Jim Greenhaw’s robot stops, stutters and lets out a loud grinding noise.

Greenhaw has spent a year building it, but now, in the heat of a competition and with half a million dollars at stake, part of its transmission has blown. The stripped gears gnash past one another like a car with a worn-out clutch.

Greenhaw, of Arroyo Grande, gathered with about 20 other robotics teams from around the country at California Polytechnic State University, San Luis Obispo, on Aug. 2 to show off his moon-dust excavating robot and compete for a $500,000 prize.

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