reducing the possibility of fatigue." Fatigue models indicated potential trouble areas that were later validated by instrumenting prototypes. The short development period called for more digital prototypes and few physical models to prove the design.

Simulation of CAD models up-front has advantages, says Lewis. "It let teams on both sides evaluate numerous alternatives and choose one early in the process," he says. Designs were tested and validated in the computer using actual field data which significantly reduces the amount of testing on physical prototypes. Test models sailed through durability test. In fact, the OEM was so confident of the design that it began tooling for production before the prototypes had finished testing.

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