On October 3, 1994, at 1520 central daylight time, a Bell 47G2, N5193B, sustained substantial damage following a loss of engine power and subsequent forced landing on a road near Decatur, Illinois. The commercial pilot and passenger reported minor injuries. The local 14 CFR Part 91 flight operated in visual meteorological conditions without a flight plan. The pilot stated he began a climbing left turn to return to Decatur Airport after completing a line patrol. At approximately 50 to 75 feet above the ground, the pilot stated he heard a strange sound, the engine began to overspeed, and the helicopter began to descend. He lowered the collective and placed the helicopter into an autorotation to make a forced landing. The helicopter landed hard on a road. The helicopter logbook shows the last inspection was a 100 hour inspection performed on September 23, 1994. The helicopter has logged 33 hours since the inspection. A postaccident examination of the engine and transmission revealed 13 of the 32 roller springs were broken inside the freewheeling unit of the transmission. Eight of the 13 broken springs were located in the 270 degree to 360 degree quadrant; a total of 10 were located in the 180 degree to 360 degree quadrant; and the remaining three were dispersed throughout the one degree to 180 degree quadrant. All pieces of the broken springs were self-contained within the freewheeling unit. There were no discrepancies noted to any other component.