Here's an example of a MGB coming apart and then getting a chip light.
NTSB Identification: NYC94FA106 .
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14 CFR Part 91: General Aviation
Accident occurred Thursday, June 30, 1994 in NEWPORT, RI
Probable Cause Approval Date: 3/27/1995
Aircraft: SIKORSKY S-76B, registration: N76UT
Injuries: 3 Uninjured.
AFTER DISCHARGING PASSENGERS ON AN ISLAND, THE S-76 HELICOPTER DEPARTED FOR A RETURN FLIGHT TO THE MAINLAND. ONE MINUTE AFTER TAKEOFF, AT 500 FEET ABOVE THE WATER, A VIBRATION FOLLOWED BY A LOUD BANG OCCURRED. THE MAIN GEAR BOX (MGB) CHIP LIGHT ILLUMINATED AND THE NUMBER ONE ENG DROPPED OFF LINE. A FORCED LANDING WAS INITIATED, THE EMERGENCY FLOATS WERE INFLATED AND A DITCHING TO WATER WAS ACCOMPLISHED. THE NUMBER ONE ENG WAS SHUT DOWN, AND THE ACFT WAS WATER TAXIED ON THE SECOND ENG. AFTER A SECOND BANG WAS HEARD THE NUMBER TWO ENG WAS SHUT DOWN. EXAMINATION REVEALED THAT THE LEFT HAND (NUMBER ONE ENG) HELICAL GEAR IN THE MGB, FAILED FROM FATIGUE CRACKS ACROSS THE 12 ATTACHING BOLT HOLES. THE TORQUE REQUIRED ON THE 12 NUTS WAS 4OO INCH POUNDS. THREE NUTS HAD LESS THEN 60 INCH LBS, THE REMAINING NINE HAD NO TORQUE. BOTH MGB OIL PUMPS WERE SEIZED FROM FOD THAT ENTERED THE PUMPS THROUGH AN ALTERNATE OIL PATH, THAT BY-PASSED THE OIL SCREENS. THE NUMBER TWO ENG PINION GEAR IN THE MGB, FAILED DUE TO OVERHEATING FROM THE LACK OF LUBRICATION. ELAPSED TIME WAS LESS THAN 6 MINUTES FROM INITIAL FORCED LANDING TO SECOND ENGINE SHUT DOWN.
The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
the failure of the left hand helical gear in the main gear box (MGB), due to undertorqued attaching bolts installed by the manufacturer, which resulted in a partial MGB failure. This failure resulted in the loss of the number one engine, illumination of the MGB chip light and a ditching to salt water on emergency floats.
Full narrative available