On February 9, 2001, about 1900 hours Pacific standard time, a Bell 47-G3B1, N3084G, landed hard near Winterhaven, California, following a drive train failure during a night agricultural spraying operation. The helicopter was owned and operated by Sundown Helicopters, Inc., of Yuma, Arizona, and was substantially damaged. The certificated commercial pilot was not injured. Visual meteorological conditions prevailed for the flight, which was operating under 14 CFR Part 137. The flight was originating at the time of the occurrence. The pilot reported that he was applying chemicals to a field of lettuce and had just taken off from his truck platform with 60 gallons of chemicals. He said that he was about 20 feet above ground level and had a forward air speed of approximately 30 miles per hour when he heard a loud screeching sound, followed by a yaw to the left, a rapid decay of rotor rpm, and an increase/overspeed in engine rpm. The helicopter landed hard, collapsing the skids. Post accident examination of the transmission was conducted on February 10, 2001. Examination of the free wheeling unit revealed the outer surface of the inner gear assembly was worn below allowable limits at the roller/gear operating surface. No other discrepancies were noted.